Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 9162 insertions, 0 deletions

diff --git a/dom/bindings/parser/WebIDL.py b/dom/bindings/parser/WebIDL.py
new file mode 100644
index 0000000000..9101e14cf2
--- /dev/null
+++ b/dom/bindings/parser/WebIDL.py
@@ -0,0 +1,9162 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+""" A WebIDL parser. """
+
+import copy
+import math
+import os
+import re
+import string
+import traceback
+from collections import OrderedDict, defaultdict
+from itertools import chain
+
+from ply import lex, yacc
+
+# Machinery
+
+
+def parseInt(literal):
+    string = literal
+    sign = 0
+    base = 0
+
+    if string[0] == "-":
+        sign = -1
+        string = string[1:]
+    else:
+        sign = 1
+
+    if string[0] == "0" and len(string) > 1:
+        if string[1] == "x" or string[1] == "X":
+            base = 16
+            string = string[2:]
+        else:
+            base = 8
+            string = string[1:]
+    else:
+        base = 10
+
+    value = int(string, base)
+    return value * sign
+
+
+def enum(*names, **kw):
+    class Foo(object):
+        attrs = OrderedDict()
+
+        def __init__(self, names):
+            for v, k in enumerate(names):
+                self.attrs[k] = v
+
+        def __getattr__(self, attr):
+            if attr in self.attrs:
+                return self.attrs[attr]
+            raise AttributeError
+
+        def __setattr__(self, name, value):  # this makes it read-only
+            raise NotImplementedError
+
+    if "base" not in kw:
+        return Foo(names)
+    return Foo(chain(kw["base"].attrs.keys(), names))
+
+
+class WebIDLError(Exception):
+    def __init__(self, message, locations, warning=False):
+        self.message = message
+        self.locations = [str(loc) for loc in locations]
+        self.warning = warning
+
+    def __str__(self):
+        return "%s: %s%s%s" % (
+            self.warning and "warning" or "error",
+            self.message,
+            ", " if len(self.locations) != 0 else "",
+            "\n".join(self.locations),
+        )
+
+
+class Location(object):
+    def __init__(self, lexer, lineno, lexpos, filename):
+        self._line = None
+        self._lineno = lineno
+        self._lexpos = lexpos
+        self._lexdata = lexer.lexdata
+        self._file = filename if filename else "<unknown>"
+
+    def __eq__(self, other):
+        return self._lexpos == other._lexpos and self._file == other._file
+
+    def filename(self):
+        return self._file
+
+    def resolve(self):
+        if self._line:
+            return
+
+        startofline = self._lexdata.rfind("\n", 0, self._lexpos) + 1
+        endofline = self._lexdata.find("\n", self._lexpos, self._lexpos + 80)
+        if endofline != -1:
+            self._line = self._lexdata[startofline:endofline]
+        else:
+            self._line = self._lexdata[startofline:]
+        self._colno = self._lexpos - startofline
+
+        # Our line number seems to point to the start of self._lexdata
+        self._lineno += self._lexdata.count("\n", 0, startofline)
+
+    def get(self):
+        self.resolve()
+        return "%s line %s:%s" % (self._file, self._lineno, self._colno)
+
+    def _pointerline(self):
+        return " " * self._colno + "^"
+
+    def __str__(self):
+        self.resolve()
+        return "%s line %s:%s\n%s\n%s" % (
+            self._file,
+            self._lineno,
+            self._colno,
+            self._line,
+            self._pointerline(),
+        )
+
+
+class BuiltinLocation(object):
+    def __init__(self, text):
+        self.msg = text + "\n"
+
+    def __eq__(self, other):
+        return isinstance(other, BuiltinLocation) and self.msg == other.msg
+
+    def filename(self):
+        return "<builtin>"
+
+    def resolve(self):
+        pass
+
+    def get(self):
+        return self.msg
+
+    def __str__(self):
+        return self.get()
+
+
+# Data Model
+
+
+class IDLObject(object):
+    def __init__(self, location):
+        self.location = location
+        self.userData = dict()
+
+    def filename(self):
+        return self.location.filename()
+
+    def isInterface(self):
+        return False
+
+    def isNamespace(self):
+        return False
+
+    def isInterfaceMixin(self):
+        return False
+
+    def isEnum(self):
+        return False
+
+    def isCallback(self):
+        return False
+
+    def isType(self):
+        return False
+
+    def isDictionary(self):
+        return False
+
+    def isUnion(self):
+        return False
+
+    def isTypedef(self):
+        return False
+
+    def getUserData(self, key, default):
+        return self.userData.get(key, default)
+
+    def setUserData(self, key, value):
+        self.userData[key] = value
+
+    def addExtendedAttributes(self, attrs):
+        assert False  # Override me!
+
+    def handleExtendedAttribute(self, attr):
+        assert False  # Override me!
+
+    def _getDependentObjects(self):
+        assert False  # Override me!
+
+    def getDeps(self, visited=None):
+        """Return a set of files that this object depends on.  If any of
+        these files are changed the parser needs to be rerun to regenerate
+        a new IDLObject.
+
+        The visited argument is a set of all the objects already visited.
+        We must test to see if we are in it, and if so, do nothing.  This
+        prevents infinite recursion."""
+
+        # NB: We can't use visited=set() above because the default value is
+        # evaluated when the def statement is evaluated, not when the function
+        # is executed, so there would be one set for all invocations.
+        if visited is None:
+            visited = set()
+
+        if self in visited:
+            return set()
+
+        visited.add(self)
+
+        deps = set()
+        if self.filename() != "<builtin>":
+            deps.add(self.filename())
+
+        for d in self._getDependentObjects():
+            deps.update(d.getDeps(visited))
+
+        return deps
+
+
+class IDLScope(IDLObject):
+    def __init__(self, location, parentScope, identifier):
+        IDLObject.__init__(self, location)
+
+        self.parentScope = parentScope
+        if identifier:
+            assert isinstance(identifier, IDLIdentifier)
+            self._name = identifier
+        else:
+            self._name = None
+
+        self._dict = {}
+        self.globalNames = set()
+        # A mapping from global name to the set of global interfaces
+        # that have that global name.
+        self.globalNameMapping = defaultdict(set)
+
+    def __str__(self):
+        return self.QName()
+
+    def QName(self):
+        # It's possible for us to be called before __init__ has been called, for
+        # the IDLObjectWithScope case.  In that case, self._name won't be set yet.
+        if hasattr(self, "_name"):
+            name = self._name
+        else:
+            name = None
+        if name:
+            return name.QName() + "::"
+        return "::"
+
+    def ensureUnique(self, identifier, object):
+        """
+        Ensure that there is at most one 'identifier' in scope ('self').
+        Note that object can be None.  This occurs if we end up here for an
+        interface type we haven't seen yet.
+        """
+        assert isinstance(identifier, IDLUnresolvedIdentifier)
+        assert not object or isinstance(object, IDLObjectWithIdentifier)
+        assert not object or object.identifier == identifier
+
+        if identifier.name in self._dict:
+            if not object:
+                return
+
+            # ensureUnique twice with the same object is not allowed
+            assert id(object) != id(self._dict[identifier.name])
+
+            replacement = self.resolveIdentifierConflict(
+                self, identifier, self._dict[identifier.name], object
+            )
+            self._dict[identifier.name] = replacement
+            return
+
+        self.addNewIdentifier(identifier, object)
+
+    def addNewIdentifier(self, identifier, object):
+        assert object
+
+        self._dict[identifier.name] = object
+
+    def resolveIdentifierConflict(self, scope, identifier, originalObject, newObject):
+        if (
+            isinstance(originalObject, IDLExternalInterface)
+            and isinstance(newObject, IDLExternalInterface)
+            and originalObject.identifier.name == newObject.identifier.name
+        ):
+            return originalObject
+
+        if isinstance(originalObject, IDLExternalInterface) or isinstance(
+            newObject, IDLExternalInterface
+        ):
+            raise WebIDLError(
+                "Name collision between "
+                "interface declarations for identifier '%s' at '%s' and '%s'"
+                % (identifier.name, originalObject.location, newObject.location),
+                [],
+            )
+
+        if isinstance(originalObject, IDLDictionary) or isinstance(
+            newObject, IDLDictionary
+        ):
+            raise WebIDLError(
+                "Name collision between dictionary declarations for "
+                "identifier '%s'.\n%s\n%s"
+                % (identifier.name, originalObject.location, newObject.location),
+                [],
+            )
+
+        # We do the merging of overloads here as opposed to in IDLInterface
+        # because we need to merge overloads of LegacyFactoryFunctions and we need to
+        # detect conflicts in those across interfaces. See also the comment in
+        # IDLInterface.addExtendedAttributes for "LegacyFactoryFunction".
+        if isinstance(originalObject, IDLMethod) and isinstance(newObject, IDLMethod):
+            return originalObject.addOverload(newObject)
+
+        # Default to throwing, derived classes can override.
+        raise self.createIdentifierConflictError(identifier, originalObject, newObject)
+
+    def createIdentifierConflictError(self, identifier, originalObject, newObject):
+        conflictdesc = "\n\t%s at %s\n\t%s at %s" % (
+            originalObject,
+            originalObject.location,
+            newObject,
+            newObject.location,
+        )
+
+        return WebIDLError(
+            "Multiple unresolvable definitions of identifier '%s' in scope '%s'%s"
+            % (identifier.name, str(self), conflictdesc),
+            [],
+        )
+
+    def _lookupIdentifier(self, identifier):
+        return self._dict[identifier.name]
+
+    def lookupIdentifier(self, identifier):
+        assert isinstance(identifier, IDLIdentifier)
+        assert identifier.scope == self
+        return self._lookupIdentifier(identifier)
+
+    def addIfaceGlobalNames(self, interfaceName, globalNames):
+        """Record the global names (from |globalNames|) that can be used in
+        [Exposed] to expose things in a global named |interfaceName|"""
+        self.globalNames.update(globalNames)
+        for name in globalNames:
+            self.globalNameMapping[name].add(interfaceName)
+
+
+class IDLIdentifier(IDLObject):
+    def __init__(self, location, scope, name):
+        IDLObject.__init__(self, location)
+
+        self.name = name
+        assert isinstance(scope, IDLScope)
+        self.scope = scope
+
+    def __str__(self):
+        return self.QName()
+
+    def QName(self):
+        return self.scope.QName() + self.name
+
+    def __hash__(self):
+        return self.QName().__hash__()
+
+    def __eq__(self, other):
+        return self.QName() == other.QName()
+
+    def object(self):
+        return self.scope.lookupIdentifier(self)
+
+
+class IDLUnresolvedIdentifier(IDLObject):
+    def __init__(
+        self, location, name, allowDoubleUnderscore=False, allowForbidden=False
+    ):
+        IDLObject.__init__(self, location)
+
+        assert len(name) > 0
+
+        if name == "__noSuchMethod__":
+            raise WebIDLError("__noSuchMethod__ is deprecated", [location])
+
+        if name[:2] == "__" and not allowDoubleUnderscore:
+            raise WebIDLError("Identifiers beginning with __ are reserved", [location])
+        if name[0] == "_" and not allowDoubleUnderscore:
+            name = name[1:]
+        if name in ["constructor", "toString"] and not allowForbidden:
+            raise WebIDLError(
+                "Cannot use reserved identifier '%s'" % (name), [location]
+            )
+
+        self.name = name
+
+    def __str__(self):
+        return self.QName()
+
+    def QName(self):
+        return "<unresolved scope>::" + self.name
+
+    def resolve(self, scope, object):
+        assert isinstance(scope, IDLScope)
+        assert not object or isinstance(object, IDLObjectWithIdentifier)
+        assert not object or object.identifier == self
+
+        scope.ensureUnique(self, object)
+
+        identifier = IDLIdentifier(self.location, scope, self.name)
+        if object:
+            object.identifier = identifier
+        return identifier
+
+    def finish(self):
+        assert False  # Should replace with a resolved identifier first.
+
+
+class IDLObjectWithIdentifier(IDLObject):
+    def __init__(self, location, parentScope, identifier):
+        IDLObject.__init__(self, location)
+
+        assert isinstance(identifier, IDLUnresolvedIdentifier)
+
+        self.identifier = identifier
+
+        if parentScope:
+            self.resolve(parentScope)
+
+    def resolve(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        assert isinstance(self.identifier, IDLUnresolvedIdentifier)
+        self.identifier.resolve(parentScope, self)
+
+
+class IDLObjectWithScope(IDLObjectWithIdentifier, IDLScope):
+    def __init__(self, location, parentScope, identifier):
+        assert isinstance(identifier, IDLUnresolvedIdentifier)
+
+        IDLObjectWithIdentifier.__init__(self, location, parentScope, identifier)
+        IDLScope.__init__(self, location, parentScope, self.identifier)
+
+
+class IDLIdentifierPlaceholder(IDLObjectWithIdentifier):
+    def __init__(self, location, identifier):
+        assert isinstance(identifier, IDLUnresolvedIdentifier)
+        IDLObjectWithIdentifier.__init__(self, location, None, identifier)
+
+    def finish(self, scope):
+        try:
+            scope._lookupIdentifier(self.identifier)
+        except Exception:
+            raise WebIDLError(
+                "Unresolved type '%s'." % self.identifier, [self.location]
+            )
+
+        obj = self.identifier.resolve(scope, None)
+        return scope.lookupIdentifier(obj)
+
+
+class IDLExposureMixins:
+    def __init__(self, location):
+        # _exposureGlobalNames are the global names listed in our [Exposed]
+        # extended attribute.  exposureSet is the exposure set as defined in the
+        # Web IDL spec: it contains interface names.
+        self._exposureGlobalNames = set()
+        self.exposureSet = set()
+        self._location = location
+        self._globalScope = None
+
+    def finish(self, scope):
+        assert scope.parentScope is None
+        self._globalScope = scope
+
+        if "*" in self._exposureGlobalNames:
+            self._exposureGlobalNames = scope.globalNames
+        else:
+            # Verify that our [Exposed] value, if any, makes sense.
+            for globalName in self._exposureGlobalNames:
+                if globalName not in scope.globalNames:
+                    raise WebIDLError(
+                        "Unknown [Exposed] value %s" % globalName, [self._location]
+                    )
+
+        # Verify that we are exposed _somwhere_ if we have some place to be
+        # exposed.  We don't want to assert that we're definitely exposed
+        # because a lot of our parser tests have small-enough IDL snippets that
+        # they don't include any globals, and we don't really want to go through
+        # and add global interfaces and [Exposed] annotations to all those
+        # tests.
+        if len(scope.globalNames) != 0 and len(self._exposureGlobalNames) == 0:
+            raise WebIDLError(
+                (
+                    "'%s' is not exposed anywhere even though we have "
+                    "globals to be exposed to"
+                )
+                % self,
+                [self.location],
+            )
+
+        globalNameSetToExposureSet(scope, self._exposureGlobalNames, self.exposureSet)
+
+    def isExposedInWindow(self):
+        return "Window" in self.exposureSet
+
+    def isExposedInAnyWorker(self):
+        return len(self.getWorkerExposureSet()) > 0
+
+    def isExposedInWorkerDebugger(self):
+        return len(self.getWorkerDebuggerExposureSet()) > 0
+
+    def isExposedInAnyWorklet(self):
+        return len(self.getWorkletExposureSet()) > 0
+
+    def isExposedInSomeButNotAllWorkers(self):
+        """
+        Returns true if the Exposed extended attribute for this interface
+        exposes it in some worker globals but not others.  The return value does
+        not depend on whether the interface is exposed in Window or System
+        globals.
+        """
+        if not self.isExposedInAnyWorker():
+            return False
+        workerScopes = self.parentScope.globalNameMapping["Worker"]
+        return len(workerScopes.difference(self.exposureSet)) > 0
+
+    def isExposedInShadowRealms(self):
+        return "ShadowRealmGlobalScope" in self.exposureSet
+
+    def getWorkerExposureSet(self):
+        workerScopes = self._globalScope.globalNameMapping["Worker"]
+        return workerScopes.intersection(self.exposureSet)
+
+    def getWorkletExposureSet(self):
+        workletScopes = self._globalScope.globalNameMapping["Worklet"]
+        return workletScopes.intersection(self.exposureSet)
+
+    def getWorkerDebuggerExposureSet(self):
+        workerDebuggerScopes = self._globalScope.globalNameMapping["WorkerDebugger"]
+        return workerDebuggerScopes.intersection(self.exposureSet)
+
+
+class IDLExternalInterface(IDLObjectWithIdentifier):
+    def __init__(self, location, parentScope, identifier):
+        assert isinstance(identifier, IDLUnresolvedIdentifier)
+        assert isinstance(parentScope, IDLScope)
+        self.parent = None
+        IDLObjectWithIdentifier.__init__(self, location, parentScope, identifier)
+        IDLObjectWithIdentifier.resolve(self, parentScope)
+
+    def finish(self, scope):
+        pass
+
+    def validate(self):
+        pass
+
+    def isIteratorInterface(self):
+        return False
+
+    def isAsyncIteratorInterface(self):
+        return False
+
+    def isExternal(self):
+        return True
+
+    def isInterface(self):
+        return True
+
+    def addExtendedAttributes(self, attrs):
+        if len(attrs) != 0:
+            raise WebIDLError(
+                "There are no extended attributes that are "
+                "allowed on external interfaces",
+                [attrs[0].location, self.location],
+            )
+
+    def resolve(self, parentScope):
+        pass
+
+    def getJSImplementation(self):
+        return None
+
+    def isJSImplemented(self):
+        return False
+
+    def hasProbablyShortLivingWrapper(self):
+        return False
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLPartialDictionary(IDLObject):
+    def __init__(self, location, name, members, nonPartialDictionary):
+        assert isinstance(name, IDLUnresolvedIdentifier)
+
+        IDLObject.__init__(self, location)
+        self.identifier = name
+        self.members = members
+        self._nonPartialDictionary = nonPartialDictionary
+        self._finished = False
+        nonPartialDictionary.addPartialDictionary(self)
+
+    def addExtendedAttributes(self, attrs):
+        pass
+
+    def finish(self, scope):
+        if self._finished:
+            return
+        self._finished = True
+
+        # Need to make sure our non-partial dictionary gets
+        # finished so it can report cases when we only have partial
+        # dictionaries.
+        self._nonPartialDictionary.finish(scope)
+
+    def validate(self):
+        pass
+
+
+class IDLPartialInterfaceOrNamespace(IDLObject):
+    def __init__(self, location, name, members, nonPartialInterfaceOrNamespace):
+        assert isinstance(name, IDLUnresolvedIdentifier)
+
+        IDLObject.__init__(self, location)
+        self.identifier = name
+        self.members = members
+        # propagatedExtendedAttrs are the ones that should get
+        # propagated to our non-partial interface.
+        self.propagatedExtendedAttrs = []
+        self._haveSecureContextExtendedAttribute = False
+        self._nonPartialInterfaceOrNamespace = nonPartialInterfaceOrNamespace
+        self._finished = False
+        nonPartialInterfaceOrNamespace.addPartial(self)
+
+    def addExtendedAttributes(self, attrs):
+        for attr in attrs:
+            identifier = attr.identifier()
+
+            if identifier == "LegacyFactoryFunction":
+                self.propagatedExtendedAttrs.append(attr)
+            elif identifier == "SecureContext":
+                self._haveSecureContextExtendedAttribute = True
+                # This gets propagated to all our members.
+                for member in self.members:
+                    if member.getExtendedAttribute("SecureContext"):
+                        raise WebIDLError(
+                            "[SecureContext] specified on both a "
+                            "partial interface member and on the "
+                            "partial interface itself",
+                            [member.location, attr.location],
+                        )
+                    member.addExtendedAttributes([attr])
+            elif identifier == "Exposed":
+                # This just gets propagated to all our members.
+                for member in self.members:
+                    if len(member._exposureGlobalNames) != 0:
+                        raise WebIDLError(
+                            "[Exposed] specified on both a "
+                            "partial interface member and on the "
+                            "partial interface itself",
+                            [member.location, attr.location],
+                        )
+                    member.addExtendedAttributes([attr])
+            else:
+                raise WebIDLError(
+                    "Unknown extended attribute %s on partial "
+                    "interface" % identifier,
+                    [attr.location],
+                )
+
+    def finish(self, scope):
+        if self._finished:
+            return
+        self._finished = True
+        if (
+            not self._haveSecureContextExtendedAttribute
+            and self._nonPartialInterfaceOrNamespace.getExtendedAttribute(
+                "SecureContext"
+            )
+        ):
+            # This gets propagated to all our members.
+            for member in self.members:
+                if member.getExtendedAttribute("SecureContext"):
+                    raise WebIDLError(
+                        "[SecureContext] specified on both a "
+                        "partial interface member and on the "
+                        "non-partial interface",
+                        [
+                            member.location,
+                            self._nonPartialInterfaceOrNamespace.location,
+                        ],
+                    )
+                member.addExtendedAttributes(
+                    [
+                        IDLExtendedAttribute(
+                            self._nonPartialInterfaceOrNamespace.location,
+                            ("SecureContext",),
+                        )
+                    ]
+                )
+        # Need to make sure our non-partial interface or namespace gets
+        # finished so it can report cases when we only have partial
+        # interfaces/namespaces.
+        self._nonPartialInterfaceOrNamespace.finish(scope)
+
+    def validate(self):
+        pass
+
+
+def convertExposedAttrToGlobalNameSet(exposedAttr, targetSet):
+    assert len(targetSet) == 0
+    if exposedAttr.hasValue():
+        targetSet.add(exposedAttr.value())
+    else:
+        assert exposedAttr.hasArgs()
+        targetSet.update(exposedAttr.args())
+
+
+def globalNameSetToExposureSet(globalScope, nameSet, exposureSet):
+    for name in nameSet:
+        exposureSet.update(globalScope.globalNameMapping[name])
+
+
+# Because WebIDL allows static and regular operations with the same identifier
+# we use a special class to be able to store them both in the scope for the
+# same identifier.
+class IDLOperations:
+    def __init__(self, static=None, regular=None):
+        self.static = static
+        self.regular = regular
+
+
+class IDLInterfaceOrInterfaceMixinOrNamespace(IDLObjectWithScope, IDLExposureMixins):
+    def __init__(self, location, parentScope, name):
+        assert isinstance(parentScope, IDLScope)
+        assert isinstance(name, IDLUnresolvedIdentifier)
+
+        self._finished = False
+        self.members = []
+        self._partials = []
+        self._extendedAttrDict = {}
+        self._isKnownNonPartial = False
+
+        IDLObjectWithScope.__init__(self, location, parentScope, name)
+        IDLExposureMixins.__init__(self, location)
+
+    def finish(self, scope):
+        if not self._isKnownNonPartial:
+            raise WebIDLError(
+                "%s does not have a non-partial declaration" % str(self),
+                [self.location],
+            )
+
+        IDLExposureMixins.finish(self, scope)
+
+        # Now go ahead and merge in our partials.
+        for partial in self._partials:
+            partial.finish(scope)
+            self.addExtendedAttributes(partial.propagatedExtendedAttrs)
+            self.members.extend(partial.members)
+
+    def addNewIdentifier(self, identifier, object):
+        if isinstance(object, IDLMethod):
+            if object.isStatic():
+                object = IDLOperations(static=object)
+            else:
+                object = IDLOperations(regular=object)
+
+        IDLScope.addNewIdentifier(self, identifier, object)
+
+    def resolveIdentifierConflict(self, scope, identifier, originalObject, newObject):
+        assert isinstance(scope, IDLScope)
+        assert isinstance(newObject, IDLInterfaceMember)
+
+        # The identifier of a regular operation or static operation must not be
+        # the same as the identifier of a constant or attribute.
+        if isinstance(newObject, IDLMethod) != isinstance(
+            originalObject, IDLOperations
+        ):
+            if isinstance(originalObject, IDLOperations):
+                if originalObject.regular is not None:
+                    originalObject = originalObject.regular
+                else:
+                    assert originalObject.static is not None
+                    originalObject = originalObject.static
+
+            raise self.createIdentifierConflictError(
+                identifier, originalObject, newObject
+            )
+
+        if isinstance(newObject, IDLMethod):
+            originalOperations = originalObject
+            if newObject.isStatic():
+                if originalOperations.static is None:
+                    originalOperations.static = newObject
+                    return originalOperations
+
+                originalObject = originalOperations.static
+            else:
+                if originalOperations.regular is None:
+                    originalOperations.regular = newObject
+                    return originalOperations
+
+                originalObject = originalOperations.regular
+
+            assert isinstance(originalObject, IDLMethod)
+        else:
+            assert isinstance(originalObject, IDLInterfaceMember)
+
+        retval = IDLScope.resolveIdentifierConflict(
+            self, scope, identifier, originalObject, newObject
+        )
+
+        if isinstance(newObject, IDLMethod):
+            if newObject.isStatic():
+                originalOperations.static = retval
+            else:
+                originalOperations.regular = retval
+
+            retval = originalOperations
+
+        # Might be a ctor, which isn't in self.members
+        if newObject in self.members:
+            self.members.remove(newObject)
+        return retval
+
+    def typeName(self):
+        if self.isInterface():
+            return "interface"
+        if self.isNamespace():
+            return "namespace"
+        assert self.isInterfaceMixin()
+        return "interface mixin"
+
+    def getExtendedAttribute(self, name):
+        return self._extendedAttrDict.get(name, None)
+
+    def setNonPartial(self, location, members):
+        if self._isKnownNonPartial:
+            raise WebIDLError(
+                "Two non-partial definitions for the " "same %s" % self.typeName(),
+                [location, self.location],
+            )
+        self._isKnownNonPartial = True
+        # Now make it look like we were parsed at this new location, since
+        # that's the place where the interface is "really" defined
+        self.location = location
+        # Put the new members at the beginning
+        self.members = members + self.members
+
+    def addPartial(self, partial):
+        assert self.identifier.name == partial.identifier.name
+        self._partials.append(partial)
+
+    def getPartials(self):
+        # Don't let people mutate our guts.
+        return list(self._partials)
+
+    def finishMembers(self, scope):
+        # Assuming we've merged in our partials, set the _exposureGlobalNames on
+        # any members that don't have it set yet.  Note that any partial
+        # interfaces that had [Exposed] set have already set up
+        # _exposureGlobalNames on all the members coming from them, so this is
+        # just implementing the "members default to interface or interface mixin
+        # that defined them" and "partial interfaces or interface mixins default
+        # to interface or interface mixin they're a partial for" rules from the
+        # spec.
+        for m in self.members:
+            # If m, or the partial m came from, had [Exposed]
+            # specified, it already has a nonempty exposure global names set.
+            if len(m._exposureGlobalNames) == 0:
+                m._exposureGlobalNames.update(self._exposureGlobalNames)
+            if m.isAttr() and m.stringifier:
+                m.expand(self.members)
+
+        # resolve() will modify self.members, so we need to iterate
+        # over a copy of the member list here.
+        for member in list(self.members):
+            member.resolve(self)
+
+        for member in self.members:
+            member.finish(scope)
+
+        # Now that we've finished our members, which has updated their exposure
+        # sets, make sure they aren't exposed in places where we are not.
+        for member in self.members:
+            if not member.exposureSet.issubset(self.exposureSet):
+                raise WebIDLError(
+                    "Interface or interface mixin member has "
+                    "larger exposure set than its container",
+                    [member.location, self.location],
+                )
+
+    def isExternal(self):
+        return False
+
+
+class IDLInterfaceMixin(IDLInterfaceOrInterfaceMixinOrNamespace):
+    def __init__(self, location, parentScope, name, members, isKnownNonPartial):
+        self.actualExposureGlobalNames = set()
+
+        assert isKnownNonPartial or len(members) == 0
+        IDLInterfaceOrInterfaceMixinOrNamespace.__init__(
+            self, location, parentScope, name
+        )
+
+        if isKnownNonPartial:
+            self.setNonPartial(location, members)
+
+    def __str__(self):
+        return "Interface mixin '%s'" % self.identifier.name
+
+    def isInterfaceMixin(self):
+        return True
+
+    def finish(self, scope):
+        if self._finished:
+            return
+        self._finished = True
+
+        # Expose to the globals of interfaces that includes this mixin if this
+        # mixin has no explicit [Exposed] so that its members can be exposed
+        # based on the base interface exposure set.
+        #
+        # Make sure this is done before IDLExposureMixins.finish call, since
+        # that converts our set of exposure global names to an actual exposure
+        # set.
+        hasImplicitExposure = len(self._exposureGlobalNames) == 0
+        if hasImplicitExposure:
+            self._exposureGlobalNames.update(self.actualExposureGlobalNames)
+
+        IDLInterfaceOrInterfaceMixinOrNamespace.finish(self, scope)
+
+        self.finishMembers(scope)
+
+    def validate(self):
+        for member in self.members:
+            if member.isAttr():
+                if member.inherit:
+                    raise WebIDLError(
+                        "Interface mixin member cannot include "
+                        "an inherited attribute",
+                        [member.location, self.location],
+                    )
+                if member.isStatic():
+                    raise WebIDLError(
+                        "Interface mixin member cannot include " "a static member",
+                        [member.location, self.location],
+                    )
+
+            if member.isMethod():
+                if member.isStatic():
+                    raise WebIDLError(
+                        "Interface mixin member cannot include " "a static operation",
+                        [member.location, self.location],
+                    )
+                if (
+                    member.isGetter()
+                    or member.isSetter()
+                    or member.isDeleter()
+                    or member.isLegacycaller()
+                ):
+                    raise WebIDLError(
+                        "Interface mixin member cannot include a " "special operation",
+                        [member.location, self.location],
+                    )
+
+    def addExtendedAttributes(self, attrs):
+        for attr in attrs:
+            identifier = attr.identifier()
+
+            if identifier == "SecureContext":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must take no arguments" % identifier, [attr.location]
+                    )
+                # This gets propagated to all our members.
+                for member in self.members:
+                    if member.getExtendedAttribute("SecureContext"):
+                        raise WebIDLError(
+                            "[SecureContext] specified on both "
+                            "an interface mixin member and on"
+                            "the interface mixin itself",
+                            [member.location, attr.location],
+                        )
+                    member.addExtendedAttributes([attr])
+            elif identifier == "Exposed":
+                convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+            else:
+                raise WebIDLError(
+                    "Unknown extended attribute %s on interface" % identifier,
+                    [attr.location],
+                )
+
+            attrlist = attr.listValue()
+            self._extendedAttrDict[identifier] = attrlist if len(attrlist) else True
+
+    def _getDependentObjects(self):
+        return set(self.members)
+
+
+class IDLInterfaceOrNamespace(IDLInterfaceOrInterfaceMixinOrNamespace):
+    def __init__(self, location, parentScope, name, parent, members, isKnownNonPartial):
+        assert isKnownNonPartial or not parent
+        assert isKnownNonPartial or len(members) == 0
+
+        self.parent = None
+        self._callback = False
+        self.maplikeOrSetlikeOrIterable = None
+        # legacyFactoryFunctions needs deterministic ordering because bindings code
+        # outputs the constructs in the order that legacyFactoryFunctions enumerates
+        # them.
+        self.legacyFactoryFunctions = list()
+        self.legacyWindowAliases = []
+        self.includedMixins = set()
+        # self.interfacesBasedOnSelf is the set of interfaces that inherit from
+        # self, including self itself.
+        # Used for distinguishability checking.
+        self.interfacesBasedOnSelf = set([self])
+        self._hasChildInterfaces = False
+        self._isOnGlobalProtoChain = False
+
+        # Tracking of the number of reserved slots we need for our
+        # members and those of ancestor interfaces.
+        self.totalMembersInSlots = 0
+        # Tracking of the number of own own members we have in slots
+        self._ownMembersInSlots = 0
+        # If this is an iterator interface, we need to know what iterable
+        # interface we're iterating for in order to get its nativeType.
+        self.iterableInterface = None
+        self.asyncIterableInterface = None
+        # True if we have cross-origin members.
+        self.hasCrossOriginMembers = False
+        # True if some descendant (including ourselves) has cross-origin members
+        self.hasDescendantWithCrossOriginMembers = False
+
+        IDLInterfaceOrInterfaceMixinOrNamespace.__init__(
+            self, location, parentScope, name
+        )
+
+        if isKnownNonPartial:
+            self.setNonPartial(location, parent, members)
+
+    def ctor(self):
+        identifier = IDLUnresolvedIdentifier(
+            self.location, "constructor", allowForbidden=True
+        )
+        try:
+            return self._lookupIdentifier(identifier).static
+        except Exception:
+            return None
+
+    def isIterable(self):
+        return (
+            self.maplikeOrSetlikeOrIterable
+            and self.maplikeOrSetlikeOrIterable.isIterable()
+        )
+
+    def isAsyncIterable(self):
+        return (
+            self.maplikeOrSetlikeOrIterable
+            and self.maplikeOrSetlikeOrIterable.isAsyncIterable()
+        )
+
+    def isIteratorInterface(self):
+        return self.iterableInterface is not None
+
+    def isAsyncIteratorInterface(self):
+        return self.asyncIterableInterface is not None
+
+    def getClassName(self):
+        return self.identifier.name
+
+    def finish(self, scope):
+        if self._finished:
+            return
+
+        self._finished = True
+
+        IDLInterfaceOrInterfaceMixinOrNamespace.finish(self, scope)
+
+        if len(self.legacyWindowAliases) > 0:
+            if not self.hasInterfaceObject():
+                raise WebIDLError(
+                    "Interface %s unexpectedly has [LegacyWindowAlias] "
+                    "and [LegacyNoInterfaceObject] together" % self.identifier.name,
+                    [self.location],
+                )
+            if not self.isExposedInWindow():
+                raise WebIDLError(
+                    "Interface %s has [LegacyWindowAlias] "
+                    "but not exposed in Window" % self.identifier.name,
+                    [self.location],
+                )
+
+        # Generate maplike/setlike interface members. Since generated members
+        # need to be treated like regular interface members, do this before
+        # things like exposure setting.
+        for member in self.members:
+            if member.isMaplikeOrSetlikeOrIterable():
+                if self.isJSImplemented():
+                    raise WebIDLError(
+                        "%s declaration used on "
+                        "interface that is implemented in JS"
+                        % (member.maplikeOrSetlikeOrIterableType),
+                        [member.location],
+                    )
+                if member.valueType.isObservableArray() or (
+                    member.hasKeyType() and member.keyType.isObservableArray()
+                ):
+                    raise WebIDLError(
+                        "%s declaration uses ObservableArray as value or key type"
+                        % (member.maplikeOrSetlikeOrIterableType),
+                        [member.location],
+                    )
+                # Check that we only have one interface declaration (currently
+                # there can only be one maplike/setlike declaration per
+                # interface)
+                if self.maplikeOrSetlikeOrIterable:
+                    raise WebIDLError(
+                        "%s declaration used on "
+                        "interface that already has %s "
+                        "declaration"
+                        % (
+                            member.maplikeOrSetlikeOrIterableType,
+                            self.maplikeOrSetlikeOrIterable.maplikeOrSetlikeOrIterableType,
+                        ),
+                        [self.maplikeOrSetlikeOrIterable.location, member.location],
+                    )
+                self.maplikeOrSetlikeOrIterable = member
+                # If we've got a maplike or setlike declaration, we'll be building all of
+                # our required methods in Codegen. Generate members now.
+                self.maplikeOrSetlikeOrIterable.expand(self.members)
+
+        assert not self.parent or isinstance(self.parent, IDLIdentifierPlaceholder)
+        parent = self.parent.finish(scope) if self.parent else None
+        if parent and isinstance(parent, IDLExternalInterface):
+            raise WebIDLError(
+                "%s inherits from %s which does not have "
+                "a definition" % (self.identifier.name, self.parent.identifier.name),
+                [self.location],
+            )
+        if parent and not isinstance(parent, IDLInterface):
+            raise WebIDLError(
+                "%s inherits from %s which is not an interface "
+                % (self.identifier.name, self.parent.identifier.name),
+                [self.location, parent.location],
+            )
+
+        self.parent = parent
+
+        assert iter(self.members)
+
+        if self.isNamespace():
+            assert not self.parent
+            for m in self.members:
+                if m.isAttr() or m.isMethod():
+                    if m.isStatic():
+                        raise WebIDLError(
+                            "Don't mark things explicitly static " "in namespaces",
+                            [self.location, m.location],
+                        )
+                    # Just mark all our methods/attributes as static.  The other
+                    # option is to duplicate the relevant InterfaceMembers
+                    # production bits but modified to produce static stuff to
+                    # start with, but that sounds annoying.
+                    m.forceStatic()
+
+        if self.parent:
+            self.parent.finish(scope)
+            self.parent._hasChildInterfaces = True
+
+            self.totalMembersInSlots = self.parent.totalMembersInSlots
+
+            # Interfaces with [Global] must not have anything inherit from them
+            if self.parent.getExtendedAttribute("Global"):
+                # Note: This is not a self.parent.isOnGlobalProtoChain() check
+                # because ancestors of a [Global] interface can have other
+                # descendants.
+                raise WebIDLError(
+                    "[Global] interface has another interface " "inheriting from it",
+                    [self.location, self.parent.location],
+                )
+
+            # Make sure that we're not exposed in places where our parent is not
+            if not self.exposureSet.issubset(self.parent.exposureSet):
+                raise WebIDLError(
+                    "Interface %s is exposed in globals where its "
+                    "parent interface %s is not exposed."
+                    % (self.identifier.name, self.parent.identifier.name),
+                    [self.location, self.parent.location],
+                )
+
+            # Callbacks must not inherit from non-callbacks.
+            # XXXbz Can non-callbacks inherit from callbacks?  Spec issue pending.
+            if self.isCallback():
+                if not self.parent.isCallback():
+                    raise WebIDLError(
+                        "Callback interface %s inheriting from "
+                        "non-callback interface %s"
+                        % (self.identifier.name, self.parent.identifier.name),
+                        [self.location, self.parent.location],
+                    )
+            elif self.parent.isCallback():
+                raise WebIDLError(
+                    "Non-callback interface %s inheriting from "
+                    "callback interface %s"
+                    % (self.identifier.name, self.parent.identifier.name),
+                    [self.location, self.parent.location],
+                )
+
+            # Interfaces which have interface objects can't inherit
+            # from [LegacyNoInterfaceObject] interfaces.
+            if self.parent.getExtendedAttribute(
+                "LegacyNoInterfaceObject"
+            ) and not self.getExtendedAttribute("LegacyNoInterfaceObject"):
+                raise WebIDLError(
+                    "Interface %s does not have "
+                    "[LegacyNoInterfaceObject] but inherits from "
+                    "interface %s which does"
+                    % (self.identifier.name, self.parent.identifier.name),
+                    [self.location, self.parent.location],
+                )
+
+            # Interfaces that are not [SecureContext] can't inherit
+            # from [SecureContext] interfaces.
+            if self.parent.getExtendedAttribute(
+                "SecureContext"
+            ) and not self.getExtendedAttribute("SecureContext"):
+                raise WebIDLError(
+                    "Interface %s does not have "
+                    "[SecureContext] but inherits from "
+                    "interface %s which does"
+                    % (self.identifier.name, self.parent.identifier.name),
+                    [self.location, self.parent.location],
+                )
+
+        for mixin in self.includedMixins:
+            mixin.finish(scope)
+
+        cycleInGraph = self.findInterfaceLoopPoint(self)
+        if cycleInGraph:
+            raise WebIDLError(
+                "Interface %s has itself as ancestor" % self.identifier.name,
+                [self.location, cycleInGraph.location],
+            )
+
+        self.finishMembers(scope)
+
+        ctor = self.ctor()
+        if ctor is not None:
+            if not self.hasInterfaceObject():
+                raise WebIDLError(
+                    "Can't have both a constructor and [LegacyNoInterfaceObject]",
+                    [self.location, ctor.location],
+                )
+
+            if self.globalNames:
+                raise WebIDLError(
+                    "Can't have both a constructor and [Global]",
+                    [self.location, ctor.location],
+                )
+
+            assert ctor._exposureGlobalNames == self._exposureGlobalNames
+            ctor._exposureGlobalNames.update(self._exposureGlobalNames)
+            # Remove the constructor operation from our member list so
+            # it doesn't get in the way later.
+            self.members.remove(ctor)
+
+        for ctor in self.legacyFactoryFunctions:
+            if self.globalNames:
+                raise WebIDLError(
+                    "Can't have both a legacy factory function and [Global]",
+                    [self.location, ctor.location],
+                )
+            assert len(ctor._exposureGlobalNames) == 0
+            ctor._exposureGlobalNames.update(self._exposureGlobalNames)
+            ctor.finish(scope)
+
+        # Make a copy of our member list, so things that implement us
+        # can get those without all the stuff we implement ourselves
+        # admixed.
+        self.originalMembers = list(self.members)
+
+        for mixin in sorted(self.includedMixins, key=lambda x: x.identifier.name):
+            for mixinMember in mixin.members:
+                for member in self.members:
+                    if mixinMember.identifier.name == member.identifier.name and (
+                        not mixinMember.isMethod()
+                        or not member.isMethod()
+                        or mixinMember.isStatic() == member.isStatic()
+                    ):
+                        raise WebIDLError(
+                            "Multiple definitions of %s on %s coming from 'includes' statements"
+                            % (member.identifier.name, self),
+                            [mixinMember.location, member.location],
+                        )
+            self.members.extend(mixin.members)
+
+        for ancestor in self.getInheritedInterfaces():
+            ancestor.interfacesBasedOnSelf.add(self)
+            if (
+                ancestor.maplikeOrSetlikeOrIterable is not None
+                and self.maplikeOrSetlikeOrIterable is not None
+            ):
+                raise WebIDLError(
+                    "Cannot have maplike/setlike on %s that "
+                    "inherits %s, which is already "
+                    "maplike/setlike"
+                    % (self.identifier.name, ancestor.identifier.name),
+                    [
+                        self.maplikeOrSetlikeOrIterable.location,
+                        ancestor.maplikeOrSetlikeOrIterable.location,
+                    ],
+                )
+
+        # Deal with interfaces marked [LegacyUnforgeable], now that we have our full
+        # member list, except unforgeables pulled in from parents.  We want to
+        # do this before we set "originatingInterface" on our unforgeable
+        # members.
+        if self.getExtendedAttribute("LegacyUnforgeable"):
+            # Check that the interface already has all the things the
+            # spec would otherwise require us to synthesize and is
+            # missing the ones we plan to synthesize.
+            if not any(m.isMethod() and m.isStringifier() for m in self.members):
+                raise WebIDLError(
+                    "LegacyUnforgeable interface %s does not have a "
+                    "stringifier" % self.identifier.name,
+                    [self.location],
+                )
+
+            for m in self.members:
+                if m.identifier.name == "toJSON":
+                    raise WebIDLError(
+                        "LegacyUnforgeable interface %s has a "
+                        "toJSON so we won't be able to add "
+                        "one ourselves" % self.identifier.name,
+                        [self.location, m.location],
+                    )
+
+                if m.identifier.name == "valueOf" and not m.isStatic():
+                    raise WebIDLError(
+                        "LegacyUnforgeable interface %s has a valueOf "
+                        "member so we won't be able to add one "
+                        "ourselves" % self.identifier.name,
+                        [self.location, m.location],
+                    )
+
+        for member in self.members:
+            if (
+                (member.isAttr() or member.isMethod())
+                and member.isLegacyUnforgeable()
+                and not hasattr(member, "originatingInterface")
+            ):
+                member.originatingInterface = self
+
+        for member in self.members:
+            if (
+                member.isMethod() and member.getExtendedAttribute("CrossOriginCallable")
+            ) or (
+                member.isAttr()
+                and (
+                    member.getExtendedAttribute("CrossOriginReadable")
+                    or member.getExtendedAttribute("CrossOriginWritable")
+                )
+            ):
+                self.hasCrossOriginMembers = True
+                break
+
+        if self.hasCrossOriginMembers:
+            parent = self
+            while parent:
+                parent.hasDescendantWithCrossOriginMembers = True
+                parent = parent.parent
+
+        # Compute slot indices for our members before we pull in unforgeable
+        # members from our parent. Also, maplike/setlike declarations get a
+        # slot to hold their backing object.
+        for member in self.members:
+            if (
+                member.isAttr()
+                and (
+                    member.getExtendedAttribute("StoreInSlot")
+                    or member.getExtendedAttribute("Cached")
+                    or member.type.isObservableArray()
+                )
+            ) or member.isMaplikeOrSetlike():
+                if self.isJSImplemented() and not member.isMaplikeOrSetlike():
+                    raise WebIDLError(
+                        "Interface %s is JS-implemented and we "
+                        "don't support [Cached] or [StoreInSlot] or ObservableArray "
+                        "on JS-implemented interfaces" % self.identifier.name,
+                        [self.location, member.location],
+                    )
+                if member.slotIndices is None:
+                    member.slotIndices = dict()
+                member.slotIndices[self.identifier.name] = self.totalMembersInSlots
+                self.totalMembersInSlots += 1
+                if member.getExtendedAttribute("StoreInSlot"):
+                    self._ownMembersInSlots += 1
+
+        if self.parent:
+            # Make sure we don't shadow any of the [LegacyUnforgeable] attributes on our
+            # ancestor interfaces.  We don't have to worry about mixins here, because
+            # those have already been imported into the relevant .members lists.  And
+            # we don't have to worry about anything other than our parent, because it
+            # has already imported its ancestors' unforgeable attributes into its
+            # member list.
+            for unforgeableMember in (
+                member
+                for member in self.parent.members
+                if (member.isAttr() or member.isMethod())
+                and member.isLegacyUnforgeable()
+            ):
+                shadows = [
+                    m
+                    for m in self.members
+                    if (m.isAttr() or m.isMethod())
+                    and not m.isStatic()
+                    and m.identifier.name == unforgeableMember.identifier.name
+                ]
+                if len(shadows) != 0:
+                    locs = [unforgeableMember.location] + [s.location for s in shadows]
+                    raise WebIDLError(
+                        "Interface %s shadows [LegacyUnforgeable] "
+                        "members of %s"
+                        % (self.identifier.name, ancestor.identifier.name),
+                        locs,
+                    )
+                # And now just stick it in our members, since we won't be
+                # inheriting this down the proto chain.  If we really cared we
+                # could try to do something where we set up the unforgeable
+                # attributes/methods of ancestor interfaces, with their
+                # corresponding getters, on our interface, but that gets pretty
+                # complicated and seems unnecessary.
+                self.members.append(unforgeableMember)
+
+        # At this point, we have all of our members. If the current interface
+        # uses maplike/setlike, check for collisions anywhere in the current
+        # interface or higher in the inheritance chain.
+        if self.maplikeOrSetlikeOrIterable:
+            testInterface = self
+            isAncestor = False
+            while testInterface:
+                self.maplikeOrSetlikeOrIterable.checkCollisions(
+                    testInterface.members, isAncestor
+                )
+                isAncestor = True
+                testInterface = testInterface.parent
+
+        # Ensure that there's at most one of each {named,indexed}
+        # {getter,setter,deleter}, at most one stringifier,
+        # and at most one legacycaller.  Note that this last is not
+        # quite per spec, but in practice no one overloads
+        # legacycallers.  Also note that in practice we disallow
+        # indexed deleters, but it simplifies some other code to
+        # treat deleter analogously to getter/setter by
+        # prefixing it with "named".
+        specialMembersSeen = {}
+        for member in self.members:
+            if not member.isMethod():
+                continue
+
+            if member.isGetter():
+                memberType = "getters"
+            elif member.isSetter():
+                memberType = "setters"
+            elif member.isDeleter():
+                memberType = "deleters"
+            elif member.isStringifier():
+                memberType = "stringifiers"
+            elif member.isLegacycaller():
+                memberType = "legacycallers"
+            else:
+                continue
+
+            if memberType != "stringifiers" and memberType != "legacycallers":
+                if member.isNamed():
+                    memberType = "named " + memberType
+                else:
+                    assert member.isIndexed()
+                    memberType = "indexed " + memberType
+
+            if memberType in specialMembersSeen:
+                raise WebIDLError(
+                    "Multiple " + memberType + " on %s" % (self),
+                    [
+                        self.location,
+                        specialMembersSeen[memberType].location,
+                        member.location,
+                    ],
+                )
+
+            specialMembersSeen[memberType] = member
+
+        if self.getExtendedAttribute("LegacyUnenumerableNamedProperties"):
+            # Check that we have a named getter.
+            if "named getters" not in specialMembersSeen:
+                raise WebIDLError(
+                    "Interface with [LegacyUnenumerableNamedProperties] does "
+                    "not have a named getter",
+                    [self.location],
+                )
+            ancestor = self.parent
+            while ancestor:
+                if ancestor.getExtendedAttribute("LegacyUnenumerableNamedProperties"):
+                    raise WebIDLError(
+                        "Interface with [LegacyUnenumerableNamedProperties] "
+                        "inherits from another interface with "
+                        "[LegacyUnenumerableNamedProperties]",
+                        [self.location, ancestor.location],
+                    )
+                ancestor = ancestor.parent
+
+        if self._isOnGlobalProtoChain:
+            # Make sure we have no named setters or deleters
+            for memberType in ["setter", "deleter"]:
+                memberId = "named " + memberType + "s"
+                if memberId in specialMembersSeen:
+                    raise WebIDLError(
+                        "Interface with [Global] has a named %s" % memberType,
+                        [self.location, specialMembersSeen[memberId].location],
+                    )
+            # Make sure we're not [LegacyOverrideBuiltIns]
+            if self.getExtendedAttribute("LegacyOverrideBuiltIns"):
+                raise WebIDLError(
+                    "Interface with [Global] also has " "[LegacyOverrideBuiltIns]",
+                    [self.location],
+                )
+            # Mark all of our ancestors as being on the global's proto chain too
+            parent = self.parent
+            while parent:
+                # Must not inherit from an interface with [LegacyOverrideBuiltIns]
+                if parent.getExtendedAttribute("LegacyOverrideBuiltIns"):
+                    raise WebIDLError(
+                        "Interface with [Global] inherits from "
+                        "interface with [LegacyOverrideBuiltIns]",
+                        [self.location, parent.location],
+                    )
+                parent._isOnGlobalProtoChain = True
+                parent = parent.parent
+
+    def validate(self):
+        def checkDuplicateNames(member, name, attributeName):
+            for m in self.members:
+                if m.identifier.name == name:
+                    raise WebIDLError(
+                        "[%s=%s] has same name as interface member"
+                        % (attributeName, name),
+                        [member.location, m.location],
+                    )
+                if m.isMethod() and m != member and name in m.aliases:
+                    raise WebIDLError(
+                        "conflicting [%s=%s] definitions" % (attributeName, name),
+                        [member.location, m.location],
+                    )
+                if m.isAttr() and m != member and name in m.bindingAliases:
+                    raise WebIDLError(
+                        "conflicting [%s=%s] definitions" % (attributeName, name),
+                        [member.location, m.location],
+                    )
+
+        # We also don't support inheriting from unforgeable interfaces.
+        if self.getExtendedAttribute("LegacyUnforgeable") and self.hasChildInterfaces():
+            locations = [self.location] + list(
+                i.location for i in self.interfacesBasedOnSelf if i.parent == self
+            )
+            raise WebIDLError(
+                "%s is an unforgeable ancestor interface" % self.identifier.name,
+                locations,
+            )
+
+        ctor = self.ctor()
+        if ctor is not None:
+            ctor.validate()
+        for namedCtor in self.legacyFactoryFunctions:
+            namedCtor.validate()
+
+        indexedGetter = None
+        hasLengthAttribute = False
+        for member in self.members:
+            member.validate()
+
+            if self.isCallback() and member.getExtendedAttribute("Replaceable"):
+                raise WebIDLError(
+                    "[Replaceable] used on an attribute on "
+                    "interface %s which is a callback interface" % self.identifier.name,
+                    [self.location, member.location],
+                )
+
+            # Check that PutForwards refers to another attribute and that no
+            # cycles exist in forwarded assignments.  Also check for a
+            # integer-typed "length" attribute.
+            if member.isAttr():
+                if member.identifier.name == "length" and member.type.isInteger():
+                    hasLengthAttribute = True
+
+                iface = self
+                attr = member
+                putForwards = attr.getExtendedAttribute("PutForwards")
+                if putForwards and self.isCallback():
+                    raise WebIDLError(
+                        "[PutForwards] used on an attribute "
+                        "on interface %s which is a callback "
+                        "interface" % self.identifier.name,
+                        [self.location, member.location],
+                    )
+
+                while putForwards is not None:
+                    forwardIface = attr.type.unroll().inner
+                    fowardAttr = None
+
+                    for forwardedMember in forwardIface.members:
+                        if (
+                            not forwardedMember.isAttr()
+                            or forwardedMember.identifier.name != putForwards[0]
+                        ):
+                            continue
+                        if forwardedMember == member:
+                            raise WebIDLError(
+                                "Cycle detected in forwarded "
+                                "assignments for attribute %s on "
+                                "%s" % (member.identifier.name, self),
+                                [member.location],
+                            )
+                        fowardAttr = forwardedMember
+                        break
+
+                    if fowardAttr is None:
+                        raise WebIDLError(
+                            "Attribute %s on %s forwards to "
+                            "missing attribute %s"
+                            % (attr.identifier.name, iface, putForwards),
+                            [attr.location],
+                        )
+
+                    iface = forwardIface
+                    attr = fowardAttr
+                    putForwards = attr.getExtendedAttribute("PutForwards")
+
+            # Check that the name of an [Alias] doesn't conflict with an
+            # interface member and whether we support indexed properties.
+            if member.isMethod():
+                if member.isGetter() and member.isIndexed():
+                    indexedGetter = member
+
+                for alias in member.aliases:
+                    if self.isOnGlobalProtoChain():
+                        raise WebIDLError(
+                            "[Alias] must not be used on a "
+                            "[Global] interface operation",
+                            [member.location],
+                        )
+                    if (
+                        member.getExtendedAttribute("Exposed")
+                        or member.getExtendedAttribute("ChromeOnly")
+                        or member.getExtendedAttribute("Pref")
+                        or member.getExtendedAttribute("Func")
+                        or member.getExtendedAttribute("Trial")
+                        or member.getExtendedAttribute("SecureContext")
+                    ):
+                        raise WebIDLError(
+                            "[Alias] must not be used on a "
+                            "conditionally exposed operation",
+                            [member.location],
+                        )
+                    if member.isStatic():
+                        raise WebIDLError(
+                            "[Alias] must not be used on a " "static operation",
+                            [member.location],
+                        )
+                    if member.isIdentifierLess():
+                        raise WebIDLError(
+                            "[Alias] must not be used on an "
+                            "identifierless operation",
+                            [member.location],
+                        )
+                    if member.isLegacyUnforgeable():
+                        raise WebIDLError(
+                            "[Alias] must not be used on an "
+                            "[LegacyUnforgeable] operation",
+                            [member.location],
+                        )
+
+                    checkDuplicateNames(member, alias, "Alias")
+
+            # Check that the name of a [BindingAlias] doesn't conflict with an
+            # interface member.
+            if member.isAttr():
+                for bindingAlias in member.bindingAliases:
+                    checkDuplicateNames(member, bindingAlias, "BindingAlias")
+
+        # Conditional exposure makes no sense for interfaces with no
+        # interface object.
+        # And SecureContext makes sense for interfaces with no interface object,
+        # since it is also propagated to interface members.
+        if (
+            self.isExposedConditionally(exclusions=["SecureContext"])
+            and not self.hasInterfaceObject()
+        ):
+            raise WebIDLError(
+                "Interface with no interface object is " "exposed conditionally",
+                [self.location],
+            )
+
+        # Value iterators are only allowed on interfaces with indexed getters,
+        # and pair iterators are only allowed on interfaces without indexed
+        # getters.
+        if self.isIterable():
+            iterableDecl = self.maplikeOrSetlikeOrIterable
+            if iterableDecl.isValueIterator():
+                if not indexedGetter:
+                    raise WebIDLError(
+                        "Interface with value iterator does not "
+                        "support indexed properties",
+                        [self.location, iterableDecl.location],
+                    )
+
+                if iterableDecl.valueType != indexedGetter.signatures()[0][0]:
+                    raise WebIDLError(
+                        "Iterable type does not match indexed " "getter type",
+                        [iterableDecl.location, indexedGetter.location],
+                    )
+
+                if not hasLengthAttribute:
+                    raise WebIDLError(
+                        "Interface with value iterator does not "
+                        'have an integer-typed "length" attribute',
+                        [self.location, iterableDecl.location],
+                    )
+            else:
+                assert iterableDecl.isPairIterator()
+                if indexedGetter:
+                    raise WebIDLError(
+                        "Interface with pair iterator supports " "indexed properties",
+                        [self.location, iterableDecl.location, indexedGetter.location],
+                    )
+
+        if indexedGetter and not hasLengthAttribute:
+            raise WebIDLError(
+                "Interface with an indexed getter does not have "
+                'an integer-typed "length" attribute',
+                [self.location, indexedGetter.location],
+            )
+
+    def setCallback(self, value):
+        self._callback = value
+
+    def isCallback(self):
+        return self._callback
+
+    def isSingleOperationInterface(self):
+        assert self.isCallback() or self.isJSImplemented()
+        return (
+            # JS-implemented things should never need the
+            # this-handling weirdness of single-operation interfaces.
+            not self.isJSImplemented()
+            and
+            # Not inheriting from another interface
+            not self.parent
+            and
+            # No attributes of any kinds
+            not any(m.isAttr() for m in self.members)
+            and
+            # There is at least one regular operation, and all regular
+            # operations have the same identifier
+            len(
+                set(
+                    m.identifier.name
+                    for m in self.members
+                    if m.isMethod() and not m.isStatic()
+                )
+            )
+            == 1
+        )
+
+    def inheritanceDepth(self):
+        depth = 0
+        parent = self.parent
+        while parent:
+            depth = depth + 1
+            parent = parent.parent
+        return depth
+
+    def hasConstants(self):
+        return any(m.isConst() for m in self.members)
+
+    def hasInterfaceObject(self):
+        if self.isCallback():
+            return self.hasConstants()
+        return not hasattr(self, "_noInterfaceObject")
+
+    def hasInterfacePrototypeObject(self):
+        return (
+            not self.isCallback()
+            and not self.isNamespace()
+            and self.getUserData("hasConcreteDescendant", False)
+        )
+
+    def addIncludedMixin(self, includedMixin):
+        assert isinstance(includedMixin, IDLInterfaceMixin)
+        self.includedMixins.add(includedMixin)
+
+    def getInheritedInterfaces(self):
+        """
+        Returns a list of the interfaces this interface inherits from
+        (not including this interface itself).  The list is in order
+        from most derived to least derived.
+        """
+        assert self._finished
+        if not self.parent:
+            return []
+        parentInterfaces = self.parent.getInheritedInterfaces()
+        parentInterfaces.insert(0, self.parent)
+        return parentInterfaces
+
+    def findInterfaceLoopPoint(self, otherInterface):
+        """
+        Finds an interface amongst our ancestors that inherits from otherInterface.
+        If there is no such interface, returns None.
+        """
+        if self.parent:
+            if self.parent == otherInterface:
+                return self
+            loopPoint = self.parent.findInterfaceLoopPoint(otherInterface)
+            if loopPoint:
+                return loopPoint
+        return None
+
+    def setNonPartial(self, location, parent, members):
+        assert not parent or isinstance(parent, IDLIdentifierPlaceholder)
+        IDLInterfaceOrInterfaceMixinOrNamespace.setNonPartial(self, location, members)
+        assert not self.parent
+        self.parent = parent
+
+    def getJSImplementation(self):
+        classId = self.getExtendedAttribute("JSImplementation")
+        if not classId:
+            return classId
+        assert isinstance(classId, list)
+        assert len(classId) == 1
+        return classId[0]
+
+    def isJSImplemented(self):
+        return bool(self.getJSImplementation())
+
+    def hasProbablyShortLivingWrapper(self):
+        current = self
+        while current:
+            if current.getExtendedAttribute("ProbablyShortLivingWrapper"):
+                return True
+            current = current.parent
+        return False
+
+    def hasChildInterfaces(self):
+        return self._hasChildInterfaces
+
+    def isOnGlobalProtoChain(self):
+        return self._isOnGlobalProtoChain
+
+    def _getDependentObjects(self):
+        deps = set(self.members)
+        deps.update(self.includedMixins)
+        if self.parent:
+            deps.add(self.parent)
+        return deps
+
+    def hasMembersInSlots(self):
+        return self._ownMembersInSlots != 0
+
+    conditionExtendedAttributes = [
+        "Pref",
+        "ChromeOnly",
+        "Func",
+        "Trial",
+        "SecureContext",
+    ]
+
+    def isExposedConditionally(self, exclusions=[]):
+        return any(
+            ((a not in exclusions) and self.getExtendedAttribute(a))
+            for a in self.conditionExtendedAttributes
+        )
+
+
+class IDLInterface(IDLInterfaceOrNamespace):
+    def __init__(
+        self,
+        location,
+        parentScope,
+        name,
+        parent,
+        members,
+        isKnownNonPartial,
+        classNameOverride=None,
+    ):
+        IDLInterfaceOrNamespace.__init__(
+            self, location, parentScope, name, parent, members, isKnownNonPartial
+        )
+        self.classNameOverride = classNameOverride
+
+    def __str__(self):
+        return "Interface '%s'" % self.identifier.name
+
+    def isInterface(self):
+        return True
+
+    def getClassName(self):
+        if self.classNameOverride:
+            return self.classNameOverride
+        return IDLInterfaceOrNamespace.getClassName(self)
+
+    def addExtendedAttributes(self, attrs):
+        for attr in attrs:
+            identifier = attr.identifier()
+
+            # Special cased attrs
+            if identifier == "TreatNonCallableAsNull":
+                raise WebIDLError(
+                    "TreatNonCallableAsNull cannot be specified on interfaces",
+                    [attr.location, self.location],
+                )
+            if identifier == "LegacyTreatNonObjectAsNull":
+                raise WebIDLError(
+                    "LegacyTreatNonObjectAsNull cannot be specified on interfaces",
+                    [attr.location, self.location],
+                )
+            elif identifier == "LegacyNoInterfaceObject":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[LegacyNoInterfaceObject] must take no arguments",
+                        [attr.location],
+                    )
+
+                self._noInterfaceObject = True
+            elif identifier == "LegacyFactoryFunction":
+                if not attr.hasValue():
+                    raise WebIDLError(
+                        (
+                            "LegacyFactoryFunction must either take an "
+                            "identifier or take a named argument list"
+                        ),
+                        [attr.location],
+                    )
+
+                args = attr.args() if attr.hasArgs() else []
+
+                method = IDLConstructor(attr.location, args, attr.value())
+                method.reallyInit(self)
+
+                # Legacy factory functions are always assumed to be able to
+                # throw (since there's no way to indicate otherwise).
+                method.addExtendedAttributes(
+                    [IDLExtendedAttribute(self.location, ("Throws",))]
+                )
+
+                # We need to detect conflicts for LegacyFactoryFunctions across
+                # interfaces. We first call resolve on the parentScope,
+                # which will merge all LegacyFactoryFunctions with the same
+                # identifier accross interfaces as overloads.
+                method.resolve(self.parentScope)
+
+                # Then we look up the identifier on the parentScope. If the
+                # result is the same as the method we're adding then it
+                # hasn't been added as an overload and it's the first time
+                # we've encountered a LegacyFactoryFunction with that identifier.
+                # If the result is not the same as the method we're adding
+                # then it has been added as an overload and we need to check
+                # whether the result is actually one of our existing
+                # LegacyFactoryFunctions.
+                newMethod = self.parentScope.lookupIdentifier(method.identifier)
+                if newMethod == method:
+                    self.legacyFactoryFunctions.append(method)
+                elif newMethod not in self.legacyFactoryFunctions:
+                    raise WebIDLError(
+                        "LegacyFactoryFunction conflicts with a "
+                        "LegacyFactoryFunction of a different interface",
+                        [method.location, newMethod.location],
+                    )
+            elif identifier == "ExceptionClass":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[ExceptionClass] must take no arguments", [attr.location]
+                    )
+                if self.parent:
+                    raise WebIDLError(
+                        "[ExceptionClass] must not be specified on "
+                        "an interface with inherited interfaces",
+                        [attr.location, self.location],
+                    )
+            elif identifier == "Global":
+                if attr.hasValue():
+                    self.globalNames = [attr.value()]
+                elif attr.hasArgs():
+                    self.globalNames = attr.args()
+                else:
+                    self.globalNames = [self.identifier.name]
+                self.parentScope.addIfaceGlobalNames(
+                    self.identifier.name, self.globalNames
+                )
+                self._isOnGlobalProtoChain = True
+            elif identifier == "LegacyWindowAlias":
+                if attr.hasValue():
+                    self.legacyWindowAliases = [attr.value()]
+                elif attr.hasArgs():
+                    self.legacyWindowAliases = attr.args()
+                else:
+                    raise WebIDLError(
+                        "[%s] must either take an identifier "
+                        "or take an identifier list" % identifier,
+                        [attr.location],
+                    )
+                for alias in self.legacyWindowAliases:
+                    unresolved = IDLUnresolvedIdentifier(attr.location, alias)
+                    IDLObjectWithIdentifier(attr.location, self.parentScope, unresolved)
+            elif identifier == "SecureContext":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must take no arguments" % identifier, [attr.location]
+                    )
+                # This gets propagated to all our members.
+                for member in self.members:
+                    if member.getExtendedAttribute("SecureContext"):
+                        raise WebIDLError(
+                            "[SecureContext] specified on both "
+                            "an interface member and on the "
+                            "interface itself",
+                            [member.location, attr.location],
+                        )
+                    member.addExtendedAttributes([attr])
+            elif (
+                identifier == "NeedResolve"
+                or identifier == "LegacyOverrideBuiltIns"
+                or identifier == "ChromeOnly"
+                or identifier == "LegacyUnforgeable"
+                or identifier == "LegacyEventInit"
+                or identifier == "ProbablyShortLivingWrapper"
+                or identifier == "LegacyUnenumerableNamedProperties"
+                or identifier == "RunConstructorInCallerCompartment"
+                or identifier == "WantsEventListenerHooks"
+                or identifier == "Serializable"
+            ):
+                # Known extended attributes that do not take values
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must take no arguments" % identifier, [attr.location]
+                    )
+            elif identifier == "Exposed":
+                convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+            elif (
+                identifier == "Pref"
+                or identifier == "JSImplementation"
+                or identifier == "HeaderFile"
+                or identifier == "Func"
+                or identifier == "Trial"
+                or identifier == "Deprecated"
+            ):
+                # Known extended attributes that take a string value
+                if not attr.hasValue():
+                    raise WebIDLError(
+                        "[%s] must have a value" % identifier, [attr.location]
+                    )
+            elif identifier == "InstrumentedProps":
+                # Known extended attributes that take a list
+                if not attr.hasArgs():
+                    raise WebIDLError(
+                        "[%s] must have arguments" % identifier, [attr.location]
+                    )
+            else:
+                raise WebIDLError(
+                    "Unknown extended attribute %s on interface" % identifier,
+                    [attr.location],
+                )
+
+            attrlist = attr.listValue()
+            self._extendedAttrDict[identifier] = attrlist if len(attrlist) else True
+
+    def validate(self):
+        IDLInterfaceOrNamespace.validate(self)
+        if self.parent and self.isSerializable() and not self.parent.isSerializable():
+            raise WebIDLError(
+                "Serializable interface inherits from non-serializable "
+                "interface.  Per spec, that means the object should not be "
+                "serializable, so chances are someone made a mistake here "
+                "somewhere.",
+                [self.location, self.parent.location],
+            )
+
+    def isSerializable(self):
+        return self.getExtendedAttribute("Serializable")
+
+    def setNonPartial(self, location, parent, members):
+        # Before we do anything else, finish initializing any constructors that
+        # might be in "members", so we don't have partially-initialized objects
+        # hanging around.  We couldn't do it before now because we needed to have
+        # to have the IDLInterface on hand to properly set the return type.
+        for member in members:
+            if isinstance(member, IDLConstructor):
+                member.reallyInit(self)
+
+        IDLInterfaceOrNamespace.setNonPartial(self, location, parent, members)
+
+
+class IDLNamespace(IDLInterfaceOrNamespace):
+    def __init__(self, location, parentScope, name, members, isKnownNonPartial):
+        IDLInterfaceOrNamespace.__init__(
+            self, location, parentScope, name, None, members, isKnownNonPartial
+        )
+
+    def __str__(self):
+        return "Namespace '%s'" % self.identifier.name
+
+    def isNamespace(self):
+        return True
+
+    def addExtendedAttributes(self, attrs):
+        # The set of things namespaces support is small enough it's simpler
+        # to factor out into a separate method than it is to sprinkle
+        # isNamespace() checks all through
+        # IDLInterfaceOrNamespace.addExtendedAttributes.
+        for attr in attrs:
+            identifier = attr.identifier()
+
+            if identifier == "Exposed":
+                convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+            elif identifier == "ClassString":
+                # Takes a string value to override the default "Object" if
+                # desired.
+                if not attr.hasValue():
+                    raise WebIDLError(
+                        "[%s] must have a value" % identifier, [attr.location]
+                    )
+            elif identifier == "ProtoObjectHack" or identifier == "ChromeOnly":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must not have arguments" % identifier, [attr.location]
+                    )
+            elif (
+                identifier == "Pref"
+                or identifier == "HeaderFile"
+                or identifier == "Func"
+                or identifier == "Trial"
+            ):
+                # Known extended attributes that take a string value
+                if not attr.hasValue():
+                    raise WebIDLError(
+                        "[%s] must have a value" % identifier, [attr.location]
+                    )
+            else:
+                raise WebIDLError(
+                    "Unknown extended attribute %s on namespace" % identifier,
+                    [attr.location],
+                )
+
+            attrlist = attr.listValue()
+            self._extendedAttrDict[identifier] = attrlist if len(attrlist) else True
+
+    def isSerializable(self):
+        return False
+
+
+class IDLDictionary(IDLObjectWithScope):
+    def __init__(self, location, parentScope, name, parent, members):
+        assert isinstance(parentScope, IDLScope)
+        assert isinstance(name, IDLUnresolvedIdentifier)
+        assert not parent or isinstance(parent, IDLIdentifierPlaceholder)
+
+        self.parent = parent
+        self._finished = False
+        self.members = list(members)
+        self._partialDictionaries = []
+        self._extendedAttrDict = {}
+        self.needsConversionToJS = False
+        self.needsConversionFromJS = False
+
+        IDLObjectWithScope.__init__(self, location, parentScope, name)
+
+    def __str__(self):
+        return "Dictionary '%s'" % self.identifier.name
+
+    def isDictionary(self):
+        return True
+
+    def canBeEmpty(self):
+        """
+        Returns true if this dictionary can be empty (that is, it has no
+        required members and neither do any of its ancestors).
+        """
+        return all(member.optional for member in self.members) and (
+            not self.parent or self.parent.canBeEmpty()
+        )
+
+    def finish(self, scope):
+        if self._finished:
+            return
+
+        self._finished = True
+
+        if self.parent:
+            assert isinstance(self.parent, IDLIdentifierPlaceholder)
+            oldParent = self.parent
+            self.parent = self.parent.finish(scope)
+            if not isinstance(self.parent, IDLDictionary):
+                raise WebIDLError(
+                    "Dictionary %s has parent that is not a dictionary"
+                    % self.identifier.name,
+                    [oldParent.location, self.parent.location],
+                )
+
+            # Make sure the parent resolves all its members before we start
+            # looking at them.
+            self.parent.finish(scope)
+
+        # Now go ahead and merge in our partial dictionaries.
+        for partial in self._partialDictionaries:
+            partial.finish(scope)
+            self.members.extend(partial.members)
+
+        for member in self.members:
+            member.resolve(self)
+            if not member.isComplete():
+                member.complete(scope)
+                assert member.type.isComplete()
+
+        # Members of a dictionary are sorted in lexicographic order,
+        # unless the dictionary opts out.
+        if not self.getExtendedAttribute("Unsorted"):
+            self.members.sort(key=lambda x: x.identifier.name)
+
+        inheritedMembers = []
+        ancestor = self.parent
+        while ancestor:
+            if ancestor == self:
+                raise WebIDLError(
+                    "Dictionary %s has itself as an ancestor" % self.identifier.name,
+                    [self.identifier.location],
+                )
+            inheritedMembers.extend(ancestor.members)
+            ancestor = ancestor.parent
+
+        # Catch name duplication
+        for inheritedMember in inheritedMembers:
+            for member in self.members:
+                if member.identifier.name == inheritedMember.identifier.name:
+                    raise WebIDLError(
+                        "Dictionary %s has two members with name %s"
+                        % (self.identifier.name, member.identifier.name),
+                        [member.location, inheritedMember.location],
+                    )
+
+    def validate(self):
+        def typeContainsDictionary(memberType, dictionary):
+            """
+            Returns a tuple whose:
+
+                - First element is a Boolean value indicating whether
+                  memberType contains dictionary.
+
+                - Second element is:
+                    A list of locations that leads from the type that was passed in
+                    the memberType argument, to the dictionary being validated,
+                    if the boolean value in the first element is True.
+
+                    None, if the boolean value in the first element is False.
+            """
+
+            if (
+                memberType.nullable()
+                or memberType.isSequence()
+                or memberType.isRecord()
+            ):
+                return typeContainsDictionary(memberType.inner, dictionary)
+
+            if memberType.isDictionary():
+                if memberType.inner == dictionary:
+                    return (True, [memberType.location])
+
+                (contains, locations) = dictionaryContainsDictionary(
+                    memberType.inner, dictionary
+                )
+                if contains:
+                    return (True, [memberType.location] + locations)
+
+            if memberType.isUnion():
+                for member in memberType.flatMemberTypes:
+                    (contains, locations) = typeContainsDictionary(member, dictionary)
+                    if contains:
+                        return (True, locations)
+
+            return (False, None)
+
+        def dictionaryContainsDictionary(dictMember, dictionary):
+            for member in dictMember.members:
+                (contains, locations) = typeContainsDictionary(member.type, dictionary)
+                if contains:
+                    return (True, [member.location] + locations)
+
+            if dictMember.parent:
+                if dictMember.parent == dictionary:
+                    return (True, [dictMember.location])
+                else:
+                    (contains, locations) = dictionaryContainsDictionary(
+                        dictMember.parent, dictionary
+                    )
+                    if contains:
+                        return (True, [dictMember.location] + locations)
+
+            return (False, None)
+
+        for member in self.members:
+            if member.type.isDictionary() and member.type.nullable():
+                raise WebIDLError(
+                    "Dictionary %s has member with nullable "
+                    "dictionary type" % self.identifier.name,
+                    [member.location],
+                )
+            (contains, locations) = typeContainsDictionary(member.type, self)
+            if contains:
+                raise WebIDLError(
+                    "Dictionary %s has member with itself as type."
+                    % self.identifier.name,
+                    [member.location] + locations,
+                )
+
+            if member.type.isUndefined():
+                raise WebIDLError(
+                    "Dictionary %s has member with undefined as its type."
+                    % self.identifier.name,
+                    [member.location],
+                )
+            elif member.type.isUnion():
+                for unionMember in member.type.unroll().flatMemberTypes:
+                    if unionMember.isUndefined():
+                        raise WebIDLError(
+                            "Dictionary %s has member with a union containing "
+                            "undefined as a type." % self.identifier.name,
+                            [unionMember.location],
+                        )
+
+    def getExtendedAttribute(self, name):
+        return self._extendedAttrDict.get(name, None)
+
+    def addExtendedAttributes(self, attrs):
+        for attr in attrs:
+            identifier = attr.identifier()
+
+            if identifier == "GenerateInitFromJSON" or identifier == "GenerateInit":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must not have arguments" % identifier, [attr.location]
+                    )
+                self.needsConversionFromJS = True
+            elif (
+                identifier == "GenerateConversionToJS" or identifier == "GenerateToJSON"
+            ):
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[%s] must not have arguments" % identifier, [attr.location]
+                    )
+                # ToJSON methods require to-JS conversion, because we
+                # implement ToJSON by converting to a JS object and
+                # then using JSON.stringify.
+                self.needsConversionToJS = True
+            elif identifier == "Unsorted":
+                if not attr.noArguments():
+                    raise WebIDLError(
+                        "[Unsorted] must take no arguments", [attr.location]
+                    )
+            else:
+                raise WebIDLError(
+                    "[%s] extended attribute not allowed on "
+                    "dictionaries" % identifier,
+                    [attr.location],
+                )
+
+            self._extendedAttrDict[identifier] = True
+
+    def _getDependentObjects(self):
+        deps = set(self.members)
+        if self.parent:
+            deps.add(self.parent)
+        return deps
+
+    def addPartialDictionary(self, partial):
+        assert self.identifier.name == partial.identifier.name
+        self._partialDictionaries.append(partial)
+
+
+class IDLEnum(IDLObjectWithIdentifier):
+    def __init__(self, location, parentScope, name, values):
+        assert isinstance(parentScope, IDLScope)
+        assert isinstance(name, IDLUnresolvedIdentifier)
+
+        if len(values) != len(set(values)):
+            raise WebIDLError(
+                "Enum %s has multiple identical strings" % name.name, [location]
+            )
+
+        IDLObjectWithIdentifier.__init__(self, location, parentScope, name)
+        self._values = values
+
+    def values(self):
+        return self._values
+
+    def finish(self, scope):
+        pass
+
+    def validate(self):
+        pass
+
+    def isEnum(self):
+        return True
+
+    def addExtendedAttributes(self, attrs):
+        if len(attrs) != 0:
+            raise WebIDLError(
+                "There are no extended attributes that are " "allowed on enums",
+                [attrs[0].location, self.location],
+            )
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLType(IDLObject):
+    Tags = enum(
+        # The integer types
+        "int8",
+        "uint8",
+        "int16",
+        "uint16",
+        "int32",
+        "uint32",
+        "int64",
+        "uint64",
+        # Additional primitive types
+        "bool",
+        "unrestricted_float",
+        "float",
+        "unrestricted_double",
+        # "double" last primitive type to match IDLBuiltinType
+        "double",
+        # Other types
+        "any",
+        "undefined",
+        "domstring",
+        "bytestring",
+        "usvstring",
+        "utf8string",
+        "jsstring",
+        "object",
+        # Funny stuff
+        "interface",
+        "dictionary",
+        "enum",
+        "callback",
+        "union",
+        "sequence",
+        "record",
+        "promise",
+        "observablearray",
+    )
+
+    def __init__(self, location, name):
+        IDLObject.__init__(self, location)
+        self.name = name
+        self.builtin = False
+        self.legacyNullToEmptyString = False
+        self._clamp = False
+        self._enforceRange = False
+        self._allowShared = False
+        self._extendedAttrDict = {}
+
+    def __hash__(self):
+        return (
+            hash(self.builtin)
+            + hash(self.name)
+            + hash(self._clamp)
+            + hash(self._enforceRange)
+            + hash(self.legacyNullToEmptyString)
+            + hash(self._allowShared)
+        )
+
+    def __eq__(self, other):
+        return (
+            other
+            and self.builtin == other.builtin
+            and self.name == other.name
+            and self._clamp == other.hasClamp()
+            and self._enforceRange == other.hasEnforceRange()
+            and self.legacyNullToEmptyString == other.legacyNullToEmptyString
+            and self._allowShared == other.hasAllowShared()
+        )
+
+    def __ne__(self, other):
+        return not self == other
+
+    def __str__(self):
+        return str(self.name)
+
+    def prettyName(self):
+        """
+        A name that looks like what this type is named in the IDL spec.  By default
+        this is just our .name, but types that have more interesting spec
+        representations should override this.
+        """
+        return str(self.name)
+
+    def isType(self):
+        return True
+
+    def nullable(self):
+        return False
+
+    def isPrimitive(self):
+        return False
+
+    def isBoolean(self):
+        return False
+
+    def isNumeric(self):
+        return False
+
+    def isString(self):
+        return False
+
+    def isByteString(self):
+        return False
+
+    def isDOMString(self):
+        return False
+
+    def isUSVString(self):
+        return False
+
+    def isUTF8String(self):
+        return False
+
+    def isJSString(self):
+        return False
+
+    def isUndefined(self):
+        return False
+
+    def isSequence(self):
+        return False
+
+    def isRecord(self):
+        return False
+
+    def isArrayBuffer(self):
+        return False
+
+    def isArrayBufferView(self):
+        return False
+
+    def isTypedArray(self):
+        return False
+
+    def isBufferSource(self):
+        return self.isArrayBuffer() or self.isArrayBufferView() or self.isTypedArray()
+
+    def isCallbackInterface(self):
+        return False
+
+    def isNonCallbackInterface(self):
+        return False
+
+    def isGeckoInterface(self):
+        """Returns a boolean indicating whether this type is an 'interface'
+        type that is implemented in Gecko. At the moment, this returns
+        true for all interface types that are not types from the TypedArray
+        spec."""
+        return self.isInterface() and not self.isSpiderMonkeyInterface()
+
+    def isSpiderMonkeyInterface(self):
+        """Returns a boolean indicating whether this type is an 'interface'
+        type that is implemented in SpiderMonkey."""
+        return self.isInterface() and self.isBufferSource()
+
+    def isAny(self):
+        return self.tag() == IDLType.Tags.any
+
+    def isObject(self):
+        return self.tag() == IDLType.Tags.object
+
+    def isPromise(self):
+        return False
+
+    def isComplete(self):
+        return True
+
+    def includesRestrictedFloat(self):
+        return False
+
+    def isFloat(self):
+        return False
+
+    def isUnrestricted(self):
+        # Should only call this on float types
+        assert self.isFloat()
+
+    def isJSONType(self):
+        return False
+
+    def isObservableArray(self):
+        return False
+
+    def isDictionaryLike(self):
+        return self.isDictionary() or self.isRecord() or self.isCallbackInterface()
+
+    def hasClamp(self):
+        return self._clamp
+
+    def hasEnforceRange(self):
+        return self._enforceRange
+
+    def hasAllowShared(self):
+        return self._allowShared
+
+    def tag(self):
+        assert False  # Override me!
+
+    def treatNonCallableAsNull(self):
+        assert self.tag() == IDLType.Tags.callback
+        return self.nullable() and self.inner.callback._treatNonCallableAsNull
+
+    def treatNonObjectAsNull(self):
+        assert self.tag() == IDLType.Tags.callback
+        return self.nullable() and self.inner.callback._treatNonObjectAsNull
+
+    def withExtendedAttributes(self, attrs):
+        if len(attrs) > 0:
+            raise WebIDLError(
+                "Extended attributes on types only supported for builtins",
+                [attrs[0].location, self.location],
+            )
+        return self
+
+    def getExtendedAttribute(self, name):
+        return self._extendedAttrDict.get(name, None)
+
+    def resolveType(self, parentScope):
+        pass
+
+    def unroll(self):
+        return self
+
+    def isDistinguishableFrom(self, other):
+        raise TypeError(
+            "Can't tell whether a generic type is or is not "
+            "distinguishable from other things"
+        )
+
+    def isExposedInAllOf(self, exposureSet):
+        return True
+
+
+class IDLUnresolvedType(IDLType):
+    """
+    Unresolved types are interface types
+    """
+
+    def __init__(self, location, name, attrs=[]):
+        IDLType.__init__(self, location, name)
+        self.extraTypeAttributes = attrs
+
+    def isComplete(self):
+        return False
+
+    def complete(self, scope):
+        obj = None
+        try:
+            obj = scope._lookupIdentifier(self.name)
+        except Exception:
+            raise WebIDLError("Unresolved type '%s'." % self.name, [self.location])
+
+        assert obj
+        assert not obj.isType()
+        if obj.isTypedef():
+            assert self.name.name == obj.identifier.name
+            typedefType = IDLTypedefType(self.location, obj.innerType, obj.identifier)
+            assert not typedefType.isComplete()
+            return typedefType.complete(scope).withExtendedAttributes(
+                self.extraTypeAttributes
+            )
+        elif obj.isCallback() and not obj.isInterface():
+            assert self.name.name == obj.identifier.name
+            return IDLCallbackType(self.location, obj)
+
+        return IDLWrapperType(self.location, obj)
+
+    def withExtendedAttributes(self, attrs):
+        return IDLUnresolvedType(self.location, self.name, attrs)
+
+    def isDistinguishableFrom(self, other):
+        raise TypeError(
+            "Can't tell whether an unresolved type is or is not "
+            "distinguishable from other things"
+        )
+
+
+class IDLParametrizedType(IDLType):
+    def __init__(self, location, name, innerType):
+        IDLType.__init__(self, location, name)
+        self.builtin = False
+        self.inner = innerType
+
+    def includesRestrictedFloat(self):
+        return self.inner.includesRestrictedFloat()
+
+    def resolveType(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        self.inner.resolveType(parentScope)
+
+    def isComplete(self):
+        return self.inner.isComplete()
+
+    def unroll(self):
+        return self.inner.unroll()
+
+    def _getDependentObjects(self):
+        return self.inner._getDependentObjects()
+
+
+class IDLNullableType(IDLParametrizedType):
+    def __init__(self, location, innerType):
+        assert not innerType == BuiltinTypes[IDLBuiltinType.Types.any]
+
+        IDLParametrizedType.__init__(self, location, None, innerType)
+
+    def __hash__(self):
+        return hash(self.inner)
+
+    def __eq__(self, other):
+        return isinstance(other, IDLNullableType) and self.inner == other.inner
+
+    def __str__(self):
+        return self.inner.__str__() + "OrNull"
+
+    def prettyName(self):
+        return self.inner.prettyName() + "?"
+
+    def nullable(self):
+        return True
+
+    def isCallback(self):
+        return self.inner.isCallback()
+
+    def isPrimitive(self):
+        return self.inner.isPrimitive()
+
+    def isBoolean(self):
+        return self.inner.isBoolean()
+
+    def isNumeric(self):
+        return self.inner.isNumeric()
+
+    def isString(self):
+        return self.inner.isString()
+
+    def isByteString(self):
+        return self.inner.isByteString()
+
+    def isDOMString(self):
+        return self.inner.isDOMString()
+
+    def isUSVString(self):
+        return self.inner.isUSVString()
+
+    def isUTF8String(self):
+        return self.inner.isUTF8String()
+
+    def isJSString(self):
+        return self.inner.isJSString()
+
+    def isFloat(self):
+        return self.inner.isFloat()
+
+    def isUnrestricted(self):
+        return self.inner.isUnrestricted()
+
+    def isInteger(self):
+        return self.inner.isInteger()
+
+    def isUndefined(self):
+        return self.inner.isUndefined()
+
+    def isSequence(self):
+        return self.inner.isSequence()
+
+    def isRecord(self):
+        return self.inner.isRecord()
+
+    def isArrayBuffer(self):
+        return self.inner.isArrayBuffer()
+
+    def isArrayBufferView(self):
+        return self.inner.isArrayBufferView()
+
+    def isTypedArray(self):
+        return self.inner.isTypedArray()
+
+    def isDictionary(self):
+        return self.inner.isDictionary()
+
+    def isInterface(self):
+        return self.inner.isInterface()
+
+    def isPromise(self):
+        # There is no such thing as a nullable Promise.
+        assert not self.inner.isPromise()
+        return False
+
+    def isCallbackInterface(self):
+        return self.inner.isCallbackInterface()
+
+    def isNonCallbackInterface(self):
+        return self.inner.isNonCallbackInterface()
+
+    def isEnum(self):
+        return self.inner.isEnum()
+
+    def isUnion(self):
+        return self.inner.isUnion()
+
+    def isJSONType(self):
+        return self.inner.isJSONType()
+
+    def isObservableArray(self):
+        return self.inner.isObservableArray()
+
+    def hasClamp(self):
+        return self.inner.hasClamp()
+
+    def hasEnforceRange(self):
+        return self.inner.hasEnforceRange()
+
+    def hasAllowShared(self):
+        return self.inner.hasAllowShared()
+
+    def isComplete(self):
+        return self.name is not None
+
+    def tag(self):
+        return self.inner.tag()
+
+    def complete(self, scope):
+        if not self.inner.isComplete():
+            self.inner = self.inner.complete(scope)
+        assert self.inner.isComplete()
+
+        if self.inner.nullable():
+            raise WebIDLError(
+                "The inner type of a nullable type must not be a nullable type",
+                [self.location, self.inner.location],
+            )
+        if self.inner.isUnion():
+            if self.inner.hasNullableType:
+                raise WebIDLError(
+                    "The inner type of a nullable type must not "
+                    "be a union type that itself has a nullable "
+                    "type as a member type",
+                    [self.location],
+                )
+        if self.inner.isDOMString():
+            if self.inner.legacyNullToEmptyString:
+                raise WebIDLError(
+                    "[LegacyNullToEmptyString] not allowed on a nullable DOMString",
+                    [self.location, self.inner.location],
+                )
+        if self.inner.isObservableArray():
+            raise WebIDLError(
+                "The inner type of a nullable type must not be an ObservableArray type",
+                [self.location, self.inner.location],
+            )
+
+        self.name = self.inner.name + "OrNull"
+        return self
+
+    def isDistinguishableFrom(self, other):
+        if (
+            other.nullable()
+            or other.isDictionary()
+            or (
+                other.isUnion() and (other.hasNullableType or other.hasDictionaryType())
+            )
+        ):
+            # Can't tell which type null should become
+            return False
+        return self.inner.isDistinguishableFrom(other)
+
+    def withExtendedAttributes(self, attrs):
+        # See https://github.com/heycam/webidl/issues/827#issuecomment-565131350
+        # Allowing extended attributes to apply to a nullable type is an intermediate solution.
+        # A potential longer term solution is to introduce a null type and get rid of nullables.
+        # For example, we could do `([Clamp] long or null) foo` in the future.
+        return IDLNullableType(self.location, self.inner.withExtendedAttributes(attrs))
+
+
+class IDLSequenceType(IDLParametrizedType):
+    def __init__(self, location, parameterType):
+        assert not parameterType.isUndefined()
+
+        IDLParametrizedType.__init__(self, location, parameterType.name, parameterType)
+        # Need to set self.name up front if our inner type is already complete,
+        # since in that case our .complete() won't be called.
+        if self.inner.isComplete():
+            self.name = self.inner.name + "Sequence"
+
+    def __hash__(self):
+        return hash(self.inner)
+
+    def __eq__(self, other):
+        return isinstance(other, IDLSequenceType) and self.inner == other.inner
+
+    def __str__(self):
+        return self.inner.__str__() + "Sequence"
+
+    def prettyName(self):
+        return "sequence<%s>" % self.inner.prettyName()
+
+    def isSequence(self):
+        return True
+
+    def isJSONType(self):
+        return self.inner.isJSONType()
+
+    def tag(self):
+        return IDLType.Tags.sequence
+
+    def complete(self, scope):
+        if self.inner.isObservableArray():
+            raise WebIDLError(
+                "The inner type of a sequence type must not be an ObservableArray type",
+                [self.location, self.inner.location],
+            )
+
+        self.inner = self.inner.complete(scope)
+        self.name = self.inner.name + "Sequence"
+        return self
+
+    def isDistinguishableFrom(self, other):
+        if other.isPromise():
+            return False
+        if other.isUnion():
+            # Just forward to the union; it'll deal
+            return other.isDistinguishableFrom(self)
+        return (
+            other.isUndefined()
+            or other.isPrimitive()
+            or other.isString()
+            or other.isEnum()
+            or other.isInterface()
+            or other.isDictionary()
+            or other.isCallback()
+            or other.isRecord()
+        )
+
+
+class IDLRecordType(IDLParametrizedType):
+    def __init__(self, location, keyType, valueType):
+        assert keyType.isString()
+        assert keyType.isComplete()
+
+        if valueType.isUndefined():
+            raise WebIDLError(
+                "We don't support undefined as a Record's values' type",
+                [location, valueType.location],
+            )
+
+        IDLParametrizedType.__init__(self, location, valueType.name, valueType)
+        self.keyType = keyType
+
+        # Need to set self.name up front if our inner type is already complete,
+        # since in that case our .complete() won't be called.
+        if self.inner.isComplete():
+            self.name = self.keyType.name + self.inner.name + "Record"
+
+    def __hash__(self):
+        return hash(self.inner)
+
+    def __eq__(self, other):
+        return isinstance(other, IDLRecordType) and self.inner == other.inner
+
+    def __str__(self):
+        return self.keyType.__str__() + self.inner.__str__() + "Record"
+
+    def prettyName(self):
+        return "record<%s, %s>" % (self.keyType.prettyName(), self.inner.prettyName())
+
+    def isRecord(self):
+        return True
+
+    def isJSONType(self):
+        return self.inner.isJSONType()
+
+    def tag(self):
+        return IDLType.Tags.record
+
+    def complete(self, scope):
+        if self.inner.isObservableArray():
+            raise WebIDLError(
+                "The value type of a record type must not be an ObservableArray type",
+                [self.location, self.inner.location],
+            )
+
+        self.inner = self.inner.complete(scope)
+        self.name = self.keyType.name + self.inner.name + "Record"
+        return self
+
+    def unroll(self):
+        # We do not unroll our inner.  Just stop at ourselves.  That
+        # lets us add headers for both ourselves and our inner as
+        # needed.
+        return self
+
+    def isDistinguishableFrom(self, other):
+        if other.isPromise():
+            return False
+        if other.isUnion():
+            # Just forward to the union; it'll deal
+            return other.isDistinguishableFrom(self)
+        return (
+            other.isPrimitive()
+            or other.isString()
+            or other.isEnum()
+            or other.isNonCallbackInterface()
+            or other.isSequence()
+        )
+
+    def isExposedInAllOf(self, exposureSet):
+        return self.inner.unroll().isExposedInAllOf(exposureSet)
+
+
+class IDLObservableArrayType(IDLParametrizedType):
+    def __init__(self, location, innerType):
+        assert not innerType.isUndefined()
+        IDLParametrizedType.__init__(self, location, None, innerType)
+
+    def __hash__(self):
+        return hash(self.inner)
+
+    def __eq__(self, other):
+        return isinstance(other, IDLObservableArrayType) and self.inner == other.inner
+
+    def __str__(self):
+        return self.inner.__str__() + "ObservableArray"
+
+    def prettyName(self):
+        return "ObservableArray<%s>" % self.inner.prettyName()
+
+    def isJSONType(self):
+        return self.inner.isJSONType()
+
+    def isObservableArray(self):
+        return True
+
+    def isComplete(self):
+        return self.name is not None
+
+    def tag(self):
+        return IDLType.Tags.observablearray
+
+    def complete(self, scope):
+        if not self.inner.isComplete():
+            self.inner = self.inner.complete(scope)
+        assert self.inner.isComplete()
+
+        if self.inner.isDictionary():
+            raise WebIDLError(
+                "The inner type of an ObservableArray type must not "
+                "be a dictionary type",
+                [self.location, self.inner.location],
+            )
+        if self.inner.isSequence():
+            raise WebIDLError(
+                "The inner type of an ObservableArray type must not "
+                "be a sequence type",
+                [self.location, self.inner.location],
+            )
+        if self.inner.isRecord():
+            raise WebIDLError(
+                "The inner type of an ObservableArray type must not be a record type",
+                [self.location, self.inner.location],
+            )
+        if self.inner.isObservableArray():
+            raise WebIDLError(
+                "The inner type of an ObservableArray type must not "
+                "be an ObservableArray type",
+                [self.location, self.inner.location],
+            )
+
+        self.name = self.inner.name + "ObservableArray"
+        return self
+
+    def isDistinguishableFrom(self, other):
+        # ObservableArrays are not distinguishable from anything.
+        return False
+
+
+class IDLUnionType(IDLType):
+    def __init__(self, location, memberTypes):
+        IDLType.__init__(self, location, "")
+        self.memberTypes = memberTypes
+        self.hasNullableType = False
+        self._dictionaryType = None
+        self.flatMemberTypes = None
+        self.builtin = False
+
+    def __eq__(self, other):
+        return isinstance(other, IDLUnionType) and self.memberTypes == other.memberTypes
+
+    def __hash__(self):
+        assert self.isComplete()
+        return self.name.__hash__()
+
+    def prettyName(self):
+        return "(" + " or ".join(m.prettyName() for m in self.memberTypes) + ")"
+
+    def isUnion(self):
+        return True
+
+    def isJSONType(self):
+        return all(m.isJSONType() for m in self.memberTypes)
+
+    def includesRestrictedFloat(self):
+        return any(t.includesRestrictedFloat() for t in self.memberTypes)
+
+    def tag(self):
+        return IDLType.Tags.union
+
+    def resolveType(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        for t in self.memberTypes:
+            t.resolveType(parentScope)
+
+    def isComplete(self):
+        return self.flatMemberTypes is not None
+
+    def complete(self, scope):
+        def typeName(type):
+            if isinstance(type, IDLNullableType):
+                return typeName(type.inner) + "OrNull"
+            if isinstance(type, IDLWrapperType):
+                return typeName(type._identifier.object())
+            if isinstance(type, IDLObjectWithIdentifier):
+                return typeName(type.identifier)
+            if isinstance(type, IDLBuiltinType) and type.hasAllowShared():
+                assert type.isBufferSource()
+                return "MaybeShared" + type.name
+            return type.name
+
+        for i, type in enumerate(self.memberTypes):
+            if not type.isComplete():
+                self.memberTypes[i] = type.complete(scope)
+
+        self.name = "Or".join(typeName(type) for type in self.memberTypes)
+        self.flatMemberTypes = list(self.memberTypes)
+        i = 0
+        while i < len(self.flatMemberTypes):
+            if self.flatMemberTypes[i].nullable():
+                if self.hasNullableType:
+                    raise WebIDLError(
+                        "Can't have more than one nullable types in a union",
+                        [nullableType.location, self.flatMemberTypes[i].location],
+                    )
+                if self.hasDictionaryType():
+                    raise WebIDLError(
+                        "Can't have a nullable type and a "
+                        "dictionary type in a union",
+                        [
+                            self._dictionaryType.location,
+                            self.flatMemberTypes[i].location,
+                        ],
+                    )
+                self.hasNullableType = True
+                nullableType = self.flatMemberTypes[i]
+                self.flatMemberTypes[i] = self.flatMemberTypes[i].inner
+                continue
+            if self.flatMemberTypes[i].isDictionary():
+                if self.hasNullableType:
+                    raise WebIDLError(
+                        "Can't have a nullable type and a "
+                        "dictionary type in a union",
+                        [nullableType.location, self.flatMemberTypes[i].location],
+                    )
+                self._dictionaryType = self.flatMemberTypes[i]
+                self.flatMemberTypes[i].inner.needsConversionFromJS = True
+            elif self.flatMemberTypes[i].isUnion():
+                self.flatMemberTypes[i : i + 1] = self.flatMemberTypes[i].memberTypes
+                continue
+            i += 1
+
+        for i, t in enumerate(self.flatMemberTypes[:-1]):
+            for u in self.flatMemberTypes[i + 1 :]:
+                if not t.isDistinguishableFrom(u):
+                    raise WebIDLError(
+                        "Flat member types of a union should be "
+                        "distinguishable, " + str(t) + " is not "
+                        "distinguishable from " + str(u),
+                        [self.location, t.location, u.location],
+                    )
+
+        return self
+
+    def isDistinguishableFrom(self, other):
+        if self.hasNullableType and other.nullable():
+            # Can't tell which type null should become
+            return False
+        if other.isUnion():
+            otherTypes = other.unroll().memberTypes
+        else:
+            otherTypes = [other]
+        # For every type in otherTypes, check that it's distinguishable from
+        # every type in our types
+        for u in otherTypes:
+            if any(not t.isDistinguishableFrom(u) for t in self.memberTypes):
+                return False
+        return True
+
+    def isExposedInAllOf(self, exposureSet):
+        # We could have different member types in different globals.
+        # Just make sure that each thing in exposureSet has one of our member types exposed in it.
+        for globalName in exposureSet:
+            if not any(
+                t.unroll().isExposedInAllOf(set([globalName]))
+                for t in self.flatMemberTypes
+            ):
+                return False
+        return True
+
+    def hasDictionaryType(self):
+        return self._dictionaryType is not None
+
+    def hasPossiblyEmptyDictionaryType(self):
+        return (
+            self._dictionaryType is not None and self._dictionaryType.inner.canBeEmpty()
+        )
+
+    def _getDependentObjects(self):
+        return set(self.memberTypes)
+
+
+class IDLTypedefType(IDLType):
+    def __init__(self, location, innerType, name):
+        IDLType.__init__(self, location, name)
+        self.inner = innerType
+        self.builtin = False
+
+    def __hash__(self):
+        return hash(self.inner)
+
+    def __eq__(self, other):
+        return isinstance(other, IDLTypedefType) and self.inner == other.inner
+
+    def __str__(self):
+        return self.name
+
+    def nullable(self):
+        return self.inner.nullable()
+
+    def isPrimitive(self):
+        return self.inner.isPrimitive()
+
+    def isBoolean(self):
+        return self.inner.isBoolean()
+
+    def isNumeric(self):
+        return self.inner.isNumeric()
+
+    def isString(self):
+        return self.inner.isString()
+
+    def isByteString(self):
+        return self.inner.isByteString()
+
+    def isDOMString(self):
+        return self.inner.isDOMString()
+
+    def isUSVString(self):
+        return self.inner.isUSVString()
+
+    def isUTF8String(self):
+        return self.inner.isUTF8String()
+
+    def isJSString(self):
+        return self.inner.isJSString()
+
+    def isUndefined(self):
+        return self.inner.isUndefined()
+
+    def isJSONType(self):
+        return self.inner.isJSONType()
+
+    def isSequence(self):
+        return self.inner.isSequence()
+
+    def isRecord(self):
+        return self.inner.isRecord()
+
+    def isDictionary(self):
+        return self.inner.isDictionary()
+
+    def isArrayBuffer(self):
+        return self.inner.isArrayBuffer()
+
+    def isArrayBufferView(self):
+        return self.inner.isArrayBufferView()
+
+    def isTypedArray(self):
+        return self.inner.isTypedArray()
+
+    def isInterface(self):
+        return self.inner.isInterface()
+
+    def isCallbackInterface(self):
+        return self.inner.isCallbackInterface()
+
+    def isNonCallbackInterface(self):
+        return self.inner.isNonCallbackInterface()
+
+    def isComplete(self):
+        return False
+
+    def complete(self, parentScope):
+        if not self.inner.isComplete():
+            self.inner = self.inner.complete(parentScope)
+        assert self.inner.isComplete()
+        return self.inner
+
+    # Do we need a resolveType impl?  I don't think it's particularly useful....
+
+    def tag(self):
+        return self.inner.tag()
+
+    def unroll(self):
+        return self.inner.unroll()
+
+    def isDistinguishableFrom(self, other):
+        return self.inner.isDistinguishableFrom(other)
+
+    def _getDependentObjects(self):
+        return self.inner._getDependentObjects()
+
+    def withExtendedAttributes(self, attrs):
+        return IDLTypedefType(
+            self.location, self.inner.withExtendedAttributes(attrs), self.name
+        )
+
+
+class IDLTypedef(IDLObjectWithIdentifier):
+    def __init__(self, location, parentScope, innerType, name):
+        # Set self.innerType first, because IDLObjectWithIdentifier.__init__
+        # will call our __str__, which wants to use it.
+        self.innerType = innerType
+        identifier = IDLUnresolvedIdentifier(location, name)
+        IDLObjectWithIdentifier.__init__(self, location, parentScope, identifier)
+
+    def __str__(self):
+        return "Typedef %s %s" % (self.identifier.name, self.innerType)
+
+    def finish(self, parentScope):
+        if not self.innerType.isComplete():
+            self.innerType = self.innerType.complete(parentScope)
+
+    def validate(self):
+        pass
+
+    def isTypedef(self):
+        return True
+
+    def addExtendedAttributes(self, attrs):
+        if len(attrs) != 0:
+            raise WebIDLError(
+                "There are no extended attributes that are " "allowed on typedefs",
+                [attrs[0].location, self.location],
+            )
+
+    def _getDependentObjects(self):
+        return self.innerType._getDependentObjects()
+
+
+class IDLWrapperType(IDLType):
+    def __init__(self, location, inner):
+        IDLType.__init__(self, location, inner.identifier.name)
+        self.inner = inner
+        self._identifier = inner.identifier
+        self.builtin = False
+
+    def __hash__(self):
+        return hash(self._identifier) + hash(self.builtin)
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, IDLWrapperType)
+            and self._identifier == other._identifier
+            and self.builtin == other.builtin
+        )
+
+    def __str__(self):
+        return str(self.name) + " (Wrapper)"
+
+    def isDictionary(self):
+        return isinstance(self.inner, IDLDictionary)
+
+    def isInterface(self):
+        return isinstance(self.inner, IDLInterface) or isinstance(
+            self.inner, IDLExternalInterface
+        )
+
+    def isCallbackInterface(self):
+        return self.isInterface() and self.inner.isCallback()
+
+    def isNonCallbackInterface(self):
+        return self.isInterface() and not self.inner.isCallback()
+
+    def isEnum(self):
+        return isinstance(self.inner, IDLEnum)
+
+    def isJSONType(self):
+        if self.isInterface():
+            if self.inner.isExternal():
+                return False
+            iface = self.inner
+            while iface:
+                if any(m.isMethod() and m.isToJSON() for m in iface.members):
+                    return True
+                iface = iface.parent
+            return False
+        elif self.isEnum():
+            return True
+        elif self.isDictionary():
+            dictionary = self.inner
+            while dictionary:
+                if not all(m.type.isJSONType() for m in dictionary.members):
+                    return False
+                dictionary = dictionary.parent
+            return True
+        else:
+            raise WebIDLError(
+                "IDLWrapperType wraps type %s that we don't know if "
+                "is serializable" % type(self.inner),
+                [self.location],
+            )
+
+    def resolveType(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        self.inner.resolve(parentScope)
+
+    def isComplete(self):
+        return True
+
+    def tag(self):
+        if self.isInterface():
+            return IDLType.Tags.interface
+        elif self.isEnum():
+            return IDLType.Tags.enum
+        elif self.isDictionary():
+            return IDLType.Tags.dictionary
+        else:
+            assert False
+
+    def isDistinguishableFrom(self, other):
+        if other.isPromise():
+            return False
+        if other.isUnion():
+            # Just forward to the union; it'll deal
+            return other.isDistinguishableFrom(self)
+        assert self.isInterface() or self.isEnum() or self.isDictionary()
+        if self.isEnum():
+            return (
+                other.isUndefined()
+                or other.isPrimitive()
+                or other.isInterface()
+                or other.isObject()
+                or other.isCallback()
+                or other.isDictionary()
+                or other.isSequence()
+                or other.isRecord()
+            )
+        if self.isDictionary() and (other.nullable() or other.isUndefined()):
+            return False
+        if (
+            other.isPrimitive()
+            or other.isString()
+            or other.isEnum()
+            or other.isSequence()
+        ):
+            return True
+        if self.isDictionary():
+            return other.isNonCallbackInterface()
+
+        assert self.isInterface()
+        if other.isInterface():
+            if other.isSpiderMonkeyInterface():
+                # Just let |other| handle things
+                return other.isDistinguishableFrom(self)
+            assert self.isGeckoInterface() and other.isGeckoInterface()
+            if self.inner.isExternal() or other.unroll().inner.isExternal():
+                return self != other
+            return len(
+                self.inner.interfacesBasedOnSelf
+                & other.unroll().inner.interfacesBasedOnSelf
+            ) == 0 and (self.isNonCallbackInterface() or other.isNonCallbackInterface())
+        if (
+            other.isUndefined()
+            or other.isDictionary()
+            or other.isCallback()
+            or other.isRecord()
+        ):
+            return self.isNonCallbackInterface()
+
+        # Not much else |other| can be.
+        # any is the union of all non-union types, so it's not distinguishable
+        # from other unions (because it is a union itself), or from all
+        # non-union types (because it has all of them as its members).
+        assert other.isAny() or other.isObject()
+        return False
+
+    def isExposedInAllOf(self, exposureSet):
+        if not self.isInterface():
+            return True
+        iface = self.inner
+        if iface.isExternal():
+            # Let's say true, so we don't have to implement exposure mixins on
+            # external interfaces and sprinkle [Exposed=Window] on every single
+            # external interface declaration.
+            return True
+        return iface.exposureSet.issuperset(exposureSet)
+
+    def _getDependentObjects(self):
+        # NB: The codegen for an interface type depends on
+        #  a) That the identifier is in fact an interface (as opposed to
+        #     a dictionary or something else).
+        #  b) The native type of the interface.
+        #  If we depend on the interface object we will also depend on
+        #  anything the interface depends on which is undesirable.  We
+        #  considered implementing a dependency just on the interface type
+        #  file, but then every modification to an interface would cause this
+        #  to be regenerated which is still undesirable.  We decided not to
+        #  depend on anything, reasoning that:
+        #  1) Changing the concrete type of the interface requires modifying
+        #     Bindings.conf, which is still a global dependency.
+        #  2) Changing an interface to a dictionary (or vice versa) with the
+        #     same identifier should be incredibly rare.
+        #
+        # On the other hand, if our type is a dictionary, we should
+        # depend on it, because the member types of a dictionary
+        # affect whether a method taking the dictionary as an argument
+        # takes a JSContext* argument or not.
+        if self.isDictionary():
+            return set([self.inner])
+        return set()
+
+
+class IDLPromiseType(IDLParametrizedType):
+    def __init__(self, location, innerType):
+        IDLParametrizedType.__init__(self, location, "Promise", innerType)
+
+    def __hash__(self):
+        return hash(self.promiseInnerType())
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, IDLPromiseType)
+            and self.promiseInnerType() == other.promiseInnerType()
+        )
+
+    def __str__(self):
+        return self.inner.__str__() + "Promise"
+
+    def prettyName(self):
+        return "Promise<%s>" % self.inner.prettyName()
+
+    def isPromise(self):
+        return True
+
+    def promiseInnerType(self):
+        return self.inner
+
+    def tag(self):
+        return IDLType.Tags.promise
+
+    def complete(self, scope):
+        if self.inner.isObservableArray():
+            raise WebIDLError(
+                "The inner type of a promise type must not be an ObservableArray type",
+                [self.location, self.inner.location],
+            )
+
+        self.inner = self.promiseInnerType().complete(scope)
+        return self
+
+    def unroll(self):
+        # We do not unroll our inner.  Just stop at ourselves.  That
+        # lets us add headers for both ourselves and our inner as
+        # needed.
+        return self
+
+    def isDistinguishableFrom(self, other):
+        # Promises are not distinguishable from anything.
+        return False
+
+    def isExposedInAllOf(self, exposureSet):
+        # Check the internal type
+        return self.promiseInnerType().unroll().isExposedInAllOf(exposureSet)
+
+
+class IDLBuiltinType(IDLType):
+    Types = enum(
+        # The integer types
+        "byte",
+        "octet",
+        "short",
+        "unsigned_short",
+        "long",
+        "unsigned_long",
+        "long_long",
+        "unsigned_long_long",
+        # Additional primitive types
+        "boolean",
+        "unrestricted_float",
+        "float",
+        "unrestricted_double",
+        # IMPORTANT: "double" must be the last primitive type listed
+        "double",
+        # Other types
+        "any",
+        "undefined",
+        "domstring",
+        "bytestring",
+        "usvstring",
+        "utf8string",
+        "jsstring",
+        "object",
+        # Funny stuff
+        "ArrayBuffer",
+        "ArrayBufferView",
+        "Int8Array",
+        "Uint8Array",
+        "Uint8ClampedArray",
+        "Int16Array",
+        "Uint16Array",
+        "Int32Array",
+        "Uint32Array",
+        "Float32Array",
+        "Float64Array",
+    )
+
+    TagLookup = {
+        Types.byte: IDLType.Tags.int8,
+        Types.octet: IDLType.Tags.uint8,
+        Types.short: IDLType.Tags.int16,
+        Types.unsigned_short: IDLType.Tags.uint16,
+        Types.long: IDLType.Tags.int32,
+        Types.unsigned_long: IDLType.Tags.uint32,
+        Types.long_long: IDLType.Tags.int64,
+        Types.unsigned_long_long: IDLType.Tags.uint64,
+        Types.boolean: IDLType.Tags.bool,
+        Types.unrestricted_float: IDLType.Tags.unrestricted_float,
+        Types.float: IDLType.Tags.float,
+        Types.unrestricted_double: IDLType.Tags.unrestricted_double,
+        Types.double: IDLType.Tags.double,
+        Types.any: IDLType.Tags.any,
+        Types.undefined: IDLType.Tags.undefined,
+        Types.domstring: IDLType.Tags.domstring,
+        Types.bytestring: IDLType.Tags.bytestring,
+        Types.usvstring: IDLType.Tags.usvstring,
+        Types.utf8string: IDLType.Tags.utf8string,
+        Types.jsstring: IDLType.Tags.jsstring,
+        Types.object: IDLType.Tags.object,
+        Types.ArrayBuffer: IDLType.Tags.interface,
+        Types.ArrayBufferView: IDLType.Tags.interface,
+        Types.Int8Array: IDLType.Tags.interface,
+        Types.Uint8Array: IDLType.Tags.interface,
+        Types.Uint8ClampedArray: IDLType.Tags.interface,
+        Types.Int16Array: IDLType.Tags.interface,
+        Types.Uint16Array: IDLType.Tags.interface,
+        Types.Int32Array: IDLType.Tags.interface,
+        Types.Uint32Array: IDLType.Tags.interface,
+        Types.Float32Array: IDLType.Tags.interface,
+        Types.Float64Array: IDLType.Tags.interface,
+    }
+
+    PrettyNames = {
+        Types.byte: "byte",
+        Types.octet: "octet",
+        Types.short: "short",
+        Types.unsigned_short: "unsigned short",
+        Types.long: "long",
+        Types.unsigned_long: "unsigned long",
+        Types.long_long: "long long",
+        Types.unsigned_long_long: "unsigned long long",
+        Types.boolean: "boolean",
+        Types.unrestricted_float: "unrestricted float",
+        Types.float: "float",
+        Types.unrestricted_double: "unrestricted double",
+        Types.double: "double",
+        Types.any: "any",
+        Types.undefined: "undefined",
+        Types.domstring: "DOMString",
+        Types.bytestring: "ByteString",
+        Types.usvstring: "USVString",
+        Types.utf8string: "USVString",  # That's what it is in spec terms
+        Types.jsstring: "USVString",  # Again, that's what it is in spec terms
+        Types.object: "object",
+        Types.ArrayBuffer: "ArrayBuffer",
+        Types.ArrayBufferView: "ArrayBufferView",
+        Types.Int8Array: "Int8Array",
+        Types.Uint8Array: "Uint8Array",
+        Types.Uint8ClampedArray: "Uint8ClampedArray",
+        Types.Int16Array: "Int16Array",
+        Types.Uint16Array: "Uint16Array",
+        Types.Int32Array: "Int32Array",
+        Types.Uint32Array: "Uint32Array",
+        Types.Float32Array: "Float32Array",
+        Types.Float64Array: "Float64Array",
+    }
+
+    def __init__(
+        self,
+        location,
+        name,
+        type,
+        clamp=False,
+        enforceRange=False,
+        legacyNullToEmptyString=False,
+        allowShared=False,
+        attrLocation=[],
+    ):
+        """
+        The mutually exclusive clamp/enforceRange/legacyNullToEmptyString/allowShared arguments
+        are used to create instances of this type with the appropriate attributes attached. Use
+        .clamped(), .rangeEnforced(), .withLegacyNullToEmptyString() and .withAllowShared().
+
+        attrLocation is an array of source locations of these attributes for error reporting.
+        """
+        IDLType.__init__(self, location, name)
+        self.builtin = True
+        self._typeTag = type
+        self._clamped = None
+        self._rangeEnforced = None
+        self._withLegacyNullToEmptyString = None
+        self._withAllowShared = None
+        if self.isInteger():
+            if clamp:
+                self._clamp = True
+                self.name = "Clamped" + self.name
+                self._extendedAttrDict["Clamp"] = True
+            elif enforceRange:
+                self._enforceRange = True
+                self.name = "RangeEnforced" + self.name
+                self._extendedAttrDict["EnforceRange"] = True
+        elif clamp or enforceRange:
+            raise WebIDLError(
+                "Non-integer types cannot be [Clamp] or [EnforceRange]", attrLocation
+            )
+        if self.isDOMString() or self.isUTF8String():
+            if legacyNullToEmptyString:
+                self.legacyNullToEmptyString = True
+                self.name = "NullIsEmpty" + self.name
+                self._extendedAttrDict["LegacyNullToEmptyString"] = True
+        elif legacyNullToEmptyString:
+            raise WebIDLError(
+                "Non-string types cannot be [LegacyNullToEmptyString]", attrLocation
+            )
+        if self.isBufferSource():
+            if allowShared:
+                self._allowShared = True
+                self._extendedAttrDict["AllowShared"] = True
+        elif allowShared:
+            raise WebIDLError(
+                "Types that are not buffer source types cannot be [AllowShared]",
+                attrLocation,
+            )
+
+    def __str__(self):
+        if self._allowShared:
+            assert self.isBufferSource()
+            return "MaybeShared" + str(self.name)
+        return str(self.name)
+
+    def prettyName(self):
+        return IDLBuiltinType.PrettyNames[self._typeTag]
+
+    def clamped(self, attrLocation):
+        if not self._clamped:
+            self._clamped = IDLBuiltinType(
+                self.location,
+                self.name,
+                self._typeTag,
+                clamp=True,
+                attrLocation=attrLocation,
+            )
+        return self._clamped
+
+    def rangeEnforced(self, attrLocation):
+        if not self._rangeEnforced:
+            self._rangeEnforced = IDLBuiltinType(
+                self.location,
+                self.name,
+                self._typeTag,
+                enforceRange=True,
+                attrLocation=attrLocation,
+            )
+        return self._rangeEnforced
+
+    def withLegacyNullToEmptyString(self, attrLocation):
+        if not self._withLegacyNullToEmptyString:
+            self._withLegacyNullToEmptyString = IDLBuiltinType(
+                self.location,
+                self.name,
+                self._typeTag,
+                legacyNullToEmptyString=True,
+                attrLocation=attrLocation,
+            )
+        return self._withLegacyNullToEmptyString
+
+    def withAllowShared(self, attrLocation):
+        if not self._withAllowShared:
+            self._withAllowShared = IDLBuiltinType(
+                self.location,
+                self.name,
+                self._typeTag,
+                allowShared=True,
+                attrLocation=attrLocation,
+            )
+        return self._withAllowShared
+
+    def isPrimitive(self):
+        return self._typeTag <= IDLBuiltinType.Types.double
+
+    def isBoolean(self):
+        return self._typeTag == IDLBuiltinType.Types.boolean
+
+    def isUndefined(self):
+        return self._typeTag == IDLBuiltinType.Types.undefined
+
+    def isNumeric(self):
+        return self.isPrimitive() and not self.isBoolean()
+
+    def isString(self):
+        return (
+            self._typeTag == IDLBuiltinType.Types.domstring
+            or self._typeTag == IDLBuiltinType.Types.bytestring
+            or self._typeTag == IDLBuiltinType.Types.usvstring
+            or self._typeTag == IDLBuiltinType.Types.utf8string
+            or self._typeTag == IDLBuiltinType.Types.jsstring
+        )
+
+    def isByteString(self):
+        return self._typeTag == IDLBuiltinType.Types.bytestring
+
+    def isDOMString(self):
+        return self._typeTag == IDLBuiltinType.Types.domstring
+
+    def isUSVString(self):
+        return self._typeTag == IDLBuiltinType.Types.usvstring
+
+    def isUTF8String(self):
+        return self._typeTag == IDLBuiltinType.Types.utf8string
+
+    def isJSString(self):
+        return self._typeTag == IDLBuiltinType.Types.jsstring
+
+    def isInteger(self):
+        return self._typeTag <= IDLBuiltinType.Types.unsigned_long_long
+
+    def isArrayBuffer(self):
+        return self._typeTag == IDLBuiltinType.Types.ArrayBuffer
+
+    def isArrayBufferView(self):
+        return self._typeTag == IDLBuiltinType.Types.ArrayBufferView
+
+    def isTypedArray(self):
+        return (
+            self._typeTag >= IDLBuiltinType.Types.Int8Array
+            and self._typeTag <= IDLBuiltinType.Types.Float64Array
+        )
+
+    def isInterface(self):
+        # TypedArray things are interface types per the TypedArray spec,
+        # but we handle them as builtins because SpiderMonkey implements
+        # all of it internally.
+        return self.isArrayBuffer() or self.isArrayBufferView() or self.isTypedArray()
+
+    def isNonCallbackInterface(self):
+        # All the interfaces we can be are non-callback
+        return self.isInterface()
+
+    def isFloat(self):
+        return (
+            self._typeTag == IDLBuiltinType.Types.float
+            or self._typeTag == IDLBuiltinType.Types.double
+            or self._typeTag == IDLBuiltinType.Types.unrestricted_float
+            or self._typeTag == IDLBuiltinType.Types.unrestricted_double
+        )
+
+    def isUnrestricted(self):
+        assert self.isFloat()
+        return (
+            self._typeTag == IDLBuiltinType.Types.unrestricted_float
+            or self._typeTag == IDLBuiltinType.Types.unrestricted_double
+        )
+
+    def isJSONType(self):
+        return self.isPrimitive() or self.isString() or self.isObject()
+
+    def includesRestrictedFloat(self):
+        return self.isFloat() and not self.isUnrestricted()
+
+    def tag(self):
+        return IDLBuiltinType.TagLookup[self._typeTag]
+
+    def isDistinguishableFrom(self, other):
+        if other.isPromise():
+            return False
+        if other.isUnion():
+            # Just forward to the union; it'll deal
+            return other.isDistinguishableFrom(self)
+        if self.isUndefined():
+            return not (other.isUndefined() or other.isDictionaryLike())
+        if self.isPrimitive():
+            if (
+                other.isUndefined()
+                or other.isString()
+                or other.isEnum()
+                or other.isInterface()
+                or other.isObject()
+                or other.isCallback()
+                or other.isDictionary()
+                or other.isSequence()
+                or other.isRecord()
+            ):
+                return True
+            if self.isBoolean():
+                return other.isNumeric()
+            assert self.isNumeric()
+            return other.isBoolean()
+        if self.isString():
+            return (
+                other.isUndefined()
+                or other.isPrimitive()
+                or other.isInterface()
+                or other.isObject()
+                or other.isCallback()
+                or other.isDictionary()
+                or other.isSequence()
+                or other.isRecord()
+            )
+        if self.isAny():
+            # Can't tell "any" apart from anything
+            return False
+        if self.isObject():
+            return (
+                other.isUndefined()
+                or other.isPrimitive()
+                or other.isString()
+                or other.isEnum()
+            )
+        # Not much else we could be!
+        assert self.isSpiderMonkeyInterface()
+        # Like interfaces, but we know we're not a callback
+        return (
+            other.isUndefined()
+            or other.isPrimitive()
+            or other.isString()
+            or other.isEnum()
+            or other.isCallback()
+            or other.isDictionary()
+            or other.isSequence()
+            or other.isRecord()
+            or (
+                other.isInterface()
+                and (
+                    # ArrayBuffer is distinguishable from everything
+                    # that's not an ArrayBuffer or a callback interface
+                    (self.isArrayBuffer() and not other.isArrayBuffer())
+                    or
+                    # ArrayBufferView is distinguishable from everything
+                    # that's not an ArrayBufferView or typed array.
+                    (
+                        self.isArrayBufferView()
+                        and not other.isArrayBufferView()
+                        and not other.isTypedArray()
+                    )
+                    or
+                    # Typed arrays are distinguishable from everything
+                    # except ArrayBufferView and the same type of typed
+                    # array
+                    (
+                        self.isTypedArray()
+                        and not other.isArrayBufferView()
+                        and not (other.isTypedArray() and other.name == self.name)
+                    )
+                )
+            )
+        )
+
+    def _getDependentObjects(self):
+        return set()
+
+    def withExtendedAttributes(self, attrs):
+        ret = self
+        for attribute in attrs:
+            identifier = attribute.identifier()
+            if identifier == "Clamp":
+                if not attribute.noArguments():
+                    raise WebIDLError(
+                        "[Clamp] must take no arguments", [attribute.location]
+                    )
+                if ret.hasEnforceRange() or self._enforceRange:
+                    raise WebIDLError(
+                        "[EnforceRange] and [Clamp] are mutually exclusive",
+                        [self.location, attribute.location],
+                    )
+                ret = self.clamped([self.location, attribute.location])
+            elif identifier == "EnforceRange":
+                if not attribute.noArguments():
+                    raise WebIDLError(
+                        "[EnforceRange] must take no arguments", [attribute.location]
+                    )
+                if ret.hasClamp() or self._clamp:
+                    raise WebIDLError(
+                        "[EnforceRange] and [Clamp] are mutually exclusive",
+                        [self.location, attribute.location],
+                    )
+                ret = self.rangeEnforced([self.location, attribute.location])
+            elif identifier == "LegacyNullToEmptyString":
+                if not (self.isDOMString() or self.isUTF8String()):
+                    raise WebIDLError(
+                        "[LegacyNullToEmptyString] only allowed on DOMStrings and UTF8Strings",
+                        [self.location, attribute.location],
+                    )
+                assert not self.nullable()
+                if attribute.hasValue():
+                    raise WebIDLError(
+                        "[LegacyNullToEmptyString] must take no identifier argument",
+                        [attribute.location],
+                    )
+                ret = self.withLegacyNullToEmptyString(
+                    [self.location, attribute.location]
+                )
+            elif identifier == "AllowShared":
+                if not attribute.noArguments():
+                    raise WebIDLError(
+                        "[AllowShared] must take no arguments", [attribute.location]
+                    )
+                if not self.isBufferSource():
+                    raise WebIDLError(
+                        "[AllowShared] only allowed on buffer source types",
+                        [self.location, attribute.location],
+                    )
+                ret = self.withAllowShared([self.location, attribute.location])
+
+            else:
+                raise WebIDLError(
+                    "Unhandled extended attribute on type",
+                    [self.location, attribute.location],
+                )
+        return ret
+
+
+BuiltinTypes = {
+    IDLBuiltinType.Types.byte: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Byte", IDLBuiltinType.Types.byte
+    ),
+    IDLBuiltinType.Types.octet: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Octet", IDLBuiltinType.Types.octet
+    ),
+    IDLBuiltinType.Types.short: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Short", IDLBuiltinType.Types.short
+    ),
+    IDLBuiltinType.Types.unsigned_short: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "UnsignedShort",
+        IDLBuiltinType.Types.unsigned_short,
+    ),
+    IDLBuiltinType.Types.long: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Long", IDLBuiltinType.Types.long
+    ),
+    IDLBuiltinType.Types.unsigned_long: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "UnsignedLong",
+        IDLBuiltinType.Types.unsigned_long,
+    ),
+    IDLBuiltinType.Types.long_long: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "LongLong", IDLBuiltinType.Types.long_long
+    ),
+    IDLBuiltinType.Types.unsigned_long_long: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "UnsignedLongLong",
+        IDLBuiltinType.Types.unsigned_long_long,
+    ),
+    IDLBuiltinType.Types.undefined: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Undefined", IDLBuiltinType.Types.undefined
+    ),
+    IDLBuiltinType.Types.boolean: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Boolean", IDLBuiltinType.Types.boolean
+    ),
+    IDLBuiltinType.Types.float: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Float", IDLBuiltinType.Types.float
+    ),
+    IDLBuiltinType.Types.unrestricted_float: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "UnrestrictedFloat",
+        IDLBuiltinType.Types.unrestricted_float,
+    ),
+    IDLBuiltinType.Types.double: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Double", IDLBuiltinType.Types.double
+    ),
+    IDLBuiltinType.Types.unrestricted_double: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "UnrestrictedDouble",
+        IDLBuiltinType.Types.unrestricted_double,
+    ),
+    IDLBuiltinType.Types.any: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Any", IDLBuiltinType.Types.any
+    ),
+    IDLBuiltinType.Types.domstring: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "String", IDLBuiltinType.Types.domstring
+    ),
+    IDLBuiltinType.Types.bytestring: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "ByteString", IDLBuiltinType.Types.bytestring
+    ),
+    IDLBuiltinType.Types.usvstring: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "USVString", IDLBuiltinType.Types.usvstring
+    ),
+    IDLBuiltinType.Types.utf8string: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "UTF8String", IDLBuiltinType.Types.utf8string
+    ),
+    IDLBuiltinType.Types.jsstring: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "JSString", IDLBuiltinType.Types.jsstring
+    ),
+    IDLBuiltinType.Types.object: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Object", IDLBuiltinType.Types.object
+    ),
+    IDLBuiltinType.Types.ArrayBuffer: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "ArrayBuffer",
+        IDLBuiltinType.Types.ArrayBuffer,
+    ),
+    IDLBuiltinType.Types.ArrayBufferView: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "ArrayBufferView",
+        IDLBuiltinType.Types.ArrayBufferView,
+    ),
+    IDLBuiltinType.Types.Int8Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Int8Array", IDLBuiltinType.Types.Int8Array
+    ),
+    IDLBuiltinType.Types.Uint8Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Uint8Array", IDLBuiltinType.Types.Uint8Array
+    ),
+    IDLBuiltinType.Types.Uint8ClampedArray: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "Uint8ClampedArray",
+        IDLBuiltinType.Types.Uint8ClampedArray,
+    ),
+    IDLBuiltinType.Types.Int16Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Int16Array", IDLBuiltinType.Types.Int16Array
+    ),
+    IDLBuiltinType.Types.Uint16Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "Uint16Array",
+        IDLBuiltinType.Types.Uint16Array,
+    ),
+    IDLBuiltinType.Types.Int32Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"), "Int32Array", IDLBuiltinType.Types.Int32Array
+    ),
+    IDLBuiltinType.Types.Uint32Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "Uint32Array",
+        IDLBuiltinType.Types.Uint32Array,
+    ),
+    IDLBuiltinType.Types.Float32Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "Float32Array",
+        IDLBuiltinType.Types.Float32Array,
+    ),
+    IDLBuiltinType.Types.Float64Array: IDLBuiltinType(
+        BuiltinLocation("<builtin type>"),
+        "Float64Array",
+        IDLBuiltinType.Types.Float64Array,
+    ),
+}
+
+
+integerTypeSizes = {
+    IDLBuiltinType.Types.byte: (-128, 127),
+    IDLBuiltinType.Types.octet: (0, 255),
+    IDLBuiltinType.Types.short: (-32768, 32767),
+    IDLBuiltinType.Types.unsigned_short: (0, 65535),
+    IDLBuiltinType.Types.long: (-2147483648, 2147483647),
+    IDLBuiltinType.Types.unsigned_long: (0, 4294967295),
+    IDLBuiltinType.Types.long_long: (-9223372036854775808, 9223372036854775807),
+    IDLBuiltinType.Types.unsigned_long_long: (0, 18446744073709551615),
+}
+
+
+def matchIntegerValueToType(value):
+    for type, extremes in integerTypeSizes.items():
+        (min, max) = extremes
+        if value <= max and value >= min:
+            return BuiltinTypes[type]
+
+    return None
+
+
+class NoCoercionFoundError(WebIDLError):
+    """
+    A class we use to indicate generic coercion failures because none of the
+    types worked out in IDLValue.coerceToType.
+    """
+
+
+class IDLValue(IDLObject):
+    def __init__(self, location, type, value):
+        IDLObject.__init__(self, location)
+        self.type = type
+        assert isinstance(type, IDLType)
+
+        self.value = value
+
+    def coerceToType(self, type, location):
+        if type == self.type:
+            return self  # Nothing to do
+
+        # We first check for unions to ensure that even if the union is nullable
+        # we end up with the right flat member type, not the union's type.
+        if type.isUnion():
+            # We use the flat member types here, because if we have a nullable
+            # member type, or a nested union, we want the type the value
+            # actually coerces to, not the nullable or nested union type.
+            for subtype in type.unroll().flatMemberTypes:
+                try:
+                    coercedValue = self.coerceToType(subtype, location)
+                    # Create a new IDLValue to make sure that we have the
+                    # correct float/double type.  This is necessary because we
+                    # use the value's type when it is a default value of a
+                    # union, and the union cares about the exact float type.
+                    return IDLValue(self.location, subtype, coercedValue.value)
+                except Exception as e:
+                    # Make sure to propagate out WebIDLErrors that are not the
+                    # generic "hey, we could not coerce to this type at all"
+                    # exception, because those are specific "coercion failed for
+                    # reason X" exceptions.  Note that we want to swallow
+                    # non-WebIDLErrors here, because those can just happen if
+                    # "type" is not something that can have a default value at
+                    # all.
+                    if isinstance(e, WebIDLError) and not isinstance(
+                        e, NoCoercionFoundError
+                    ):
+                        raise e
+
+        # If the type allows null, rerun this matching on the inner type, except
+        # nullable enums.  We handle those specially, because we want our
+        # default string values to stay strings even when assigned to a nullable
+        # enum.
+        elif type.nullable() and not type.isEnum():
+            innerValue = self.coerceToType(type.inner, location)
+            return IDLValue(self.location, type, innerValue.value)
+
+        elif self.type.isInteger() and type.isInteger():
+            # We're both integer types.  See if we fit.
+
+            (min, max) = integerTypeSizes[type._typeTag]
+            if self.value <= max and self.value >= min:
+                # Promote
+                return IDLValue(self.location, type, self.value)
+            else:
+                raise WebIDLError(
+                    "Value %s is out of range for type %s." % (self.value, type),
+                    [location],
+                )
+        elif self.type.isInteger() and type.isFloat():
+            # Convert an integer literal into float
+            if -(2**24) <= self.value <= 2**24:
+                return IDLValue(self.location, type, float(self.value))
+            else:
+                raise WebIDLError(
+                    "Converting value %s to %s will lose precision."
+                    % (self.value, type),
+                    [location],
+                )
+        elif self.type.isString() and type.isEnum():
+            # Just keep our string, but make sure it's a valid value for this enum
+            enum = type.unroll().inner
+            if self.value not in enum.values():
+                raise WebIDLError(
+                    "'%s' is not a valid default value for enum %s"
+                    % (self.value, enum.identifier.name),
+                    [location, enum.location],
+                )
+            return self
+        elif self.type.isFloat() and type.isFloat():
+            if not type.isUnrestricted() and (
+                self.value == float("inf")
+                or self.value == float("-inf")
+                or math.isnan(self.value)
+            ):
+                raise WebIDLError(
+                    "Trying to convert unrestricted value %s to non-unrestricted"
+                    % self.value,
+                    [location],
+                )
+            return IDLValue(self.location, type, self.value)
+        elif self.type.isString() and type.isUSVString():
+            # Allow USVStrings to use default value just like
+            # DOMString.  No coercion is required in this case as Codegen.py
+            # treats USVString just like DOMString, but with an
+            # extra normalization step.
+            assert self.type.isDOMString()
+            return self
+        elif self.type.isString() and (
+            type.isByteString() or type.isJSString() or type.isUTF8String()
+        ):
+            # Allow ByteStrings, UTF8String, and JSStrings to use a default
+            # value like DOMString.
+            # No coercion is required as Codegen.py will handle the
+            # extra steps. We want to make sure that our string contains
+            # only valid characters, so we check that here.
+            valid_ascii_lit = (
+                " " + string.ascii_letters + string.digits + string.punctuation
+            )
+            for idx, c in enumerate(self.value):
+                if c not in valid_ascii_lit:
+                    raise WebIDLError(
+                        "Coercing this string literal %s to a ByteString is not supported yet. "
+                        "Coercion failed due to an unsupported byte %d at index %d."
+                        % (self.value.__repr__(), ord(c), idx),
+                        [location],
+                    )
+
+            return IDLValue(self.location, type, self.value)
+        elif self.type.isDOMString() and type.legacyNullToEmptyString:
+            # LegacyNullToEmptyString is a different type for resolution reasons,
+            # however once you have a value it doesn't matter
+            return self
+
+        raise NoCoercionFoundError(
+            "Cannot coerce type %s to type %s." % (self.type, type), [location]
+        )
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLNullValue(IDLObject):
+    def __init__(self, location):
+        IDLObject.__init__(self, location)
+        self.type = None
+        self.value = None
+
+    def coerceToType(self, type, location):
+        if (
+            not isinstance(type, IDLNullableType)
+            and not (type.isUnion() and type.hasNullableType)
+            and not type.isAny()
+        ):
+            raise WebIDLError("Cannot coerce null value to type %s." % type, [location])
+
+        nullValue = IDLNullValue(self.location)
+        if type.isUnion() and not type.nullable() and type.hasDictionaryType():
+            # We're actually a default value for the union's dictionary member.
+            # Use its type.
+            for t in type.flatMemberTypes:
+                if t.isDictionary():
+                    nullValue.type = t
+                    return nullValue
+        nullValue.type = type
+        return nullValue
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLEmptySequenceValue(IDLObject):
+    def __init__(self, location):
+        IDLObject.__init__(self, location)
+        self.type = None
+        self.value = None
+
+    def coerceToType(self, type, location):
+        if type.isUnion():
+            # We use the flat member types here, because if we have a nullable
+            # member type, or a nested union, we want the type the value
+            # actually coerces to, not the nullable or nested union type.
+            for subtype in type.unroll().flatMemberTypes:
+                try:
+                    return self.coerceToType(subtype, location)
+                except Exception:
+                    pass
+
+        if not type.isSequence():
+            raise WebIDLError(
+                "Cannot coerce empty sequence value to type %s." % type, [location]
+            )
+
+        emptySequenceValue = IDLEmptySequenceValue(self.location)
+        emptySequenceValue.type = type
+        return emptySequenceValue
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLDefaultDictionaryValue(IDLObject):
+    def __init__(self, location):
+        IDLObject.__init__(self, location)
+        self.type = None
+        self.value = None
+
+    def coerceToType(self, type, location):
+        if type.isUnion():
+            # We use the flat member types here, because if we have a nullable
+            # member type, or a nested union, we want the type the value
+            # actually coerces to, not the nullable or nested union type.
+            for subtype in type.unroll().flatMemberTypes:
+                try:
+                    return self.coerceToType(subtype, location)
+                except Exception:
+                    pass
+
+        if not type.isDictionary():
+            raise WebIDLError(
+                "Cannot coerce default dictionary value to type %s." % type, [location]
+            )
+
+        defaultDictionaryValue = IDLDefaultDictionaryValue(self.location)
+        defaultDictionaryValue.type = type
+        return defaultDictionaryValue
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLUndefinedValue(IDLObject):
+    def __init__(self, location):
+        IDLObject.__init__(self, location)
+        self.type = None
+        self.value = None
+
+    def coerceToType(self, type, location):
+        if not type.isAny():
+            raise WebIDLError(
+                "Cannot coerce undefined value to type %s." % type, [location]
+            )
+
+        undefinedValue = IDLUndefinedValue(self.location)
+        undefinedValue.type = type
+        return undefinedValue
+
+    def _getDependentObjects(self):
+        return set()
+
+
+class IDLInterfaceMember(IDLObjectWithIdentifier, IDLExposureMixins):
+    Tags = enum(
+        "Const", "Attr", "Method", "MaplikeOrSetlike", "AsyncIterable", "Iterable"
+    )
+
+    Special = enum("Static", "Stringifier")
+
+    AffectsValues = ("Nothing", "Everything")
+    DependsOnValues = ("Nothing", "DOMState", "DeviceState", "Everything")
+
+    def __init__(self, location, identifier, tag, extendedAttrDict=None):
+        IDLObjectWithIdentifier.__init__(self, location, None, identifier)
+        IDLExposureMixins.__init__(self, location)
+        self.tag = tag
+        if extendedAttrDict is None:
+            self._extendedAttrDict = {}
+        else:
+            self._extendedAttrDict = extendedAttrDict
+
+    def isMethod(self):
+        return self.tag == IDLInterfaceMember.Tags.Method
+
+    def isAttr(self):
+        return self.tag == IDLInterfaceMember.Tags.Attr
+
+    def isConst(self):
+        return self.tag == IDLInterfaceMember.Tags.Const
+
+    def isMaplikeOrSetlikeOrIterable(self):
+        return (
+            self.tag == IDLInterfaceMember.Tags.MaplikeOrSetlike
+            or self.tag == IDLInterfaceMember.Tags.AsyncIterable
+            or self.tag == IDLInterfaceMember.Tags.Iterable
+        )
+
+    def isMaplikeOrSetlike(self):
+        return self.tag == IDLInterfaceMember.Tags.MaplikeOrSetlike
+
+    def addExtendedAttributes(self, attrs):
+        for attr in attrs:
+            self.handleExtendedAttribute(attr)
+            attrlist = attr.listValue()
+            self._extendedAttrDict[attr.identifier()] = (
+                attrlist if len(attrlist) else True
+            )
+
+    def handleExtendedAttribute(self, attr):
+        pass
+
+    def getExtendedAttribute(self, name):
+        return self._extendedAttrDict.get(name, None)
+
+    def finish(self, scope):
+        IDLExposureMixins.finish(self, scope)
+
+    def validate(self):
+        if self.isAttr() or self.isMethod():
+            if self.affects == "Everything" and self.dependsOn != "Everything":
+                raise WebIDLError(
+                    "Interface member is flagged as affecting "
+                    "everything but not depending on everything. "
+                    "That seems rather unlikely.",
+                    [self.location],
+                )
+
+        if self.getExtendedAttribute("NewObject"):
+            if self.dependsOn == "Nothing" or self.dependsOn == "DOMState":
+                raise WebIDLError(
+                    "A [NewObject] method is not idempotent, "
+                    "so it has to depend on something other than DOM state.",
+                    [self.location],
+                )
+            if self.getExtendedAttribute("Cached") or self.getExtendedAttribute(
+                "StoreInSlot"
+            ):
+                raise WebIDLError(
+                    "A [NewObject] attribute shouldnt be "
+                    "[Cached] or [StoreInSlot], since the point "
+                    "of those is to keep returning the same "
+                    "thing across multiple calls, which is not "
+                    "what [NewObject] does.",
+                    [self.location],
+                )
+
+    def _setDependsOn(self, dependsOn):
+        if self.dependsOn != "Everything":
+            raise WebIDLError(
+                "Trying to specify multiple different DependsOn, "
+                "Pure, or Constant extended attributes for "
+                "attribute",
+                [self.location],
+            )
+        if dependsOn not in IDLInterfaceMember.DependsOnValues:
+            raise WebIDLError(
+                "Invalid [DependsOn=%s] on attribute" % dependsOn, [self.location]
+            )
+        self.dependsOn = dependsOn
+
+    def _setAffects(self, affects):
+        if self.affects != "Everything":
+            raise WebIDLError(
+                "Trying to specify multiple different Affects, "
+                "Pure, or Constant extended attributes for "
+                "attribute",
+                [self.location],
+            )
+        if affects not in IDLInterfaceMember.AffectsValues:
+            raise WebIDLError(
+                "Invalid [Affects=%s] on attribute" % affects, [self.location]
+            )
+        self.affects = affects
+
+    def _addAlias(self, alias):
+        if alias in self.aliases:
+            raise WebIDLError(
+                "Duplicate [Alias=%s] on attribute" % alias, [self.location]
+            )
+        self.aliases.append(alias)
+
+    def _addBindingAlias(self, bindingAlias):
+        if bindingAlias in self.bindingAliases:
+            raise WebIDLError(
+                "Duplicate [BindingAlias=%s] on attribute" % bindingAlias,
+                [self.location],
+            )
+        self.bindingAliases.append(bindingAlias)
+
+
+class IDLMaplikeOrSetlikeOrIterableBase(IDLInterfaceMember):
+    def __init__(self, location, identifier, ifaceType, keyType, valueType, ifaceKind):
+        IDLInterfaceMember.__init__(self, location, identifier, ifaceKind)
+        if keyType is not None:
+            assert isinstance(keyType, IDLType)
+        else:
+            assert valueType is not None
+        assert ifaceType in ["maplike", "setlike", "iterable", "asynciterable"]
+        if valueType is not None:
+            assert isinstance(valueType, IDLType)
+        self.keyType = keyType
+        self.valueType = valueType
+        self.maplikeOrSetlikeOrIterableType = ifaceType
+        self.disallowedMemberNames = []
+        self.disallowedNonMethodNames = []
+
+    def isMaplike(self):
+        return self.maplikeOrSetlikeOrIterableType == "maplike"
+
+    def isSetlike(self):
+        return self.maplikeOrSetlikeOrIterableType == "setlike"
+
+    def isIterable(self):
+        return self.maplikeOrSetlikeOrIterableType == "iterable"
+
+    def isAsyncIterable(self):
+        return self.maplikeOrSetlikeOrIterableType == "asynciterable"
+
+    def hasKeyType(self):
+        return self.keyType is not None
+
+    def hasValueType(self):
+        return self.valueType is not None
+
+    def checkCollisions(self, members, isAncestor):
+        for member in members:
+            # Check that there are no disallowed members
+            if member.identifier.name in self.disallowedMemberNames and not (
+                (
+                    member.isMethod()
+                    and (
+                        member.isStatic() or member.isMaplikeOrSetlikeOrIterableMethod()
+                    )
+                )
+                or (member.isAttr() and member.isMaplikeOrSetlikeAttr())
+            ):
+                raise WebIDLError(
+                    "Member '%s' conflicts "
+                    "with reserved %s name."
+                    % (member.identifier.name, self.maplikeOrSetlikeOrIterableType),
+                    [self.location, member.location],
+                )
+            # Check that there are no disallowed non-method members.
+            # Ancestor members are always disallowed here; own members
+            # are disallowed only if they're non-methods.
+            if (
+                isAncestor or member.isAttr() or member.isConst()
+            ) and member.identifier.name in self.disallowedNonMethodNames:
+                raise WebIDLError(
+                    "Member '%s' conflicts "
+                    "with reserved %s method."
+                    % (member.identifier.name, self.maplikeOrSetlikeOrIterableType),
+                    [self.location, member.location],
+                )
+
+    def addMethod(
+        self,
+        name,
+        members,
+        allowExistingOperations,
+        returnType,
+        args=[],
+        chromeOnly=False,
+        isPure=False,
+        affectsNothing=False,
+        newObject=False,
+        isIteratorAlias=False,
+    ):
+        """
+        Create an IDLMethod based on the parameters passed in.
+
+        - members is the member list to add this function to, since this is
+          called during the member expansion portion of interface object
+          building.
+
+        - chromeOnly is only True for read-only js implemented classes, to
+        implement underscore prefixed convenience functions which would
+        otherwise not be available, unlike the case of C++ bindings.
+
+        - isPure is only True for idempotent functions, so it is not valid for
+        things like keys, values, etc. that return a new object every time.
+
+        - affectsNothing means that nothing changes due to this method, which
+          affects JIT optimization behavior
+
+        - newObject means the method creates and returns a new object.
+
+        """
+        # Only add name to lists for collision checks if it's not chrome
+        # only.
+        if chromeOnly:
+            name = "__" + name
+        else:
+            if not allowExistingOperations:
+                self.disallowedMemberNames.append(name)
+            else:
+                self.disallowedNonMethodNames.append(name)
+        # If allowExistingOperations is True, and another operation exists
+        # with the same name as the one we're trying to add, don't add the
+        # maplike/setlike operation.
+        if allowExistingOperations:
+            for m in members:
+                if m.identifier.name == name and m.isMethod() and not m.isStatic():
+                    return
+        method = IDLMethod(
+            self.location,
+            IDLUnresolvedIdentifier(
+                self.location, name, allowDoubleUnderscore=chromeOnly
+            ),
+            returnType,
+            args,
+            maplikeOrSetlikeOrIterable=self,
+        )
+        # We need to be able to throw from declaration methods
+        method.addExtendedAttributes([IDLExtendedAttribute(self.location, ("Throws",))])
+        if chromeOnly:
+            method.addExtendedAttributes(
+                [IDLExtendedAttribute(self.location, ("ChromeOnly",))]
+            )
+        if isPure:
+            method.addExtendedAttributes(
+                [IDLExtendedAttribute(self.location, ("Pure",))]
+            )
+        # Following attributes are used for keys/values/entries. Can't mark
+        # them pure, since they return a new object each time they are run.
+        if affectsNothing:
+            method.addExtendedAttributes(
+                [
+                    IDLExtendedAttribute(self.location, ("DependsOn", "Everything")),
+                    IDLExtendedAttribute(self.location, ("Affects", "Nothing")),
+                ]
+            )
+        if newObject:
+            method.addExtendedAttributes(
+                [IDLExtendedAttribute(self.location, ("NewObject",))]
+            )
+        if isIteratorAlias:
+            if not self.isAsyncIterable():
+                method.addExtendedAttributes(
+                    [IDLExtendedAttribute(self.location, ("Alias", "@@iterator"))]
+                )
+            else:
+                method.addExtendedAttributes(
+                    [IDLExtendedAttribute(self.location, ("Alias", "@@asyncIterator"))]
+                )
+        members.append(method)
+
+    def resolve(self, parentScope):
+        if self.keyType:
+            self.keyType.resolveType(parentScope)
+        if self.valueType:
+            self.valueType.resolveType(parentScope)
+
+    def finish(self, scope):
+        IDLInterfaceMember.finish(self, scope)
+        if self.keyType and not self.keyType.isComplete():
+            t = self.keyType.complete(scope)
+
+            assert not isinstance(t, IDLUnresolvedType)
+            assert not isinstance(t, IDLTypedefType)
+            assert not isinstance(t.name, IDLUnresolvedIdentifier)
+            self.keyType = t
+        if self.valueType and not self.valueType.isComplete():
+            t = self.valueType.complete(scope)
+
+            assert not isinstance(t, IDLUnresolvedType)
+            assert not isinstance(t, IDLTypedefType)
+            assert not isinstance(t.name, IDLUnresolvedIdentifier)
+            self.valueType = t
+
+    def validate(self):
+        IDLInterfaceMember.validate(self)
+
+    def handleExtendedAttribute(self, attr):
+        IDLInterfaceMember.handleExtendedAttribute(self, attr)
+
+    def _getDependentObjects(self):
+        deps = set()
+        if self.keyType:
+            deps.add(self.keyType)
+        if self.valueType:
+            deps.add(self.valueType)
+        return deps
+
+    def getForEachArguments(self):
+        return [
+            IDLArgument(
+                self.location,
+                IDLUnresolvedIdentifier(
+                    BuiltinLocation("<auto-generated-identifier>"), "callback"
+                ),
+                BuiltinTypes[IDLBuiltinType.Types.object],
+            ),
+            IDLArgument(
+                self.location,
+                IDLUnresolvedIdentifier(
+                    BuiltinLocation("<auto-generated-identifier>"), "thisArg"
+                ),
+                BuiltinTypes[IDLBuiltinType.Types.any],
+                optional=True,
+            ),
+        ]
+
+
+# Iterable adds ES6 iterator style functions and traits
+# (keys/values/entries/@@iterator) to an interface.
+class IDLIterable(IDLMaplikeOrSetlikeOrIterableBase):
+    def __init__(self, location, identifier, keyType, valueType, scope):
+        IDLMaplikeOrSetlikeOrIterableBase.__init__(
+            self,
+            location,
+            identifier,
+            "iterable",
+            keyType,
+            valueType,
+            IDLInterfaceMember.Tags.Iterable,
+        )
+        self.iteratorType = None
+
+    def __str__(self):
+        return "declared iterable with key '%s' and value '%s'" % (
+            self.keyType,
+            self.valueType,
+        )
+
+    def expand(self, members):
+        """
+        In order to take advantage of all of the method machinery in Codegen,
+        we generate our functions as if they were part of the interface
+        specification during parsing.
+        """
+        # We only need to add entries/keys/values here if we're a pair iterator.
+        # Value iterators just copy these from %ArrayPrototype% instead.
+        if not self.isPairIterator():
+            return
+
+        # object entries()
+        self.addMethod(
+            "entries",
+            members,
+            False,
+            self.iteratorType,
+            affectsNothing=True,
+            newObject=True,
+            isIteratorAlias=True,
+        )
+        # object keys()
+        self.addMethod(
+            "keys",
+            members,
+            False,
+            self.iteratorType,
+            affectsNothing=True,
+            newObject=True,
+        )
+        # object values()
+        self.addMethod(
+            "values",
+            members,
+            False,
+            self.iteratorType,
+            affectsNothing=True,
+            newObject=True,
+        )
+
+        # undefined forEach(callback(valueType, keyType), optional any thisArg)
+        self.addMethod(
+            "forEach",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.undefined],
+            self.getForEachArguments(),
+        )
+
+    def isValueIterator(self):
+        return not self.isPairIterator()
+
+    def isPairIterator(self):
+        return self.hasKeyType()
+
+
+class IDLAsyncIterable(IDLMaplikeOrSetlikeOrIterableBase):
+    def __init__(self, location, identifier, keyType, valueType, argList, scope):
+        for arg in argList:
+            if not arg.optional:
+                raise WebIDLError(
+                    "The arguments of the asynchronously iterable declaration on "
+                    "%s must all be optional arguments." % identifier,
+                    [arg.location],
+                )
+
+        IDLMaplikeOrSetlikeOrIterableBase.__init__(
+            self,
+            location,
+            identifier,
+            "asynciterable",
+            keyType,
+            valueType,
+            IDLInterfaceMember.Tags.AsyncIterable,
+        )
+        self.iteratorType = None
+        self.argList = argList
+
+    def __str__(self):
+        return "declared async iterable with key '%s' and value '%s'" % (
+            self.keyType,
+            self.valueType,
+        )
+
+    def expand(self, members):
+        """
+        In order to take advantage of all of the method machinery in Codegen,
+        we generate our functions as if they were part of the interface
+        specification during parsing.
+        """
+        # object values()
+        self.addMethod(
+            "values",
+            members,
+            False,
+            self.iteratorType,
+            self.argList,
+            affectsNothing=True,
+            newObject=True,
+            isIteratorAlias=(not self.isPairIterator()),
+        )
+
+        # We only need to add entries/keys here if we're a pair iterator.
+        if not self.isPairIterator():
+            return
+
+        # Methods can't share their IDLArguments, so we need to make copies here.
+        def copyArgList(argList):
+            return map(copy.copy, argList)
+
+        # object entries()
+        self.addMethod(
+            "entries",
+            members,
+            False,
+            self.iteratorType,
+            copyArgList(self.argList),
+            affectsNothing=True,
+            newObject=True,
+            isIteratorAlias=True,
+        )
+        # object keys()
+        self.addMethod(
+            "keys",
+            members,
+            False,
+            self.iteratorType,
+            copyArgList(self.argList),
+            affectsNothing=True,
+            newObject=True,
+        )
+
+    def isValueIterator(self):
+        return not self.isPairIterator()
+
+    def isPairIterator(self):
+        return self.hasKeyType()
+
+
+# MaplikeOrSetlike adds ES6 map-or-set-like traits to an interface.
+class IDLMaplikeOrSetlike(IDLMaplikeOrSetlikeOrIterableBase):
+    def __init__(
+        self, location, identifier, maplikeOrSetlikeType, readonly, keyType, valueType
+    ):
+        IDLMaplikeOrSetlikeOrIterableBase.__init__(
+            self,
+            location,
+            identifier,
+            maplikeOrSetlikeType,
+            keyType,
+            valueType,
+            IDLInterfaceMember.Tags.MaplikeOrSetlike,
+        )
+        self.readonly = readonly
+        self.slotIndices = None
+
+        # When generating JSAPI access code, we need to know the backing object
+        # type prefix to create the correct function. Generate here for reuse.
+        if self.isMaplike():
+            self.prefix = "Map"
+        elif self.isSetlike():
+            self.prefix = "Set"
+
+    def __str__(self):
+        return "declared '%s' with key '%s'" % (
+            self.maplikeOrSetlikeOrIterableType,
+            self.keyType,
+        )
+
+    def expand(self, members):
+        """
+        In order to take advantage of all of the method machinery in Codegen,
+        we generate our functions as if they were part of the interface
+        specification during parsing.
+        """
+        # Both maplike and setlike have a size attribute
+        members.append(
+            IDLAttribute(
+                self.location,
+                IDLUnresolvedIdentifier(
+                    BuiltinLocation("<auto-generated-identifier>"), "size"
+                ),
+                BuiltinTypes[IDLBuiltinType.Types.unsigned_long],
+                True,
+                maplikeOrSetlike=self,
+            )
+        )
+        self.reserved_ro_names = ["size"]
+        self.disallowedMemberNames.append("size")
+
+        # object entries()
+        self.addMethod(
+            "entries",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.object],
+            affectsNothing=True,
+            isIteratorAlias=self.isMaplike(),
+        )
+        # object keys()
+        self.addMethod(
+            "keys",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.object],
+            affectsNothing=True,
+        )
+        # object values()
+        self.addMethod(
+            "values",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.object],
+            affectsNothing=True,
+            isIteratorAlias=self.isSetlike(),
+        )
+
+        # undefined forEach(callback(valueType, keyType), thisVal)
+        self.addMethod(
+            "forEach",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.undefined],
+            self.getForEachArguments(),
+        )
+
+        def getKeyArg():
+            return IDLArgument(
+                self.location,
+                IDLUnresolvedIdentifier(self.location, "key"),
+                self.keyType,
+            )
+
+        # boolean has(keyType key)
+        self.addMethod(
+            "has",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.boolean],
+            [getKeyArg()],
+            isPure=True,
+        )
+
+        if not self.readonly:
+            # undefined clear()
+            self.addMethod(
+                "clear", members, True, BuiltinTypes[IDLBuiltinType.Types.undefined], []
+            )
+            # boolean delete(keyType key)
+            self.addMethod(
+                "delete",
+                members,
+                True,
+                BuiltinTypes[IDLBuiltinType.Types.boolean],
+                [getKeyArg()],
+            )
+
+        if self.isSetlike():
+            if not self.readonly:
+                # Add returns the set object it just added to.
+                # object add(keyType key)
+
+                self.addMethod(
+                    "add",
+                    members,
+                    True,
+                    BuiltinTypes[IDLBuiltinType.Types.object],
+                    [getKeyArg()],
+                )
+            return
+
+        # If we get this far, we're a maplike declaration.
+
+        # valueType get(keyType key)
+        #
+        # Note that instead of the value type, we're using any here. The
+        # validity checks should happen as things are inserted into the map,
+        # and using any as the return type makes code generation much simpler.
+        #
+        # TODO: Bug 1155340 may change this to use specific type to provide
+        # more info to JIT.
+        self.addMethod(
+            "get",
+            members,
+            False,
+            BuiltinTypes[IDLBuiltinType.Types.any],
+            [getKeyArg()],
+            isPure=True,
+        )
+
+        def getValueArg():
+            return IDLArgument(
+                self.location,
+                IDLUnresolvedIdentifier(self.location, "value"),
+                self.valueType,
+            )
+
+        if not self.readonly:
+            self.addMethod(
+                "set",
+                members,
+                True,
+                BuiltinTypes[IDLBuiltinType.Types.object],
+                [getKeyArg(), getValueArg()],
+            )
+
+
+class IDLConst(IDLInterfaceMember):
+    def __init__(self, location, identifier, type, value):
+        IDLInterfaceMember.__init__(
+            self, location, identifier, IDLInterfaceMember.Tags.Const
+        )
+
+        assert isinstance(type, IDLType)
+        if type.isDictionary():
+            raise WebIDLError(
+                "A constant cannot be of a dictionary type", [self.location]
+            )
+        if type.isRecord():
+            raise WebIDLError("A constant cannot be of a record type", [self.location])
+        self.type = type
+        self.value = value
+
+        if identifier.name == "prototype":
+            raise WebIDLError(
+                "The identifier of a constant must not be 'prototype'", [location]
+            )
+
+    def __str__(self):
+        return "'%s' const '%s'" % (self.type, self.identifier)
+
+    def finish(self, scope):
+        IDLInterfaceMember.finish(self, scope)
+
+        if not self.type.isComplete():
+            type = self.type.complete(scope)
+            if not type.isPrimitive() and not type.isString():
+                locations = [self.type.location, type.location]
+                try:
+                    locations.append(type.inner.location)
+                except Exception:
+                    pass
+                raise WebIDLError("Incorrect type for constant", locations)
+            self.type = type
+
+        # The value might not match the type
+        coercedValue = self.value.coerceToType(self.type, self.location)
+        assert coercedValue
+
+        self.value = coercedValue
+
+    def validate(self):
+        IDLInterfaceMember.validate(self)
+
+    def handleExtendedAttribute(self, attr):
+        identifier = attr.identifier()
+        if identifier == "Exposed":
+            convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+        elif (
+            identifier == "Pref"
+            or identifier == "ChromeOnly"
+            or identifier == "Func"
+            or identifier == "Trial"
+            or identifier == "SecureContext"
+            or identifier == "NonEnumerable"
+        ):
+            # Known attributes that we don't need to do anything with here
+            pass
+        else:
+            raise WebIDLError(
+                "Unknown extended attribute %s on constant" % identifier,
+                [attr.location],
+            )
+        IDLInterfaceMember.handleExtendedAttribute(self, attr)
+
+    def _getDependentObjects(self):
+        return set([self.type, self.value])
+
+
+class IDLAttribute(IDLInterfaceMember):
+    def __init__(
+        self,
+        location,
+        identifier,
+        type,
+        readonly,
+        inherit=False,
+        static=False,
+        stringifier=False,
+        maplikeOrSetlike=None,
+        extendedAttrDict=None,
+    ):
+        IDLInterfaceMember.__init__(
+            self,
+            location,
+            identifier,
+            IDLInterfaceMember.Tags.Attr,
+            extendedAttrDict=extendedAttrDict,
+        )
+
+        assert isinstance(type, IDLType)
+        self.type = type
+        self.readonly = readonly
+        self.inherit = inherit
+        self._static = static
+        self.legacyLenientThis = False
+        self._legacyUnforgeable = False
+        self.stringifier = stringifier
+        self.slotIndices = None
+        assert maplikeOrSetlike is None or isinstance(
+            maplikeOrSetlike, IDLMaplikeOrSetlike
+        )
+        self.maplikeOrSetlike = maplikeOrSetlike
+        self.dependsOn = "Everything"
+        self.affects = "Everything"
+        self.bindingAliases = []
+
+        if static and identifier.name == "prototype":
+            raise WebIDLError(
+                "The identifier of a static attribute must not be 'prototype'",
+                [location],
+            )
+
+        if readonly and inherit:
+            raise WebIDLError(
+                "An attribute cannot be both 'readonly' and 'inherit'", [self.location]
+            )
+
+    def isStatic(self):
+        return self._static
+
+    def forceStatic(self):
+        self._static = True
+
+    def __str__(self):
+        return "'%s' attribute '%s'" % (self.type, self.identifier)
+
+    def finish(self, scope):
+        IDLInterfaceMember.finish(self, scope)
+
+        if not self.type.isComplete():
+            t = self.type.complete(scope)
+
+            assert not isinstance(t, IDLUnresolvedType)
+            assert not isinstance(t, IDLTypedefType)
+            assert not isinstance(t.name, IDLUnresolvedIdentifier)
+            self.type = t
+
+        if self.readonly and (
+            self.type.hasClamp()
+            or self.type.hasEnforceRange()
+            or self.type.hasAllowShared()
+            or self.type.legacyNullToEmptyString
+        ):
+            raise WebIDLError(
+                "A readonly attribute cannot be [Clamp] or [EnforceRange] or [AllowShared]",
+                [self.location],
+            )
+        if self.type.isDictionary() and not self.getExtendedAttribute("Cached"):
+            raise WebIDLError(
+                "An attribute cannot be of a dictionary type", [self.location]
+            )
+        if self.type.isSequence() and not self.getExtendedAttribute("Cached"):
+            raise WebIDLError(
+                "A non-cached attribute cannot be of a sequence " "type",
+                [self.location],
+            )
+        if self.type.isRecord() and not self.getExtendedAttribute("Cached"):
+            raise WebIDLError(
+                "A non-cached attribute cannot be of a record " "type", [self.location]
+            )
+        if self.type.isUnion():
+            for f in self.type.unroll().flatMemberTypes:
+                if f.isDictionary():
+                    raise WebIDLError(
+                        "An attribute cannot be of a union "
+                        "type if one of its member types (or "
+                        "one of its member types's member "
+                        "types, and so on) is a dictionary "
+                        "type",
+                        [self.location, f.location],
+                    )
+                if f.isSequence():
+                    raise WebIDLError(
+                        "An attribute cannot be of a union "
+                        "type if one of its member types (or "
+                        "one of its member types's member "
+                        "types, and so on) is a sequence "
+                        "type",
+                        [self.location, f.location],
+                    )
+                if f.isRecord():
+                    raise WebIDLError(
+                        "An attribute cannot be of a union "
+                        "type if one of its member types (or "
+                        "one of its member types's member "
+                        "types, and so on) is a record "
+                        "type",
+                        [self.location, f.location],
+                    )
+        if not self.type.isInterface() and self.getExtendedAttribute("PutForwards"):
+            raise WebIDLError(
+                "An attribute with [PutForwards] must have an "
+                "interface type as its type",
+                [self.location],
+            )
+
+        if not self.type.isInterface() and self.getExtendedAttribute("SameObject"):
+            raise WebIDLError(
+                "An attribute with [SameObject] must have an "
+                "interface type as its type",
+                [self.location],
+            )
+
+        if self.type.isPromise() and not self.readonly:
+            raise WebIDLError(
+                "Promise-returning attributes must be readonly", [self.location]
+            )
+
+        if self.type.isObservableArray():
+            if self.isStatic():
+                raise WebIDLError(
+                    "A static attribute cannot have an ObservableArray type",
+                    [self.location],
+                )
+            if self.getExtendedAttribute("Cached") or self.getExtendedAttribute(
+                "StoreInSlot"
+            ):
+                raise WebIDLError(
+                    "[Cached] and [StoreInSlot] must not be used "
+                    "on an attribute whose type is ObservableArray",
+                    [self.location],
+                )
+
+    def validate(self):
+        def typeContainsChromeOnlyDictionaryMember(type):
+            if type.nullable() or type.isSequence() or type.isRecord():
+                return typeContainsChromeOnlyDictionaryMember(type.inner)
+
+            if type.isUnion():
+                for memberType in type.flatMemberTypes:
+                    (contains, location) = typeContainsChromeOnlyDictionaryMember(
+                        memberType
+                    )
+                    if contains:
+                        return (True, location)
+
+            if type.isDictionary():
+                dictionary = type.inner
+                while dictionary:
+                    (contains, location) = dictionaryContainsChromeOnlyMember(
+                        dictionary
+                    )
+                    if contains:
+                        return (True, location)
+                    dictionary = dictionary.parent
+
+            return (False, None)
+
+        def dictionaryContainsChromeOnlyMember(dictionary):
+            for member in dictionary.members:
+                if member.getExtendedAttribute("ChromeOnly"):
+                    return (True, member.location)
+                (contains, location) = typeContainsChromeOnlyDictionaryMember(
+                    member.type
+                )
+                if contains:
+                    return (True, location)
+            return (False, None)
+
+        IDLInterfaceMember.validate(self)
+
+        if self.getExtendedAttribute("Cached") or self.getExtendedAttribute(
+            "StoreInSlot"
+        ):
+            if not self.affects == "Nothing":
+                raise WebIDLError(
+                    "Cached attributes and attributes stored in "
+                    "slots must be Constant or Pure or "
+                    "Affects=Nothing, since the getter won't always "
+                    "be called.",
+                    [self.location],
+                )
+            (contains, location) = typeContainsChromeOnlyDictionaryMember(self.type)
+            if contains:
+                raise WebIDLError(
+                    "[Cached] and [StoreInSlot] must not be used "
+                    "on an attribute whose type contains a "
+                    "[ChromeOnly] dictionary member",
+                    [self.location, location],
+                )
+        if self.getExtendedAttribute("Frozen"):
+            if (
+                not self.type.isSequence()
+                and not self.type.isDictionary()
+                and not self.type.isRecord()
+            ):
+                raise WebIDLError(
+                    "[Frozen] is only allowed on "
+                    "sequence-valued, dictionary-valued, and "
+                    "record-valued attributes",
+                    [self.location],
+                )
+        if not self.type.unroll().isExposedInAllOf(self.exposureSet):
+            raise WebIDLError(
+                "Attribute returns a type that is not exposed "
+                "everywhere where the attribute is exposed",
+                [self.location],
+            )
+        if self.getExtendedAttribute("CEReactions"):
+            if self.readonly:
+                raise WebIDLError(
+                    "[CEReactions] is not allowed on " "readonly attributes",
+                    [self.location],
+                )
+
+    def handleExtendedAttribute(self, attr):
+        identifier = attr.identifier()
+        if (
+            identifier == "SetterThrows"
+            or identifier == "SetterCanOOM"
+            or identifier == "SetterNeedsSubjectPrincipal"
+        ) and self.readonly:
+            raise WebIDLError(
+                "Readonly attributes must not be flagged as " "[%s]" % identifier,
+                [self.location],
+            )
+        elif identifier == "BindingAlias":
+            if not attr.hasValue():
+                raise WebIDLError(
+                    "[BindingAlias] takes an identifier or string", [attr.location]
+                )
+            self._addBindingAlias(attr.value())
+        elif (
+            (
+                identifier == "Throws"
+                or identifier == "GetterThrows"
+                or identifier == "CanOOM"
+                or identifier == "GetterCanOOM"
+            )
+            and self.getExtendedAttribute("StoreInSlot")
+        ) or (
+            identifier == "StoreInSlot"
+            and (
+                self.getExtendedAttribute("Throws")
+                or self.getExtendedAttribute("GetterThrows")
+                or self.getExtendedAttribute("CanOOM")
+                or self.getExtendedAttribute("GetterCanOOM")
+            )
+        ):
+            raise WebIDLError("Throwing things can't be [StoreInSlot]", [attr.location])
+        elif identifier == "LegacyLenientThis":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[LegacyLenientThis] must take no arguments", [attr.location]
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[LegacyLenientThis] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("CrossOriginReadable"):
+                raise WebIDLError(
+                    "[LegacyLenientThis] is not allowed in combination "
+                    "with [CrossOriginReadable]",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("CrossOriginWritable"):
+                raise WebIDLError(
+                    "[LegacyLenientThis] is not allowed in combination "
+                    "with [CrossOriginWritable]",
+                    [attr.location, self.location],
+                )
+            self.legacyLenientThis = True
+        elif identifier == "LegacyUnforgeable":
+            if self.isStatic():
+                raise WebIDLError(
+                    "[LegacyUnforgeable] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+            self._legacyUnforgeable = True
+        elif identifier == "SameObject" and not self.readonly:
+            raise WebIDLError(
+                "[SameObject] only allowed on readonly attributes",
+                [attr.location, self.location],
+            )
+        elif identifier == "Constant" and not self.readonly:
+            raise WebIDLError(
+                "[Constant] only allowed on readonly attributes",
+                [attr.location, self.location],
+            )
+        elif identifier == "PutForwards":
+            if not self.readonly:
+                raise WebIDLError(
+                    "[PutForwards] is only allowed on readonly " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.type.isPromise():
+                raise WebIDLError(
+                    "[PutForwards] is not allowed on " "Promise-typed attributes",
+                    [attr.location, self.location],
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[PutForwards] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("Replaceable") is not None:
+                raise WebIDLError(
+                    "[PutForwards] and [Replaceable] can't both "
+                    "appear on the same attribute",
+                    [attr.location, self.location],
+                )
+            if not attr.hasValue():
+                raise WebIDLError(
+                    "[PutForwards] takes an identifier", [attr.location, self.location]
+                )
+        elif identifier == "Replaceable":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[Replaceable] must take no arguments", [attr.location]
+                )
+            if not self.readonly:
+                raise WebIDLError(
+                    "[Replaceable] is only allowed on readonly " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.type.isPromise():
+                raise WebIDLError(
+                    "[Replaceable] is not allowed on " "Promise-typed attributes",
+                    [attr.location, self.location],
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[Replaceable] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("PutForwards") is not None:
+                raise WebIDLError(
+                    "[PutForwards] and [Replaceable] can't both "
+                    "appear on the same attribute",
+                    [attr.location, self.location],
+                )
+        elif identifier == "LegacyLenientSetter":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[LegacyLenientSetter] must take no arguments", [attr.location]
+                )
+            if not self.readonly:
+                raise WebIDLError(
+                    "[LegacyLenientSetter] is only allowed on readonly " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.type.isPromise():
+                raise WebIDLError(
+                    "[LegacyLenientSetter] is not allowed on "
+                    "Promise-typed attributes",
+                    [attr.location, self.location],
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[LegacyLenientSetter] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("PutForwards") is not None:
+                raise WebIDLError(
+                    "[LegacyLenientSetter] and [PutForwards] can't both "
+                    "appear on the same attribute",
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("Replaceable") is not None:
+                raise WebIDLError(
+                    "[LegacyLenientSetter] and [Replaceable] can't both "
+                    "appear on the same attribute",
+                    [attr.location, self.location],
+                )
+        elif identifier == "LenientFloat":
+            if self.readonly:
+                raise WebIDLError(
+                    "[LenientFloat] used on a readonly attribute",
+                    [attr.location, self.location],
+                )
+            if not self.type.includesRestrictedFloat():
+                raise WebIDLError(
+                    "[LenientFloat] used on an attribute with a "
+                    "non-restricted-float type",
+                    [attr.location, self.location],
+                )
+        elif identifier == "StoreInSlot":
+            if self.getExtendedAttribute("Cached"):
+                raise WebIDLError(
+                    "[StoreInSlot] and [Cached] must not be "
+                    "specified on the same attribute",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Cached":
+            if self.getExtendedAttribute("StoreInSlot"):
+                raise WebIDLError(
+                    "[Cached] and [StoreInSlot] must not be "
+                    "specified on the same attribute",
+                    [attr.location, self.location],
+                )
+        elif identifier == "CrossOriginReadable" or identifier == "CrossOriginWritable":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[%s] must take no arguments" % identifier, [attr.location]
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[%s] is only allowed on non-static " "attributes" % identifier,
+                    [attr.location, self.location],
+                )
+            if self.getExtendedAttribute("LegacyLenientThis"):
+                raise WebIDLError(
+                    "[LegacyLenientThis] is not allowed in combination "
+                    "with [%s]" % identifier,
+                    [attr.location, self.location],
+                )
+        elif identifier == "Exposed":
+            convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+        elif identifier == "Pure":
+            if not attr.noArguments():
+                raise WebIDLError("[Pure] must take no arguments", [attr.location])
+            self._setDependsOn("DOMState")
+            self._setAffects("Nothing")
+        elif identifier == "Constant" or identifier == "SameObject":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[%s] must take no arguments" % identifier, [attr.location]
+                )
+            self._setDependsOn("Nothing")
+            self._setAffects("Nothing")
+        elif identifier == "Affects":
+            if not attr.hasValue():
+                raise WebIDLError("[Affects] takes an identifier", [attr.location])
+            self._setAffects(attr.value())
+        elif identifier == "DependsOn":
+            if not attr.hasValue():
+                raise WebIDLError("[DependsOn] takes an identifier", [attr.location])
+            if (
+                attr.value() != "Everything"
+                and attr.value() != "DOMState"
+                and not self.readonly
+            ):
+                raise WebIDLError(
+                    "[DependsOn=%s] only allowed on "
+                    "readonly attributes" % attr.value(),
+                    [attr.location, self.location],
+                )
+            self._setDependsOn(attr.value())
+        elif identifier == "UseCounter":
+            if self.stringifier:
+                raise WebIDLError(
+                    "[UseCounter] must not be used on a " "stringifier attribute",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Unscopable":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[Unscopable] must take no arguments", [attr.location]
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[Unscopable] is only allowed on non-static "
+                    "attributes and operations",
+                    [attr.location, self.location],
+                )
+        elif identifier == "CEReactions":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[CEReactions] must take no arguments", [attr.location]
+                )
+        elif (
+            identifier == "Pref"
+            or identifier == "Deprecated"
+            or identifier == "SetterThrows"
+            or identifier == "Throws"
+            or identifier == "GetterThrows"
+            or identifier == "SetterCanOOM"
+            or identifier == "CanOOM"
+            or identifier == "GetterCanOOM"
+            or identifier == "ChromeOnly"
+            or identifier == "Func"
+            or identifier == "Trial"
+            or identifier == "SecureContext"
+            or identifier == "Frozen"
+            or identifier == "NewObject"
+            or identifier == "NeedsSubjectPrincipal"
+            or identifier == "SetterNeedsSubjectPrincipal"
+            or identifier == "GetterNeedsSubjectPrincipal"
+            or identifier == "NeedsCallerType"
+            or identifier == "BinaryName"
+            or identifier == "NonEnumerable"
+            or identifier == "BindingTemplate"
+        ):
+            # Known attributes that we don't need to do anything with here
+            pass
+        else:
+            raise WebIDLError(
+                "Unknown extended attribute %s on attribute" % identifier,
+                [attr.location],
+            )
+        IDLInterfaceMember.handleExtendedAttribute(self, attr)
+
+    def getExtendedAttributes(self):
+        return self._extendedAttrDict
+
+    def resolve(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        self.type.resolveType(parentScope)
+        IDLObjectWithIdentifier.resolve(self, parentScope)
+
+    def hasLegacyLenientThis(self):
+        return self.legacyLenientThis
+
+    def isMaplikeOrSetlikeAttr(self):
+        """
+        True if this attribute was generated from an interface with
+        maplike/setlike (e.g. this is the size attribute for
+        maplike/setlike)
+        """
+        return self.maplikeOrSetlike is not None
+
+    def isLegacyUnforgeable(self):
+        return self._legacyUnforgeable
+
+    def _getDependentObjects(self):
+        return set([self.type])
+
+    def expand(self, members):
+        assert self.stringifier
+        if (
+            not self.type.isDOMString()
+            and not self.type.isUSVString()
+            and not self.type.isUTF8String()
+        ):
+            raise WebIDLError(
+                "The type of a stringifer attribute must be "
+                "either DOMString, USVString or UTF8String",
+                [self.location],
+            )
+        identifier = IDLUnresolvedIdentifier(
+            self.location, "__stringifier", allowDoubleUnderscore=True
+        )
+        method = IDLMethod(
+            self.location,
+            identifier,
+            returnType=self.type,
+            arguments=[],
+            stringifier=True,
+            underlyingAttr=self,
+        )
+        allowedExtAttrs = ["Throws", "NeedsSubjectPrincipal", "Pure"]
+        # Safe to ignore these as they are only meaningful for attributes
+        attributeOnlyExtAttrs = [
+            "CEReactions",
+            "CrossOriginWritable",
+            "SetterThrows",
+        ]
+        for key, value in self._extendedAttrDict.items():
+            if key in allowedExtAttrs:
+                if value is not True:
+                    raise WebIDLError(
+                        "[%s] with a value is currently "
+                        "unsupported in stringifier attributes, "
+                        "please file a bug to add support" % key,
+                        [self.location],
+                    )
+                method.addExtendedAttributes(
+                    [IDLExtendedAttribute(self.location, (key,))]
+                )
+            elif key not in attributeOnlyExtAttrs:
+                raise WebIDLError(
+                    "[%s] is currently unsupported in "
+                    "stringifier attributes, please file a bug "
+                    "to add support" % key,
+                    [self.location],
+                )
+        members.append(method)
+
+
+class IDLArgument(IDLObjectWithIdentifier):
+    def __init__(
+        self,
+        location,
+        identifier,
+        type,
+        optional=False,
+        defaultValue=None,
+        variadic=False,
+        dictionaryMember=False,
+        allowTypeAttributes=False,
+    ):
+        IDLObjectWithIdentifier.__init__(self, location, None, identifier)
+
+        assert isinstance(type, IDLType)
+        self.type = type
+
+        self.optional = optional
+        self.defaultValue = defaultValue
+        self.variadic = variadic
+        self.dictionaryMember = dictionaryMember
+        self._isComplete = False
+        self._allowTreatNonCallableAsNull = False
+        self._extendedAttrDict = {}
+        self.allowTypeAttributes = allowTypeAttributes
+
+        assert not variadic or optional
+        assert not variadic or not defaultValue
+
+    def addExtendedAttributes(self, attrs):
+        for attribute in attrs:
+            identifier = attribute.identifier()
+            if self.allowTypeAttributes and (
+                identifier == "EnforceRange"
+                or identifier == "Clamp"
+                or identifier == "LegacyNullToEmptyString"
+                or identifier == "AllowShared"
+            ):
+                self.type = self.type.withExtendedAttributes([attribute])
+            elif identifier == "TreatNonCallableAsNull":
+                self._allowTreatNonCallableAsNull = True
+            elif self.dictionaryMember and (
+                identifier == "ChromeOnly"
+                or identifier == "Func"
+                or identifier == "Trial"
+                or identifier == "Pref"
+            ):
+                if not self.optional:
+                    raise WebIDLError(
+                        "[%s] must not be used on a required "
+                        "dictionary member" % identifier,
+                        [attribute.location],
+                    )
+            elif self.dictionaryMember and identifier == "BinaryType":
+                if not len(attribute.listValue()) == 1:
+                    raise WebIDLError(
+                        "[%s] BinaryType must take one argument" % identifier,
+                        [attribute.location],
+                    )
+                if not self.defaultValue:
+                    raise WebIDLError(
+                        "[%s] BinaryType can't be used without default value"
+                        % identifier,
+                        [attribute.location],
+                    )
+            else:
+                raise WebIDLError(
+                    "Unhandled extended attribute on %s"
+                    % (
+                        "a dictionary member"
+                        if self.dictionaryMember
+                        else "an argument"
+                    ),
+                    [attribute.location],
+                )
+            attrlist = attribute.listValue()
+            self._extendedAttrDict[identifier] = attrlist if len(attrlist) else True
+
+    def getExtendedAttribute(self, name):
+        return self._extendedAttrDict.get(name, None)
+
+    def isComplete(self):
+        return self._isComplete
+
+    def complete(self, scope):
+        if self._isComplete:
+            return
+
+        self._isComplete = True
+
+        if not self.type.isComplete():
+            type = self.type.complete(scope)
+            assert not isinstance(type, IDLUnresolvedType)
+            assert not isinstance(type, IDLTypedefType)
+            assert not isinstance(type.name, IDLUnresolvedIdentifier)
+            self.type = type
+
+        if self.type.isUndefined():
+            raise WebIDLError(
+                "undefined must not be used as the type of an argument in any circumstance",
+                [self.location],
+            )
+
+        if self.type.isAny():
+            assert self.defaultValue is None or isinstance(
+                self.defaultValue, IDLNullValue
+            )
+            # optional 'any' values always have a default value
+            if self.optional and not self.defaultValue and not self.variadic:
+                # Set the default value to undefined, for simplicity, so the
+                # codegen doesn't have to special-case this.
+                self.defaultValue = IDLUndefinedValue(self.location)
+
+        if self.dictionaryMember and self.type.legacyNullToEmptyString:
+            raise WebIDLError(
+                "Dictionary members cannot be [LegacyNullToEmptyString]",
+                [self.location],
+            )
+        if self.type.isObservableArray():
+            raise WebIDLError(
+                "%s cannot have an ObservableArray type"
+                % ("Dictionary members" if self.dictionaryMember else "Arguments"),
+                [self.location],
+            )
+        # Now do the coercing thing; this needs to happen after the
+        # above creation of a default value.
+        if self.defaultValue:
+            self.defaultValue = self.defaultValue.coerceToType(self.type, self.location)
+            assert self.defaultValue
+
+    def allowTreatNonCallableAsNull(self):
+        return self._allowTreatNonCallableAsNull
+
+    def _getDependentObjects(self):
+        deps = set([self.type])
+        if self.defaultValue:
+            deps.add(self.defaultValue)
+        return deps
+
+    def canHaveMissingValue(self):
+        return self.optional and not self.defaultValue
+
+
+class IDLCallback(IDLObjectWithScope):
+    def __init__(
+        self, location, parentScope, identifier, returnType, arguments, isConstructor
+    ):
+        assert isinstance(returnType, IDLType)
+
+        self._returnType = returnType
+        # Clone the list
+        self._arguments = list(arguments)
+
+        IDLObjectWithScope.__init__(self, location, parentScope, identifier)
+
+        for returnType, arguments in self.signatures():
+            for argument in arguments:
+                argument.resolve(self)
+
+        self._treatNonCallableAsNull = False
+        self._treatNonObjectAsNull = False
+        self._isRunScriptBoundary = False
+        self._isConstructor = isConstructor
+
+    def isCallback(self):
+        return True
+
+    def isConstructor(self):
+        return self._isConstructor
+
+    def signatures(self):
+        return [(self._returnType, self._arguments)]
+
+    def finish(self, scope):
+        if not self._returnType.isComplete():
+            type = self._returnType.complete(scope)
+
+            assert not isinstance(type, IDLUnresolvedType)
+            assert not isinstance(type, IDLTypedefType)
+            assert not isinstance(type.name, IDLUnresolvedIdentifier)
+            self._returnType = type
+
+        for argument in self._arguments:
+            if argument.type.isComplete():
+                continue
+
+            type = argument.type.complete(scope)
+
+            assert not isinstance(type, IDLUnresolvedType)
+            assert not isinstance(type, IDLTypedefType)
+            assert not isinstance(type.name, IDLUnresolvedIdentifier)
+            argument.type = type
+
+    def validate(self):
+        for argument in self._arguments:
+            if argument.type.isUndefined():
+                raise WebIDLError(
+                    "undefined must not be used as the type of an argument in any circumstance",
+                    [self.location],
+                )
+
+    def addExtendedAttributes(self, attrs):
+        unhandledAttrs = []
+        for attr in attrs:
+            if attr.identifier() == "TreatNonCallableAsNull":
+                self._treatNonCallableAsNull = True
+            elif attr.identifier() == "LegacyTreatNonObjectAsNull":
+                if self._isConstructor:
+                    raise WebIDLError(
+                        "[LegacyTreatNonObjectAsNull] is not supported "
+                        "on constructors",
+                        [self.location],
+                    )
+                self._treatNonObjectAsNull = True
+            elif attr.identifier() == "MOZ_CAN_RUN_SCRIPT_BOUNDARY":
+                if self._isConstructor:
+                    raise WebIDLError(
+                        "[MOZ_CAN_RUN_SCRIPT_BOUNDARY] is not "
+                        "permitted on constructors",
+                        [self.location],
+                    )
+                self._isRunScriptBoundary = True
+            else:
+                unhandledAttrs.append(attr)
+        if self._treatNonCallableAsNull and self._treatNonObjectAsNull:
+            raise WebIDLError(
+                "Cannot specify both [TreatNonCallableAsNull] "
+                "and [LegacyTreatNonObjectAsNull]",
+                [self.location],
+            )
+        if len(unhandledAttrs) != 0:
+            IDLType.addExtendedAttributes(self, unhandledAttrs)
+
+    def _getDependentObjects(self):
+        return set([self._returnType] + self._arguments)
+
+    def isRunScriptBoundary(self):
+        return self._isRunScriptBoundary
+
+
+class IDLCallbackType(IDLType):
+    def __init__(self, location, callback):
+        IDLType.__init__(self, location, callback.identifier.name)
+        self.callback = callback
+
+    def isCallback(self):
+        return True
+
+    def tag(self):
+        return IDLType.Tags.callback
+
+    def isDistinguishableFrom(self, other):
+        if other.isPromise():
+            return False
+        if other.isUnion():
+            # Just forward to the union; it'll deal
+            return other.isDistinguishableFrom(self)
+        return (
+            other.isUndefined()
+            or other.isPrimitive()
+            or other.isString()
+            or other.isEnum()
+            or other.isNonCallbackInterface()
+            or other.isSequence()
+        )
+
+    def _getDependentObjects(self):
+        return self.callback._getDependentObjects()
+
+
+class IDLMethodOverload:
+    """
+    A class that represents a single overload of a WebIDL method.  This is not
+    quite the same as an element of the "effective overload set" in the spec,
+    because separate IDLMethodOverloads are not created based on arguments being
+    optional.  Rather, when multiple methods have the same name, there is an
+    IDLMethodOverload for each one, all hanging off an IDLMethod representing
+    the full set of overloads.
+    """
+
+    def __init__(self, returnType, arguments, location):
+        self.returnType = returnType
+        # Clone the list of arguments, just in case
+        self.arguments = list(arguments)
+        self.location = location
+
+    def _getDependentObjects(self):
+        deps = set(self.arguments)
+        deps.add(self.returnType)
+        return deps
+
+    def includesRestrictedFloatArgument(self):
+        return any(arg.type.includesRestrictedFloat() for arg in self.arguments)
+
+
+class IDLMethod(IDLInterfaceMember, IDLScope):
+    Special = enum(
+        "Getter", "Setter", "Deleter", "LegacyCaller", base=IDLInterfaceMember.Special
+    )
+
+    NamedOrIndexed = enum("Neither", "Named", "Indexed")
+
+    def __init__(
+        self,
+        location,
+        identifier,
+        returnType,
+        arguments,
+        static=False,
+        getter=False,
+        setter=False,
+        deleter=False,
+        specialType=NamedOrIndexed.Neither,
+        legacycaller=False,
+        stringifier=False,
+        maplikeOrSetlikeOrIterable=None,
+        underlyingAttr=None,
+    ):
+        # REVIEW: specialType is NamedOrIndexed -- wow, this is messed up.
+        IDLInterfaceMember.__init__(
+            self, location, identifier, IDLInterfaceMember.Tags.Method
+        )
+
+        self._hasOverloads = False
+
+        assert isinstance(returnType, IDLType)
+
+        # self._overloads is a list of IDLMethodOverloads
+        self._overloads = [IDLMethodOverload(returnType, arguments, location)]
+
+        assert isinstance(static, bool)
+        self._static = static
+        assert isinstance(getter, bool)
+        self._getter = getter
+        assert isinstance(setter, bool)
+        self._setter = setter
+        assert isinstance(deleter, bool)
+        self._deleter = deleter
+        assert isinstance(legacycaller, bool)
+        self._legacycaller = legacycaller
+        assert isinstance(stringifier, bool)
+        self._stringifier = stringifier
+        assert maplikeOrSetlikeOrIterable is None or isinstance(
+            maplikeOrSetlikeOrIterable, IDLMaplikeOrSetlikeOrIterableBase
+        )
+        self.maplikeOrSetlikeOrIterable = maplikeOrSetlikeOrIterable
+        self._htmlConstructor = False
+        self.underlyingAttr = underlyingAttr
+        self._specialType = specialType
+        self._legacyUnforgeable = False
+        self.dependsOn = "Everything"
+        self.affects = "Everything"
+        self.aliases = []
+
+        if static and identifier.name == "prototype":
+            raise WebIDLError(
+                "The identifier of a static operation must not be 'prototype'",
+                [location],
+            )
+
+        self.assertSignatureConstraints()
+
+    def __str__(self):
+        return "Method '%s'" % self.identifier
+
+    def assertSignatureConstraints(self):
+        if self._getter or self._deleter:
+            assert len(self._overloads) == 1
+            overload = self._overloads[0]
+            arguments = overload.arguments
+            assert len(arguments) == 1
+            assert (
+                arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.domstring]
+                or arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.unsigned_long]
+            )
+            assert not arguments[0].optional and not arguments[0].variadic
+            assert not self._getter or not overload.returnType.isUndefined()
+
+        if self._setter:
+            assert len(self._overloads) == 1
+            arguments = self._overloads[0].arguments
+            assert len(arguments) == 2
+            assert (
+                arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.domstring]
+                or arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.unsigned_long]
+            )
+            assert not arguments[0].optional and not arguments[0].variadic
+            assert not arguments[1].optional and not arguments[1].variadic
+
+        if self._stringifier:
+            assert len(self._overloads) == 1
+            overload = self._overloads[0]
+            assert len(overload.arguments) == 0
+            if not self.underlyingAttr:
+                assert (
+                    overload.returnType == BuiltinTypes[IDLBuiltinType.Types.domstring]
+                )
+
+    def isStatic(self):
+        return self._static
+
+    def forceStatic(self):
+        self._static = True
+
+    def isGetter(self):
+        return self._getter
+
+    def isSetter(self):
+        return self._setter
+
+    def isDeleter(self):
+        return self._deleter
+
+    def isNamed(self):
+        assert (
+            self._specialType == IDLMethod.NamedOrIndexed.Named
+            or self._specialType == IDLMethod.NamedOrIndexed.Indexed
+        )
+        return self._specialType == IDLMethod.NamedOrIndexed.Named
+
+    def isIndexed(self):
+        assert (
+            self._specialType == IDLMethod.NamedOrIndexed.Named
+            or self._specialType == IDLMethod.NamedOrIndexed.Indexed
+        )
+        return self._specialType == IDLMethod.NamedOrIndexed.Indexed
+
+    def isLegacycaller(self):
+        return self._legacycaller
+
+    def isStringifier(self):
+        return self._stringifier
+
+    def isToJSON(self):
+        return self.identifier.name == "toJSON"
+
+    def isDefaultToJSON(self):
+        return self.isToJSON() and self.getExtendedAttribute("Default")
+
+    def isMaplikeOrSetlikeOrIterableMethod(self):
+        """
+        True if this method was generated as part of a
+        maplike/setlike/etc interface (e.g. has/get methods)
+        """
+        return self.maplikeOrSetlikeOrIterable is not None
+
+    def isSpecial(self):
+        return (
+            self.isGetter()
+            or self.isSetter()
+            or self.isDeleter()
+            or self.isLegacycaller()
+            or self.isStringifier()
+        )
+
+    def isHTMLConstructor(self):
+        return self._htmlConstructor
+
+    def hasOverloads(self):
+        return self._hasOverloads
+
+    def isIdentifierLess(self):
+        """
+        True if the method name started with __, and if the method is not a
+        maplike/setlike method. Interfaces with maplike/setlike will generate
+        methods starting with __ for chrome only backing object access in JS
+        implemented interfaces, so while these functions use what is considered
+        an non-identifier name, they actually DO have an identifier.
+        """
+        return (
+            self.identifier.name[:2] == "__"
+            and not self.isMaplikeOrSetlikeOrIterableMethod()
+        )
+
+    def resolve(self, parentScope):
+        assert isinstance(parentScope, IDLScope)
+        IDLObjectWithIdentifier.resolve(self, parentScope)
+        IDLScope.__init__(self, self.location, parentScope, self.identifier)
+        for returnType, arguments in self.signatures():
+            for argument in arguments:
+                argument.resolve(self)
+
+    def addOverload(self, method):
+        assert len(method._overloads) == 1
+
+        if self._extendedAttrDict != method._extendedAttrDict:
+            extendedAttrDiff = set(self._extendedAttrDict.keys()) ^ set(
+                method._extendedAttrDict.keys()
+            )
+
+            if extendedAttrDiff == {"LenientFloat"}:
+                if "LenientFloat" not in self._extendedAttrDict:
+                    for overload in self._overloads:
+                        if overload.includesRestrictedFloatArgument():
+                            raise WebIDLError(
+                                "Restricted float behavior differs on different "
+                                "overloads of %s" % method.identifier,
+                                [overload.location, method.location],
+                            )
+                    self._extendedAttrDict["LenientFloat"] = method._extendedAttrDict[
+                        "LenientFloat"
+                    ]
+                elif method._overloads[0].includesRestrictedFloatArgument():
+                    raise WebIDLError(
+                        "Restricted float behavior differs on different "
+                        "overloads of %s" % method.identifier,
+                        [self.location, method.location],
+                    )
+            else:
+                raise WebIDLError(
+                    "Extended attributes differ on different "
+                    "overloads of %s" % method.identifier,
+                    [self.location, method.location],
+                )
+
+        self._overloads.extend(method._overloads)
+
+        self._hasOverloads = True
+
+        if self.isStatic() != method.isStatic():
+            raise WebIDLError(
+                "Overloaded identifier %s appears with different values of the 'static' attribute"
+                % method.identifier,
+                [method.location],
+            )
+
+        if self.isLegacycaller() != method.isLegacycaller():
+            raise WebIDLError(
+                (
+                    "Overloaded identifier %s appears with different "
+                    "values of the 'legacycaller' attribute" % method.identifier
+                ),
+                [method.location],
+            )
+
+        # Can't overload special things!
+        if (
+            self.isGetter()
+            or method.isGetter()
+            or self.isSetter()
+            or method.isSetter()
+            or self.isDeleter()
+            or method.isDeleter()
+            or self.isStringifier()
+            or method.isStringifier()
+        ):
+            raise WebIDLError(
+                ("Can't overload a special operation"),
+                [self.location, method.location],
+            )
+        if self.isHTMLConstructor() or method.isHTMLConstructor():
+            raise WebIDLError(
+                (
+                    "An interface must contain only a single operation annotated with HTMLConstructor, and no others"
+                ),
+                [self.location, method.location],
+            )
+
+        return self
+
+    def signatures(self):
+        return [
+            (overload.returnType, overload.arguments) for overload in self._overloads
+        ]
+
+    def finish(self, scope):
+        IDLInterfaceMember.finish(self, scope)
+
+        for overload in self._overloads:
+            returnType = overload.returnType
+            if not returnType.isComplete():
+                returnType = returnType.complete(scope)
+                assert not isinstance(returnType, IDLUnresolvedType)
+                assert not isinstance(returnType, IDLTypedefType)
+                assert not isinstance(returnType.name, IDLUnresolvedIdentifier)
+                overload.returnType = returnType
+
+            for argument in overload.arguments:
+                if not argument.isComplete():
+                    argument.complete(scope)
+                assert argument.type.isComplete()
+
+        # Now compute various information that will be used by the
+        # WebIDL overload resolution algorithm.
+        self.maxArgCount = max(len(s[1]) for s in self.signatures())
+        self.allowedArgCounts = [
+            i
+            for i in range(self.maxArgCount + 1)
+            if len(self.signaturesForArgCount(i)) != 0
+        ]
+
+    def validate(self):
+        IDLInterfaceMember.validate(self)
+
+        # Make sure our overloads are properly distinguishable and don't have
+        # different argument types before the distinguishing args.
+        for argCount in self.allowedArgCounts:
+            possibleOverloads = self.overloadsForArgCount(argCount)
+            if len(possibleOverloads) == 1:
+                continue
+            distinguishingIndex = self.distinguishingIndexForArgCount(argCount)
+            for idx in range(distinguishingIndex):
+                firstSigType = possibleOverloads[0].arguments[idx].type
+                for overload in possibleOverloads[1:]:
+                    if overload.arguments[idx].type != firstSigType:
+                        raise WebIDLError(
+                            "Signatures for method '%s' with %d arguments have "
+                            "different types of arguments at index %d, which "
+                            "is before distinguishing index %d"
+                            % (
+                                self.identifier.name,
+                                argCount,
+                                idx,
+                                distinguishingIndex,
+                            ),
+                            [self.location, overload.location],
+                        )
+
+        overloadWithPromiseReturnType = None
+        overloadWithoutPromiseReturnType = None
+        for overload in self._overloads:
+            returnType = overload.returnType
+            if not returnType.unroll().isExposedInAllOf(self.exposureSet):
+                raise WebIDLError(
+                    "Overload returns a type that is not exposed "
+                    "everywhere where the method is exposed",
+                    [overload.location],
+                )
+
+            variadicArgument = None
+
+            arguments = overload.arguments
+            for idx, argument in enumerate(arguments):
+                assert argument.type.isComplete()
+
+                if (
+                    argument.type.isDictionary()
+                    and argument.type.unroll().inner.canBeEmpty()
+                ) or (
+                    argument.type.isUnion()
+                    and argument.type.unroll().hasPossiblyEmptyDictionaryType()
+                ):
+                    # Optional dictionaries and unions containing optional
+                    # dictionaries at the end of the list or followed by
+                    # optional arguments must be optional.
+                    if not argument.optional and all(
+                        arg.optional for arg in arguments[idx + 1 :]
+                    ):
+                        raise WebIDLError(
+                            "Dictionary argument without any "
+                            "required fields or union argument "
+                            "containing such dictionary not "
+                            "followed by a required argument "
+                            "must be optional",
+                            [argument.location],
+                        )
+
+                    if not argument.defaultValue and all(
+                        arg.optional for arg in arguments[idx + 1 :]
+                    ):
+                        raise WebIDLError(
+                            "Dictionary argument without any "
+                            "required fields or union argument "
+                            "containing such dictionary not "
+                            "followed by a required argument "
+                            "must have a default value",
+                            [argument.location],
+                        )
+
+                # An argument cannot be a nullable dictionary or a
+                # nullable union containing a dictionary.
+                if argument.type.nullable() and (
+                    argument.type.isDictionary()
+                    or (
+                        argument.type.isUnion()
+                        and argument.type.unroll().hasDictionaryType()
+                    )
+                ):
+                    raise WebIDLError(
+                        "An argument cannot be a nullable "
+                        "dictionary or nullable union "
+                        "containing a dictionary",
+                        [argument.location],
+                    )
+
+                # Only the last argument can be variadic
+                if variadicArgument:
+                    raise WebIDLError(
+                        "Variadic argument is not last argument",
+                        [variadicArgument.location],
+                    )
+                if argument.variadic:
+                    variadicArgument = argument
+
+            if returnType.isPromise():
+                overloadWithPromiseReturnType = overload
+            else:
+                overloadWithoutPromiseReturnType = overload
+
+        # Make sure either all our overloads return Promises or none do
+        if overloadWithPromiseReturnType and overloadWithoutPromiseReturnType:
+            raise WebIDLError(
+                "We have overloads with both Promise and " "non-Promise return types",
+                [
+                    overloadWithPromiseReturnType.location,
+                    overloadWithoutPromiseReturnType.location,
+                ],
+            )
+
+        if overloadWithPromiseReturnType and self._legacycaller:
+            raise WebIDLError(
+                "May not have a Promise return type for a " "legacycaller.",
+                [overloadWithPromiseReturnType.location],
+            )
+
+        if self.getExtendedAttribute("StaticClassOverride") and not (
+            self.identifier.scope.isJSImplemented() and self.isStatic()
+        ):
+            raise WebIDLError(
+                "StaticClassOverride can be applied to static"
+                " methods on JS-implemented classes only.",
+                [self.location],
+            )
+
+        # Ensure that toJSON methods satisfy the spec constraints on them.
+        if self.identifier.name == "toJSON":
+            if len(self.signatures()) != 1:
+                raise WebIDLError(
+                    "toJSON method has multiple overloads",
+                    [self._overloads[0].location, self._overloads[1].location],
+                )
+            if len(self.signatures()[0][1]) != 0:
+                raise WebIDLError("toJSON method has arguments", [self.location])
+            if not self.signatures()[0][0].isJSONType():
+                raise WebIDLError(
+                    "toJSON method has non-JSON return type", [self.location]
+                )
+
+    def overloadsForArgCount(self, argc):
+        return [
+            overload
+            for overload in self._overloads
+            if len(overload.arguments) == argc
+            or (
+                len(overload.arguments) > argc
+                and all(arg.optional for arg in overload.arguments[argc:])
+            )
+            or (
+                len(overload.arguments) < argc
+                and len(overload.arguments) > 0
+                and overload.arguments[-1].variadic
+            )
+        ]
+
+    def signaturesForArgCount(self, argc):
+        return [
+            (overload.returnType, overload.arguments)
+            for overload in self.overloadsForArgCount(argc)
+        ]
+
+    def locationsForArgCount(self, argc):
+        return [overload.location for overload in self.overloadsForArgCount(argc)]
+
+    def distinguishingIndexForArgCount(self, argc):
+        def isValidDistinguishingIndex(idx, signatures):
+            for firstSigIndex, (firstRetval, firstArgs) in enumerate(signatures[:-1]):
+                for secondRetval, secondArgs in signatures[firstSigIndex + 1 :]:
+                    if idx < len(firstArgs):
+                        firstType = firstArgs[idx].type
+                    else:
+                        assert firstArgs[-1].variadic
+                        firstType = firstArgs[-1].type
+                    if idx < len(secondArgs):
+                        secondType = secondArgs[idx].type
+                    else:
+                        assert secondArgs[-1].variadic
+                        secondType = secondArgs[-1].type
+                    if not firstType.isDistinguishableFrom(secondType):
+                        return False
+            return True
+
+        signatures = self.signaturesForArgCount(argc)
+        for idx in range(argc):
+            if isValidDistinguishingIndex(idx, signatures):
+                return idx
+        # No valid distinguishing index.  Time to throw
+        locations = self.locationsForArgCount(argc)
+        raise WebIDLError(
+            "Signatures with %d arguments for method '%s' are not "
+            "distinguishable" % (argc, self.identifier.name),
+            locations,
+        )
+
+    def handleExtendedAttribute(self, attr):
+        identifier = attr.identifier()
+        if (
+            identifier == "GetterThrows"
+            or identifier == "SetterThrows"
+            or identifier == "GetterCanOOM"
+            or identifier == "SetterCanOOM"
+            or identifier == "SetterNeedsSubjectPrincipal"
+            or identifier == "GetterNeedsSubjectPrincipal"
+        ):
+            raise WebIDLError(
+                "Methods must not be flagged as " "[%s]" % identifier,
+                [attr.location, self.location],
+            )
+        elif identifier == "LegacyUnforgeable":
+            if self.isStatic():
+                raise WebIDLError(
+                    "[LegacyUnforgeable] is only allowed on non-static " "methods",
+                    [attr.location, self.location],
+                )
+            self._legacyUnforgeable = True
+        elif identifier == "SameObject":
+            raise WebIDLError(
+                "Methods must not be flagged as [SameObject]",
+                [attr.location, self.location],
+            )
+        elif identifier == "Constant":
+            raise WebIDLError(
+                "Methods must not be flagged as [Constant]",
+                [attr.location, self.location],
+            )
+        elif identifier == "PutForwards":
+            raise WebIDLError(
+                "Only attributes support [PutForwards]", [attr.location, self.location]
+            )
+        elif identifier == "LegacyLenientSetter":
+            raise WebIDLError(
+                "Only attributes support [LegacyLenientSetter]",
+                [attr.location, self.location],
+            )
+        elif identifier == "LenientFloat":
+            # This is called before we've done overload resolution
+            overloads = self._overloads
+            assert len(overloads) == 1
+            if not overloads[0].returnType.isUndefined():
+                raise WebIDLError(
+                    "[LenientFloat] used on a non-undefined method",
+                    [attr.location, self.location],
+                )
+            if not overloads[0].includesRestrictedFloatArgument():
+                raise WebIDLError(
+                    "[LenientFloat] used on an operation with no "
+                    "restricted float type arguments",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Exposed":
+            convertExposedAttrToGlobalNameSet(attr, self._exposureGlobalNames)
+        elif (
+            identifier == "CrossOriginCallable"
+            or identifier == "WebGLHandlesContextLoss"
+        ):
+            # Known no-argument attributes.
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[%s] must take no arguments" % identifier, [attr.location]
+                )
+            if identifier == "CrossOriginCallable" and self.isStatic():
+                raise WebIDLError(
+                    "[CrossOriginCallable] is only allowed on non-static " "attributes",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Pure":
+            if not attr.noArguments():
+                raise WebIDLError("[Pure] must take no arguments", [attr.location])
+            self._setDependsOn("DOMState")
+            self._setAffects("Nothing")
+        elif identifier == "Affects":
+            if not attr.hasValue():
+                raise WebIDLError("[Affects] takes an identifier", [attr.location])
+            self._setAffects(attr.value())
+        elif identifier == "DependsOn":
+            if not attr.hasValue():
+                raise WebIDLError("[DependsOn] takes an identifier", [attr.location])
+            self._setDependsOn(attr.value())
+        elif identifier == "Alias":
+            if not attr.hasValue():
+                raise WebIDLError(
+                    "[Alias] takes an identifier or string", [attr.location]
+                )
+            self._addAlias(attr.value())
+        elif identifier == "UseCounter":
+            if self.isSpecial():
+                raise WebIDLError(
+                    "[UseCounter] must not be used on a special " "operation",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Unscopable":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[Unscopable] must take no arguments", [attr.location]
+                )
+            if self.isStatic():
+                raise WebIDLError(
+                    "[Unscopable] is only allowed on non-static "
+                    "attributes and operations",
+                    [attr.location, self.location],
+                )
+        elif identifier == "CEReactions":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[CEReactions] must take no arguments", [attr.location]
+                )
+
+            if self.isSpecial() and not self.isSetter() and not self.isDeleter():
+                raise WebIDLError(
+                    "[CEReactions] is only allowed on operation, "
+                    "attribute, setter, and deleter",
+                    [attr.location, self.location],
+                )
+        elif identifier == "Default":
+            if not attr.noArguments():
+                raise WebIDLError("[Default] must take no arguments", [attr.location])
+
+            if not self.isToJSON():
+                raise WebIDLError(
+                    "[Default] is only allowed on toJSON operations",
+                    [attr.location, self.location],
+                )
+
+            if self.signatures()[0][0] != BuiltinTypes[IDLBuiltinType.Types.object]:
+                raise WebIDLError(
+                    "The return type of the default toJSON "
+                    "operation must be 'object'",
+                    [attr.location, self.location],
+                )
+        elif (
+            identifier == "Throws"
+            or identifier == "CanOOM"
+            or identifier == "NewObject"
+            or identifier == "ChromeOnly"
+            or identifier == "Pref"
+            or identifier == "Deprecated"
+            or identifier == "Func"
+            or identifier == "Trial"
+            or identifier == "SecureContext"
+            or identifier == "BinaryName"
+            or identifier == "NeedsSubjectPrincipal"
+            or identifier == "NeedsCallerType"
+            or identifier == "StaticClassOverride"
+            or identifier == "NonEnumerable"
+            or identifier == "Unexposed"
+            or identifier == "WebExtensionStub"
+        ):
+            # Known attributes that we don't need to do anything with here
+            pass
+        else:
+            raise WebIDLError(
+                "Unknown extended attribute %s on method" % identifier, [attr.location]
+            )
+        IDLInterfaceMember.handleExtendedAttribute(self, attr)
+
+    def returnsPromise(self):
+        return self._overloads[0].returnType.isPromise()
+
+    def isLegacyUnforgeable(self):
+        return self._legacyUnforgeable
+
+    def _getDependentObjects(self):
+        deps = set()
+        for overload in self._overloads:
+            deps.update(overload._getDependentObjects())
+        return deps
+
+
+class IDLConstructor(IDLMethod):
+    def __init__(self, location, args, name):
+        # We can't actually init our IDLMethod yet, because we do not know the
+        # return type yet.  Just save the info we have for now and we will init
+        # it later.
+        self._initLocation = location
+        self._initArgs = args
+        self._initName = name
+        self._inited = False
+        self._initExtendedAttrs = []
+
+    def addExtendedAttributes(self, attrs):
+        if self._inited:
+            return IDLMethod.addExtendedAttributes(self, attrs)
+        self._initExtendedAttrs.extend(attrs)
+
+    def handleExtendedAttribute(self, attr):
+        identifier = attr.identifier()
+        if (
+            identifier == "BinaryName"
+            or identifier == "ChromeOnly"
+            or identifier == "NewObject"
+            or identifier == "SecureContext"
+            or identifier == "Throws"
+            or identifier == "Func"
+            or identifier == "Trial"
+            or identifier == "Pref"
+            or identifier == "UseCounter"
+        ):
+            IDLMethod.handleExtendedAttribute(self, attr)
+        elif identifier == "HTMLConstructor":
+            if not attr.noArguments():
+                raise WebIDLError(
+                    "[HTMLConstructor] must take no arguments", [attr.location]
+                )
+            # We shouldn't end up here for legacy factory functions.
+            assert self.identifier.name == "constructor"
+
+            if any(len(sig[1]) != 0 for sig in self.signatures()):
+                raise WebIDLError(
+                    "[HTMLConstructor] must not be applied to a "
+                    "constructor operation that has arguments.",
+                    [attr.location],
+                )
+            self._htmlConstructor = True
+        else:
+            raise WebIDLError(
+                "Unknown extended attribute %s on method" % identifier, [attr.location]
+            )
+
+    def reallyInit(self, parentInterface):
+        name = self._initName
+        location = self._initLocation
+        identifier = IDLUnresolvedIdentifier(location, name, allowForbidden=True)
+        retType = IDLWrapperType(parentInterface.location, parentInterface)
+        IDLMethod.__init__(
+            self, location, identifier, retType, self._initArgs, static=True
+        )
+        self._inited = True
+        # Propagate through whatever extended attributes we already had
+        self.addExtendedAttributes(self._initExtendedAttrs)
+        self._initExtendedAttrs = []
+        # Constructors are always NewObject.  Whether they throw or not is
+        # indicated by [Throws] annotations in the usual way.
+        self.addExtendedAttributes(
+            [IDLExtendedAttribute(self.location, ("NewObject",))]
+        )
+
+
+class IDLIncludesStatement(IDLObject):
+    def __init__(self, location, interface, mixin):
+        IDLObject.__init__(self, location)
+        self.interface = interface
+        self.mixin = mixin
+        self._finished = False
+
+    def finish(self, scope):
+        if self._finished:
+            return
+        self._finished = True
+        assert isinstance(self.interface, IDLIdentifierPlaceholder)
+        assert isinstance(self.mixin, IDLIdentifierPlaceholder)
+        interface = self.interface.finish(scope)
+        mixin = self.mixin.finish(scope)
+        # NOTE: we depend on not setting self.interface and
+        # self.mixin here to keep track of the original
+        # locations.
+        if not isinstance(interface, IDLInterface):
+            raise WebIDLError(
+                "Left-hand side of 'includes' is not an " "interface",
+                [self.interface.location, interface.location],
+            )
+        if interface.isCallback():
+            raise WebIDLError(
+                "Left-hand side of 'includes' is a callback " "interface",
+                [self.interface.location, interface.location],
+            )
+        if not isinstance(mixin, IDLInterfaceMixin):
+            raise WebIDLError(
+                "Right-hand side of 'includes' is not an " "interface mixin",
+                [self.mixin.location, mixin.location],
+            )
+
+        mixin.actualExposureGlobalNames.update(interface._exposureGlobalNames)
+
+        interface.addIncludedMixin(mixin)
+        self.interface = interface
+        self.mixin = mixin
+
+    def validate(self):
+        pass
+
+    def addExtendedAttributes(self, attrs):
+        if len(attrs) != 0:
+            raise WebIDLError(
+                "There are no extended attributes that are "
+                "allowed on includes statements",
+                [attrs[0].location, self.location],
+            )
+
+
+class IDLExtendedAttribute(IDLObject):
+    """
+    A class to represent IDL extended attributes so we can give them locations
+    """
+
+    def __init__(self, location, tuple):
+        IDLObject.__init__(self, location)
+        self._tuple = tuple
+
+    def identifier(self):
+        return self._tuple[0]
+
+    def noArguments(self):
+        return len(self._tuple) == 1
+
+    def hasValue(self):
+        return len(self._tuple) >= 2 and isinstance(self._tuple[1], str)
+
+    def value(self):
+        assert self.hasValue()
+        return self._tuple[1]
+
+    def hasArgs(self):
+        return (
+            len(self._tuple) == 2
+            and isinstance(self._tuple[1], list)
+            or len(self._tuple) == 3
+        )
+
+    def args(self):
+        assert self.hasArgs()
+        # Our args are our last element
+        return self._tuple[-1]
+
+    def listValue(self):
+        """
+        Backdoor for storing random data in _extendedAttrDict
+        """
+        return list(self._tuple)[1:]
+
+
+# Parser
+
+
+class Tokenizer(object):
+    tokens = ["INTEGER", "FLOATLITERAL", "IDENTIFIER", "STRING", "WHITESPACE", "OTHER"]
+
+    def t_FLOATLITERAL(self, t):
+        r"(-?(([0-9]+\.[0-9]*|[0-9]*\.[0-9]+)([Ee][+-]?[0-9]+)?|[0-9]+[Ee][+-]?[0-9]+|Infinity))|NaN"
+        t.value = float(t.value)
+        return t
+
+    def t_INTEGER(self, t):
+        r"-?(0([0-7]+|[Xx][0-9A-Fa-f]+)?|[1-9][0-9]*)"
+        try:
+            # Can't use int(), because that doesn't handle octal properly.
+            t.value = parseInt(t.value)
+        except Exception:
+            raise WebIDLError(
+                "Invalid integer literal",
+                [
+                    Location(
+                        lexer=self.lexer,
+                        lineno=self.lexer.lineno,
+                        lexpos=self.lexer.lexpos,
+                        filename=self._filename,
+                    )
+                ],
+            )
+        return t
+
+    def t_IDENTIFIER(self, t):
+        r"[_-]?[A-Za-z][0-9A-Z_a-z-]*"
+        t.type = self.keywords.get(t.value, "IDENTIFIER")
+        return t
+
+    def t_STRING(self, t):
+        r'"[^"]*"'
+        t.value = t.value[1:-1]
+        return t
+
+    def t_WHITESPACE(self, t):
+        r"[\t\n\r ]+|[\t\n\r ]*((//[^\n]*|/\*.*?\*/)[\t\n\r ]*)+"
+        pass
+
+    def t_ELLIPSIS(self, t):
+        r"\.\.\."
+        t.type = self.keywords.get(t.value)
+        return t
+
+    def t_OTHER(self, t):
+        r"[^\t\n\r 0-9A-Z_a-z]"
+        t.type = self.keywords.get(t.value, "OTHER")
+        return t
+
+    keywords = {
+        "interface": "INTERFACE",
+        "partial": "PARTIAL",
+        "mixin": "MIXIN",
+        "dictionary": "DICTIONARY",
+        "exception": "EXCEPTION",
+        "enum": "ENUM",
+        "callback": "CALLBACK",
+        "typedef": "TYPEDEF",
+        "includes": "INCLUDES",
+        "const": "CONST",
+        "null": "NULL",
+        "true": "TRUE",
+        "false": "FALSE",
+        "serializer": "SERIALIZER",
+        "stringifier": "STRINGIFIER",
+        "unrestricted": "UNRESTRICTED",
+        "attribute": "ATTRIBUTE",
+        "readonly": "READONLY",
+        "inherit": "INHERIT",
+        "static": "STATIC",
+        "getter": "GETTER",
+        "setter": "SETTER",
+        "deleter": "DELETER",
+        "legacycaller": "LEGACYCALLER",
+        "optional": "OPTIONAL",
+        "...": "ELLIPSIS",
+        "::": "SCOPE",
+        "DOMString": "DOMSTRING",
+        "ByteString": "BYTESTRING",
+        "USVString": "USVSTRING",
+        "JSString": "JSSTRING",
+        "UTF8String": "UTF8STRING",
+        "any": "ANY",
+        "boolean": "BOOLEAN",
+        "byte": "BYTE",
+        "double": "DOUBLE",
+        "float": "FLOAT",
+        "long": "LONG",
+        "object": "OBJECT",
+        "ObservableArray": "OBSERVABLEARRAY",
+        "octet": "OCTET",
+        "Promise": "PROMISE",
+        "required": "REQUIRED",
+        "sequence": "SEQUENCE",
+        "record": "RECORD",
+        "short": "SHORT",
+        "unsigned": "UNSIGNED",
+        "undefined": "UNDEFINED",
+        ":": "COLON",
+        ";": "SEMICOLON",
+        "{": "LBRACE",
+        "}": "RBRACE",
+        "(": "LPAREN",
+        ")": "RPAREN",
+        "[": "LBRACKET",
+        "]": "RBRACKET",
+        "?": "QUESTIONMARK",
+        "*": "ASTERISK",
+        ",": "COMMA",
+        "=": "EQUALS",
+        "<": "LT",
+        ">": "GT",
+        "ArrayBuffer": "ARRAYBUFFER",
+        "or": "OR",
+        "maplike": "MAPLIKE",
+        "setlike": "SETLIKE",
+        "iterable": "ITERABLE",
+        "namespace": "NAMESPACE",
+        "constructor": "CONSTRUCTOR",
+        "symbol": "SYMBOL",
+        "async": "ASYNC",
+    }
+
+    tokens.extend(keywords.values())
+
+    def t_error(self, t):
+        raise WebIDLError(
+            "Unrecognized Input",
+            [
+                Location(
+                    lexer=self.lexer,
+                    lineno=self.lexer.lineno,
+                    lexpos=self.lexer.lexpos,
+                    filename=self._filename,
+                )
+            ],
+        )
+
+    def __init__(self, outputdir, lexer=None):
+        if lexer:
+            self.lexer = lexer
+        else:
+            self.lexer = lex.lex(object=self, reflags=re.DOTALL)
+
+
+class SqueakyCleanLogger(object):
+    errorWhitelist = [
+        # Web IDL defines the WHITESPACE token, but doesn't actually
+        # use it ... so far.
+        "Token 'WHITESPACE' defined, but not used",
+        # And that means we have an unused token
+        "There is 1 unused token",
+        # Web IDL defines a OtherOrComma rule that's only used in
+        # ExtendedAttributeInner, which we don't use yet.
+        "Rule 'OtherOrComma' defined, but not used",
+        # And an unused rule
+        "There is 1 unused rule",
+        # And the OtherOrComma grammar symbol is unreachable.
+        "Symbol 'OtherOrComma' is unreachable",
+        # Which means the Other symbol is unreachable.
+        "Symbol 'Other' is unreachable",
+    ]
+
+    def __init__(self):
+        self.errors = []
+
+    def debug(self, msg, *args, **kwargs):
+        pass
+
+    info = debug
+
+    def warning(self, msg, *args, **kwargs):
+        if (
+            msg == "%s:%d: Rule %r defined, but not used"
+            or msg == "%s:%d: Rule '%s' defined, but not used"
+        ):
+            # Munge things so we don't have to hardcode filenames and
+            # line numbers in our whitelist.
+            whitelistmsg = "Rule %r defined, but not used"
+            whitelistargs = args[2:]
+        else:
+            whitelistmsg = msg
+            whitelistargs = args
+        if (whitelistmsg % whitelistargs) not in SqueakyCleanLogger.errorWhitelist:
+            self.errors.append(msg % args)
+
+    error = warning
+
+    def reportGrammarErrors(self):
+        if self.errors:
+            raise WebIDLError("\n".join(self.errors), [])
+
+
+class Parser(Tokenizer):
+    def getLocation(self, p, i):
+        return Location(self.lexer, p.lineno(i), p.lexpos(i), self._filename)
+
+    def globalScope(self):
+        return self._globalScope
+
+    # The p_Foo functions here must match the WebIDL spec's grammar.
+    # It's acceptable to split things at '|' boundaries.
+    def p_Definitions(self, p):
+        """
+        Definitions : ExtendedAttributeList Definition Definitions
+        """
+        if p[2]:
+            p[0] = [p[2]]
+            p[2].addExtendedAttributes(p[1])
+        else:
+            assert not p[1]
+            p[0] = []
+
+        p[0].extend(p[3])
+
+    def p_DefinitionsEmpty(self, p):
+        """
+        Definitions :
+        """
+        p[0] = []
+
+    def p_Definition(self, p):
+        """
+        Definition : CallbackOrInterfaceOrMixin
+                   | Namespace
+                   | Partial
+                   | Dictionary
+                   | Exception
+                   | Enum
+                   | Typedef
+                   | IncludesStatement
+        """
+        p[0] = p[1]
+        assert p[1]  # We might not have implemented something ...
+
+    def p_CallbackOrInterfaceOrMixinCallback(self, p):
+        """
+        CallbackOrInterfaceOrMixin : CALLBACK CallbackRestOrInterface
+        """
+        if p[2].isInterface():
+            assert isinstance(p[2], IDLInterface)
+            p[2].setCallback(True)
+
+        p[0] = p[2]
+
+    def p_CallbackOrInterfaceOrMixinInterfaceOrMixin(self, p):
+        """
+        CallbackOrInterfaceOrMixin : INTERFACE InterfaceOrMixin
+        """
+        p[0] = p[2]
+
+    def p_CallbackRestOrInterface(self, p):
+        """
+        CallbackRestOrInterface : CallbackRest
+                                | CallbackConstructorRest
+                                | CallbackInterface
+        """
+        assert p[1]
+        p[0] = p[1]
+
+    def handleNonPartialObject(
+        self, location, identifier, constructor, constructorArgs, nonPartialArgs
+    ):
+        """
+        This handles non-partial objects (interfaces, namespaces and
+        dictionaries) by checking for an existing partial object, and promoting
+        it to non-partial as needed.  The return value is the non-partial
+        object.
+
+        constructorArgs are all the args for the constructor except the last
+        one: isKnownNonPartial.
+
+        nonPartialArgs are the args for the setNonPartial call.
+        """
+        # The name of the class starts with "IDL", so strip that off.
+        # Also, starts with a capital letter after that, so nix that
+        # as well.
+        prettyname = constructor.__name__[3:].lower()
+
+        try:
+            existingObj = self.globalScope()._lookupIdentifier(identifier)
+            if existingObj:
+                if not isinstance(existingObj, constructor):
+                    raise WebIDLError(
+                        "%s has the same name as "
+                        "non-%s object" % (prettyname.capitalize(), prettyname),
+                        [location, existingObj.location],
+                    )
+                existingObj.setNonPartial(*nonPartialArgs)
+                return existingObj
+        except Exception as ex:
+            if isinstance(ex, WebIDLError):
+                raise ex
+            pass
+
+        # True for isKnownNonPartial
+        return constructor(*(constructorArgs + [True]))
+
+    def p_InterfaceOrMixin(self, p):
+        """
+        InterfaceOrMixin : InterfaceRest
+                         | MixinRest
+        """
+        p[0] = p[1]
+
+    def p_CallbackInterface(self, p):
+        """
+        CallbackInterface : INTERFACE InterfaceRest
+        """
+        p[0] = p[2]
+
+    def p_InterfaceRest(self, p):
+        """
+        InterfaceRest : IDENTIFIER Inheritance LBRACE InterfaceMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(location, p[1])
+        members = p[4]
+        parent = p[2]
+
+        p[0] = self.handleNonPartialObject(
+            location,
+            identifier,
+            IDLInterface,
+            [location, self.globalScope(), identifier, parent, members],
+            [location, parent, members],
+        )
+
+    def p_InterfaceForwardDecl(self, p):
+        """
+        InterfaceRest : IDENTIFIER SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(location, p[1])
+
+        try:
+            if self.globalScope()._lookupIdentifier(identifier):
+                p[0] = self.globalScope()._lookupIdentifier(identifier)
+                if not isinstance(p[0], IDLExternalInterface):
+                    raise WebIDLError(
+                        "Name collision between external "
+                        "interface declaration for identifier "
+                        "%s and %s" % (identifier.name, p[0]),
+                        [location, p[0].location],
+                    )
+                return
+        except Exception as ex:
+            if isinstance(ex, WebIDLError):
+                raise ex
+            pass
+
+        p[0] = IDLExternalInterface(location, self.globalScope(), identifier)
+
+    def p_MixinRest(self, p):
+        """
+        MixinRest : MIXIN IDENTIFIER LBRACE MixinMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[4]
+
+        p[0] = self.handleNonPartialObject(
+            location,
+            identifier,
+            IDLInterfaceMixin,
+            [location, self.globalScope(), identifier, members],
+            [location, members],
+        )
+
+    def p_Namespace(self, p):
+        """
+        Namespace : NAMESPACE IDENTIFIER LBRACE InterfaceMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[4]
+
+        p[0] = self.handleNonPartialObject(
+            location,
+            identifier,
+            IDLNamespace,
+            [location, self.globalScope(), identifier, members],
+            [location, None, members],
+        )
+
+    def p_Partial(self, p):
+        """
+        Partial : PARTIAL PartialDefinition
+        """
+        p[0] = p[2]
+
+    def p_PartialDefinitionInterface(self, p):
+        """
+        PartialDefinition : INTERFACE PartialInterfaceOrPartialMixin
+        """
+        p[0] = p[2]
+
+    def p_PartialDefinition(self, p):
+        """
+        PartialDefinition : PartialNamespace
+                          | PartialDictionary
+        """
+        p[0] = p[1]
+
+    def handlePartialObject(
+        self,
+        location,
+        identifier,
+        nonPartialConstructor,
+        nonPartialConstructorArgs,
+        partialConstructorArgs,
+    ):
+        """
+        This handles partial objects (interfaces, namespaces and dictionaries)
+        by checking for an existing non-partial object, and adding ourselves to
+        it as needed.  The return value is our partial object.  We use
+        IDLPartialInterfaceOrNamespace for partial interfaces or namespaces,
+        and IDLPartialDictionary for partial dictionaries.
+
+        nonPartialConstructorArgs are all the args for the non-partial
+        constructor except the last two: members and isKnownNonPartial.
+
+        partialConstructorArgs are the arguments for the partial object
+        constructor, except the last one (the non-partial object).
+        """
+        # The name of the class starts with "IDL", so strip that off.
+        # Also, starts with a capital letter after that, so nix that
+        # as well.
+        prettyname = nonPartialConstructor.__name__[3:].lower()
+
+        nonPartialObject = None
+        try:
+            nonPartialObject = self.globalScope()._lookupIdentifier(identifier)
+            if nonPartialObject:
+                if not isinstance(nonPartialObject, nonPartialConstructor):
+                    raise WebIDLError(
+                        "Partial %s has the same name as "
+                        "non-%s object" % (prettyname, prettyname),
+                        [location, nonPartialObject.location],
+                    )
+        except Exception as ex:
+            if isinstance(ex, WebIDLError):
+                raise ex
+            pass
+
+        if not nonPartialObject:
+            nonPartialObject = nonPartialConstructor(
+                # No members, False for isKnownNonPartial
+                *(nonPartialConstructorArgs),
+                members=[],
+                isKnownNonPartial=False
+            )
+
+        partialObject = None
+        if isinstance(nonPartialObject, IDLDictionary):
+            partialObject = IDLPartialDictionary(
+                *(partialConstructorArgs + [nonPartialObject])
+            )
+        elif isinstance(
+            nonPartialObject, (IDLInterface, IDLInterfaceMixin, IDLNamespace)
+        ):
+            partialObject = IDLPartialInterfaceOrNamespace(
+                *(partialConstructorArgs + [nonPartialObject])
+            )
+        else:
+            raise WebIDLError(
+                "Unknown partial object type %s" % type(partialObject), [location]
+            )
+
+        return partialObject
+
+    def p_PartialInterfaceOrPartialMixin(self, p):
+        """
+        PartialInterfaceOrPartialMixin : PartialInterfaceRest
+                                       | PartialMixinRest
+        """
+        p[0] = p[1]
+
+    def p_PartialInterfaceRest(self, p):
+        """
+        PartialInterfaceRest : IDENTIFIER LBRACE PartialInterfaceMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(location, p[1])
+        members = p[3]
+
+        p[0] = self.handlePartialObject(
+            location,
+            identifier,
+            IDLInterface,
+            [location, self.globalScope(), identifier, None],
+            [location, identifier, members],
+        )
+
+    def p_PartialMixinRest(self, p):
+        """
+        PartialMixinRest : MIXIN IDENTIFIER LBRACE MixinMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[4]
+
+        p[0] = self.handlePartialObject(
+            location,
+            identifier,
+            IDLInterfaceMixin,
+            [location, self.globalScope(), identifier],
+            [location, identifier, members],
+        )
+
+    def p_PartialNamespace(self, p):
+        """
+        PartialNamespace : NAMESPACE IDENTIFIER LBRACE InterfaceMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[4]
+
+        p[0] = self.handlePartialObject(
+            location,
+            identifier,
+            IDLNamespace,
+            [location, self.globalScope(), identifier],
+            [location, identifier, members],
+        )
+
+    def p_PartialDictionary(self, p):
+        """
+        PartialDictionary : DICTIONARY IDENTIFIER LBRACE DictionaryMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[4]
+
+        p[0] = self.handlePartialObject(
+            location,
+            identifier,
+            IDLDictionary,
+            [location, self.globalScope(), identifier],
+            [location, identifier, members],
+        )
+
+    def p_Inheritance(self, p):
+        """
+        Inheritance : COLON ScopedName
+        """
+        p[0] = IDLIdentifierPlaceholder(self.getLocation(p, 2), p[2])
+
+    def p_InheritanceEmpty(self, p):
+        """
+        Inheritance :
+        """
+        pass
+
+    def p_InterfaceMembers(self, p):
+        """
+        InterfaceMembers : ExtendedAttributeList InterfaceMember InterfaceMembers
+        """
+        p[0] = [p[2]]
+
+        assert not p[1] or p[2]
+        p[2].addExtendedAttributes(p[1])
+
+        p[0].extend(p[3])
+
+    def p_InterfaceMembersEmpty(self, p):
+        """
+        InterfaceMembers :
+        """
+        p[0] = []
+
+    def p_InterfaceMember(self, p):
+        """
+        InterfaceMember : PartialInterfaceMember
+                        | Constructor
+        """
+        p[0] = p[1]
+
+    def p_Constructor(self, p):
+        """
+        Constructor : CONSTRUCTOR LPAREN ArgumentList RPAREN SEMICOLON
+        """
+        p[0] = IDLConstructor(self.getLocation(p, 1), p[3], "constructor")
+
+    def p_PartialInterfaceMembers(self, p):
+        """
+        PartialInterfaceMembers : ExtendedAttributeList PartialInterfaceMember PartialInterfaceMembers
+        """
+        p[0] = [p[2]]
+
+        assert not p[1] or p[2]
+        p[2].addExtendedAttributes(p[1])
+
+        p[0].extend(p[3])
+
+    def p_PartialInterfaceMembersEmpty(self, p):
+        """
+        PartialInterfaceMembers :
+        """
+        p[0] = []
+
+    def p_PartialInterfaceMember(self, p):
+        """
+        PartialInterfaceMember : Const
+                               | AttributeOrOperationOrMaplikeOrSetlikeOrIterable
+        """
+        p[0] = p[1]
+
+    def p_MixinMembersEmpty(self, p):
+        """
+        MixinMembers :
+        """
+        p[0] = []
+
+    def p_MixinMembers(self, p):
+        """
+        MixinMembers : ExtendedAttributeList MixinMember MixinMembers
+        """
+        p[0] = [p[2]]
+
+        assert not p[1] or p[2]
+        p[2].addExtendedAttributes(p[1])
+
+        p[0].extend(p[3])
+
+    def p_MixinMember(self, p):
+        """
+        MixinMember : Const
+                    | Attribute
+                    | Operation
+        """
+        p[0] = p[1]
+
+    def p_Dictionary(self, p):
+        """
+        Dictionary : DICTIONARY IDENTIFIER Inheritance LBRACE DictionaryMembers RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        members = p[5]
+        p[0] = IDLDictionary(location, self.globalScope(), identifier, p[3], members)
+
+    def p_DictionaryMembers(self, p):
+        """
+        DictionaryMembers : ExtendedAttributeList DictionaryMember DictionaryMembers
+                         |
+        """
+        if len(p) == 1:
+            # We're at the end of the list
+            p[0] = []
+            return
+        p[2].addExtendedAttributes(p[1])
+        p[0] = [p[2]]
+        p[0].extend(p[3])
+
+    def p_DictionaryMemberRequired(self, p):
+        """
+        DictionaryMember : REQUIRED TypeWithExtendedAttributes IDENTIFIER SEMICOLON
+        """
+        # These quack a lot like required arguments, so just treat them that way.
+        t = p[2]
+        assert isinstance(t, IDLType)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 3), p[3])
+
+        p[0] = IDLArgument(
+            self.getLocation(p, 3),
+            identifier,
+            t,
+            optional=False,
+            defaultValue=None,
+            variadic=False,
+            dictionaryMember=True,
+        )
+
+    def p_DictionaryMember(self, p):
+        """
+        DictionaryMember : Type IDENTIFIER Default SEMICOLON
+        """
+        # These quack a lot like optional arguments, so just treat them that way.
+        t = p[1]
+        assert isinstance(t, IDLType)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        defaultValue = p[3]
+
+        # Any attributes that precede this may apply to the type, so
+        # we configure the argument to forward type attributes down instead of producing
+        # a parse error
+        p[0] = IDLArgument(
+            self.getLocation(p, 2),
+            identifier,
+            t,
+            optional=True,
+            defaultValue=defaultValue,
+            variadic=False,
+            dictionaryMember=True,
+            allowTypeAttributes=True,
+        )
+
+    def p_Default(self, p):
+        """
+        Default : EQUALS DefaultValue
+                |
+        """
+        if len(p) > 1:
+            p[0] = p[2]
+        else:
+            p[0] = None
+
+    def p_DefaultValue(self, p):
+        """
+        DefaultValue : ConstValue
+                     | LBRACKET RBRACKET
+                     | LBRACE RBRACE
+        """
+        if len(p) == 2:
+            p[0] = p[1]
+        else:
+            assert len(p) == 3  # Must be [] or {}
+            if p[1] == "[":
+                p[0] = IDLEmptySequenceValue(self.getLocation(p, 1))
+            else:
+                assert p[1] == "{"
+                p[0] = IDLDefaultDictionaryValue(self.getLocation(p, 1))
+
+    def p_DefaultValueNull(self, p):
+        """
+        DefaultValue : NULL
+        """
+        p[0] = IDLNullValue(self.getLocation(p, 1))
+
+    def p_DefaultValueUndefined(self, p):
+        """
+        DefaultValue : UNDEFINED
+        """
+        p[0] = IDLUndefinedValue(self.getLocation(p, 1))
+
+    def p_Exception(self, p):
+        """
+        Exception : EXCEPTION IDENTIFIER Inheritance LBRACE ExceptionMembers RBRACE SEMICOLON
+        """
+        pass
+
+    def p_Enum(self, p):
+        """
+        Enum : ENUM IDENTIFIER LBRACE EnumValueList RBRACE SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+
+        values = p[4]
+        assert values
+        p[0] = IDLEnum(location, self.globalScope(), identifier, values)
+
+    def p_EnumValueList(self, p):
+        """
+        EnumValueList : STRING EnumValueListComma
+        """
+        p[0] = [p[1]]
+        p[0].extend(p[2])
+
+    def p_EnumValueListComma(self, p):
+        """
+        EnumValueListComma : COMMA EnumValueListString
+        """
+        p[0] = p[2]
+
+    def p_EnumValueListCommaEmpty(self, p):
+        """
+        EnumValueListComma :
+        """
+        p[0] = []
+
+    def p_EnumValueListString(self, p):
+        """
+        EnumValueListString : STRING EnumValueListComma
+        """
+        p[0] = [p[1]]
+        p[0].extend(p[2])
+
+    def p_EnumValueListStringEmpty(self, p):
+        """
+        EnumValueListString :
+        """
+        p[0] = []
+
+    def p_CallbackRest(self, p):
+        """
+        CallbackRest : IDENTIFIER EQUALS Type LPAREN ArgumentList RPAREN SEMICOLON
+        """
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 1), p[1])
+        p[0] = IDLCallback(
+            self.getLocation(p, 1),
+            self.globalScope(),
+            identifier,
+            p[3],
+            p[5],
+            isConstructor=False,
+        )
+
+    def p_CallbackConstructorRest(self, p):
+        """
+        CallbackConstructorRest : CONSTRUCTOR IDENTIFIER EQUALS Type LPAREN ArgumentList RPAREN SEMICOLON
+        """
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 2), p[2])
+        p[0] = IDLCallback(
+            self.getLocation(p, 2),
+            self.globalScope(),
+            identifier,
+            p[4],
+            p[6],
+            isConstructor=True,
+        )
+
+    def p_ExceptionMembers(self, p):
+        """
+        ExceptionMembers : ExtendedAttributeList ExceptionMember ExceptionMembers
+                         |
+        """
+        pass
+
+    def p_Typedef(self, p):
+        """
+        Typedef : TYPEDEF TypeWithExtendedAttributes IDENTIFIER SEMICOLON
+        """
+        typedef = IDLTypedef(self.getLocation(p, 1), self.globalScope(), p[2], p[3])
+        p[0] = typedef
+
+    def p_IncludesStatement(self, p):
+        """
+        IncludesStatement : ScopedName INCLUDES ScopedName SEMICOLON
+        """
+        assert p[2] == "includes"
+        interface = IDLIdentifierPlaceholder(self.getLocation(p, 1), p[1])
+        mixin = IDLIdentifierPlaceholder(self.getLocation(p, 3), p[3])
+        p[0] = IDLIncludesStatement(self.getLocation(p, 1), interface, mixin)
+
+    def p_Const(self, p):
+        """
+        Const : CONST ConstType IDENTIFIER EQUALS ConstValue SEMICOLON
+        """
+        location = self.getLocation(p, 1)
+        type = p[2]
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 3), p[3])
+        value = p[5]
+        p[0] = IDLConst(location, identifier, type, value)
+
+    def p_ConstValueBoolean(self, p):
+        """
+        ConstValue : BooleanLiteral
+        """
+        location = self.getLocation(p, 1)
+        booleanType = BuiltinTypes[IDLBuiltinType.Types.boolean]
+        p[0] = IDLValue(location, booleanType, p[1])
+
+    def p_ConstValueInteger(self, p):
+        """
+        ConstValue : INTEGER
+        """
+        location = self.getLocation(p, 1)
+
+        # We don't know ahead of time what type the integer literal is.
+        # Determine the smallest type it could possibly fit in and use that.
+        integerType = matchIntegerValueToType(p[1])
+        if integerType is None:
+            raise WebIDLError("Integer literal out of range", [location])
+
+        p[0] = IDLValue(location, integerType, p[1])
+
+    def p_ConstValueFloat(self, p):
+        """
+        ConstValue : FLOATLITERAL
+        """
+        location = self.getLocation(p, 1)
+        p[0] = IDLValue(
+            location, BuiltinTypes[IDLBuiltinType.Types.unrestricted_float], p[1]
+        )
+
+    def p_ConstValueString(self, p):
+        """
+        ConstValue : STRING
+        """
+        location = self.getLocation(p, 1)
+        stringType = BuiltinTypes[IDLBuiltinType.Types.domstring]
+        p[0] = IDLValue(location, stringType, p[1])
+
+    def p_BooleanLiteralTrue(self, p):
+        """
+        BooleanLiteral : TRUE
+        """
+        p[0] = True
+
+    def p_BooleanLiteralFalse(self, p):
+        """
+        BooleanLiteral : FALSE
+        """
+        p[0] = False
+
+    def p_AttributeOrOperationOrMaplikeOrSetlikeOrIterable(self, p):
+        """
+        AttributeOrOperationOrMaplikeOrSetlikeOrIterable : Attribute
+                                                         | Maplike
+                                                         | Setlike
+                                                         | Iterable
+                                                         | AsyncIterable
+                                                         | Operation
+        """
+        p[0] = p[1]
+
+    def p_Iterable(self, p):
+        """
+        Iterable : ITERABLE LT TypeWithExtendedAttributes GT SEMICOLON
+                 | ITERABLE LT TypeWithExtendedAttributes COMMA TypeWithExtendedAttributes GT SEMICOLON
+        """
+        location = self.getLocation(p, 2)
+        identifier = IDLUnresolvedIdentifier(
+            location, "__iterable", allowDoubleUnderscore=True
+        )
+        if len(p) > 6:
+            keyType = p[3]
+            valueType = p[5]
+        else:
+            keyType = None
+            valueType = p[3]
+
+        p[0] = IDLIterable(location, identifier, keyType, valueType, self.globalScope())
+
+    def p_AsyncIterable(self, p):
+        """
+        AsyncIterable : ASYNC ITERABLE LT TypeWithExtendedAttributes GT SEMICOLON
+                      | ASYNC ITERABLE LT TypeWithExtendedAttributes COMMA TypeWithExtendedAttributes GT SEMICOLON
+                      | ASYNC ITERABLE LT TypeWithExtendedAttributes GT LPAREN ArgumentList RPAREN SEMICOLON
+                      | ASYNC ITERABLE LT TypeWithExtendedAttributes COMMA TypeWithExtendedAttributes GT LPAREN ArgumentList RPAREN SEMICOLON
+        """
+        location = self.getLocation(p, 2)
+        identifier = IDLUnresolvedIdentifier(
+            location, "__iterable", allowDoubleUnderscore=True
+        )
+        if len(p) == 12:
+            keyType = p[4]
+            valueType = p[6]
+            argList = p[9]
+        elif len(p) == 10:
+            keyType = None
+            valueType = p[4]
+            argList = p[7]
+        elif len(p) == 9:
+            keyType = p[4]
+            valueType = p[6]
+            argList = []
+        else:
+            keyType = None
+            valueType = p[4]
+            argList = []
+
+        p[0] = IDLAsyncIterable(
+            location, identifier, keyType, valueType, argList, self.globalScope()
+        )
+
+    def p_Setlike(self, p):
+        """
+        Setlike : ReadOnly SETLIKE LT TypeWithExtendedAttributes GT SEMICOLON
+        """
+        readonly = p[1]
+        maplikeOrSetlikeType = p[2]
+        location = self.getLocation(p, 2)
+        identifier = IDLUnresolvedIdentifier(
+            location, "__setlike", allowDoubleUnderscore=True
+        )
+        keyType = p[4]
+        valueType = keyType
+        p[0] = IDLMaplikeOrSetlike(
+            location, identifier, maplikeOrSetlikeType, readonly, keyType, valueType
+        )
+
+    def p_Maplike(self, p):
+        """
+        Maplike : ReadOnly MAPLIKE LT TypeWithExtendedAttributes COMMA TypeWithExtendedAttributes GT SEMICOLON
+        """
+        readonly = p[1]
+        maplikeOrSetlikeType = p[2]
+        location = self.getLocation(p, 2)
+        identifier = IDLUnresolvedIdentifier(
+            location, "__maplike", allowDoubleUnderscore=True
+        )
+        keyType = p[4]
+        valueType = p[6]
+        p[0] = IDLMaplikeOrSetlike(
+            location, identifier, maplikeOrSetlikeType, readonly, keyType, valueType
+        )
+
+    def p_AttributeWithQualifier(self, p):
+        """
+        Attribute : Qualifier AttributeRest
+        """
+        static = IDLInterfaceMember.Special.Static in p[1]
+        stringifier = IDLInterfaceMember.Special.Stringifier in p[1]
+        (location, identifier, type, readonly) = p[2]
+        p[0] = IDLAttribute(
+            location, identifier, type, readonly, static=static, stringifier=stringifier
+        )
+
+    def p_AttributeInherited(self, p):
+        """
+        Attribute : INHERIT AttributeRest
+        """
+        (location, identifier, type, readonly) = p[2]
+        p[0] = IDLAttribute(location, identifier, type, readonly, inherit=True)
+
+    def p_Attribute(self, p):
+        """
+        Attribute : AttributeRest
+        """
+        (location, identifier, type, readonly) = p[1]
+        p[0] = IDLAttribute(location, identifier, type, readonly, inherit=False)
+
+    def p_AttributeRest(self, p):
+        """
+        AttributeRest : ReadOnly ATTRIBUTE TypeWithExtendedAttributes AttributeName SEMICOLON
+        """
+        location = self.getLocation(p, 2)
+        readonly = p[1]
+        t = p[3]
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 4), p[4])
+        p[0] = (location, identifier, t, readonly)
+
+    def p_ReadOnly(self, p):
+        """
+        ReadOnly : READONLY
+        """
+        p[0] = True
+
+    def p_ReadOnlyEmpty(self, p):
+        """
+        ReadOnly :
+        """
+        p[0] = False
+
+    def p_Operation(self, p):
+        """
+        Operation : Qualifiers OperationRest
+        """
+        qualifiers = p[1]
+
+        # Disallow duplicates in the qualifier set
+        if not len(set(qualifiers)) == len(qualifiers):
+            raise WebIDLError(
+                "Duplicate qualifiers are not allowed", [self.getLocation(p, 1)]
+            )
+
+        static = IDLInterfaceMember.Special.Static in p[1]
+        # If static is there that's all that's allowed.  This is disallowed
+        # by the parser, so we can assert here.
+        assert not static or len(qualifiers) == 1
+
+        stringifier = IDLInterfaceMember.Special.Stringifier in p[1]
+        # If stringifier is there that's all that's allowed.  This is disallowed
+        # by the parser, so we can assert here.
+        assert not stringifier or len(qualifiers) == 1
+
+        getter = True if IDLMethod.Special.Getter in p[1] else False
+        setter = True if IDLMethod.Special.Setter in p[1] else False
+        deleter = True if IDLMethod.Special.Deleter in p[1] else False
+        legacycaller = True if IDLMethod.Special.LegacyCaller in p[1] else False
+
+        if getter or deleter:
+            if setter:
+                raise WebIDLError(
+                    "getter and deleter are incompatible with setter",
+                    [self.getLocation(p, 1)],
+                )
+
+        (returnType, identifier, arguments) = p[2]
+
+        assert isinstance(returnType, IDLType)
+
+        specialType = IDLMethod.NamedOrIndexed.Neither
+
+        if getter or deleter:
+            if len(arguments) != 1:
+                raise WebIDLError(
+                    "%s has wrong number of arguments"
+                    % ("getter" if getter else "deleter"),
+                    [self.getLocation(p, 2)],
+                )
+            argType = arguments[0].type
+            if argType == BuiltinTypes[IDLBuiltinType.Types.domstring]:
+                specialType = IDLMethod.NamedOrIndexed.Named
+            elif argType == BuiltinTypes[IDLBuiltinType.Types.unsigned_long]:
+                specialType = IDLMethod.NamedOrIndexed.Indexed
+                if deleter:
+                    raise WebIDLError(
+                        "There is no such thing as an indexed deleter.",
+                        [self.getLocation(p, 1)],
+                    )
+            else:
+                raise WebIDLError(
+                    "%s has wrong argument type (must be DOMString or UnsignedLong)"
+                    % ("getter" if getter else "deleter"),
+                    [arguments[0].location],
+                )
+            if arguments[0].optional or arguments[0].variadic:
+                raise WebIDLError(
+                    "%s cannot have %s argument"
+                    % (
+                        "getter" if getter else "deleter",
+                        "optional" if arguments[0].optional else "variadic",
+                    ),
+                    [arguments[0].location],
+                )
+        if getter:
+            if returnType.isUndefined():
+                raise WebIDLError(
+                    "getter cannot have undefined return type", [self.getLocation(p, 2)]
+                )
+        if setter:
+            if len(arguments) != 2:
+                raise WebIDLError(
+                    "setter has wrong number of arguments", [self.getLocation(p, 2)]
+                )
+            argType = arguments[0].type
+            if argType == BuiltinTypes[IDLBuiltinType.Types.domstring]:
+                specialType = IDLMethod.NamedOrIndexed.Named
+            elif argType == BuiltinTypes[IDLBuiltinType.Types.unsigned_long]:
+                specialType = IDLMethod.NamedOrIndexed.Indexed
+            else:
+                raise WebIDLError(
+                    "settter has wrong argument type (must be DOMString or UnsignedLong)",
+                    [arguments[0].location],
+                )
+            if arguments[0].optional or arguments[0].variadic:
+                raise WebIDLError(
+                    "setter cannot have %s argument"
+                    % ("optional" if arguments[0].optional else "variadic"),
+                    [arguments[0].location],
+                )
+            if arguments[1].optional or arguments[1].variadic:
+                raise WebIDLError(
+                    "setter cannot have %s argument"
+                    % ("optional" if arguments[1].optional else "variadic"),
+                    [arguments[1].location],
+                )
+
+        if stringifier:
+            if len(arguments) != 0:
+                raise WebIDLError(
+                    "stringifier has wrong number of arguments",
+                    [self.getLocation(p, 2)],
+                )
+            if not returnType.isDOMString():
+                raise WebIDLError(
+                    "stringifier must have DOMString return type",
+                    [self.getLocation(p, 2)],
+                )
+
+        # identifier might be None.  This is only permitted for special methods.
+        if not identifier:
+            if (
+                not getter
+                and not setter
+                and not deleter
+                and not legacycaller
+                and not stringifier
+            ):
+                raise WebIDLError(
+                    "Identifier required for non-special methods",
+                    [self.getLocation(p, 2)],
+                )
+
+            location = BuiltinLocation("<auto-generated-identifier>")
+            identifier = IDLUnresolvedIdentifier(
+                location,
+                "__%s%s%s%s%s%s"
+                % (
+                    "named"
+                    if specialType == IDLMethod.NamedOrIndexed.Named
+                    else "indexed"
+                    if specialType == IDLMethod.NamedOrIndexed.Indexed
+                    else "",
+                    "getter" if getter else "",
+                    "setter" if setter else "",
+                    "deleter" if deleter else "",
+                    "legacycaller" if legacycaller else "",
+                    "stringifier" if stringifier else "",
+                ),
+                allowDoubleUnderscore=True,
+            )
+
+        method = IDLMethod(
+            self.getLocation(p, 2),
+            identifier,
+            returnType,
+            arguments,
+            static=static,
+            getter=getter,
+            setter=setter,
+            deleter=deleter,
+            specialType=specialType,
+            legacycaller=legacycaller,
+            stringifier=stringifier,
+        )
+        p[0] = method
+
+    def p_Stringifier(self, p):
+        """
+        Operation : STRINGIFIER SEMICOLON
+        """
+        identifier = IDLUnresolvedIdentifier(
+            BuiltinLocation("<auto-generated-identifier>"),
+            "__stringifier",
+            allowDoubleUnderscore=True,
+        )
+        method = IDLMethod(
+            self.getLocation(p, 1),
+            identifier,
+            returnType=BuiltinTypes[IDLBuiltinType.Types.domstring],
+            arguments=[],
+            stringifier=True,
+        )
+        p[0] = method
+
+    def p_QualifierStatic(self, p):
+        """
+        Qualifier : STATIC
+        """
+        p[0] = [IDLInterfaceMember.Special.Static]
+
+    def p_QualifierStringifier(self, p):
+        """
+        Qualifier : STRINGIFIER
+        """
+        p[0] = [IDLInterfaceMember.Special.Stringifier]
+
+    def p_Qualifiers(self, p):
+        """
+        Qualifiers : Qualifier
+                   | Specials
+        """
+        p[0] = p[1]
+
+    def p_Specials(self, p):
+        """
+        Specials : Special Specials
+        """
+        p[0] = [p[1]]
+        p[0].extend(p[2])
+
+    def p_SpecialsEmpty(self, p):
+        """
+        Specials :
+        """
+        p[0] = []
+
+    def p_SpecialGetter(self, p):
+        """
+        Special : GETTER
+        """
+        p[0] = IDLMethod.Special.Getter
+
+    def p_SpecialSetter(self, p):
+        """
+        Special : SETTER
+        """
+        p[0] = IDLMethod.Special.Setter
+
+    def p_SpecialDeleter(self, p):
+        """
+        Special : DELETER
+        """
+        p[0] = IDLMethod.Special.Deleter
+
+    def p_SpecialLegacyCaller(self, p):
+        """
+        Special : LEGACYCALLER
+        """
+        p[0] = IDLMethod.Special.LegacyCaller
+
+    def p_OperationRest(self, p):
+        """
+        OperationRest : Type OptionalIdentifier LPAREN ArgumentList RPAREN SEMICOLON
+        """
+        p[0] = (p[1], p[2], p[4])
+
+    def p_OptionalIdentifier(self, p):
+        """
+        OptionalIdentifier : IDENTIFIER
+        """
+        p[0] = IDLUnresolvedIdentifier(self.getLocation(p, 1), p[1])
+
+    def p_OptionalIdentifierEmpty(self, p):
+        """
+        OptionalIdentifier :
+        """
+        pass
+
+    def p_ArgumentList(self, p):
+        """
+        ArgumentList : Argument Arguments
+        """
+        p[0] = [p[1]] if p[1] else []
+        p[0].extend(p[2])
+
+    def p_ArgumentListEmpty(self, p):
+        """
+        ArgumentList :
+        """
+        p[0] = []
+
+    def p_Arguments(self, p):
+        """
+        Arguments : COMMA Argument Arguments
+        """
+        p[0] = [p[2]] if p[2] else []
+        p[0].extend(p[3])
+
+    def p_ArgumentsEmpty(self, p):
+        """
+        Arguments :
+        """
+        p[0] = []
+
+    def p_Argument(self, p):
+        """
+        Argument : ExtendedAttributeList ArgumentRest
+        """
+        p[0] = p[2]
+        p[0].addExtendedAttributes(p[1])
+
+    def p_ArgumentRestOptional(self, p):
+        """
+        ArgumentRest : OPTIONAL TypeWithExtendedAttributes ArgumentName Default
+        """
+        t = p[2]
+        assert isinstance(t, IDLType)
+        # Arg names can be reserved identifiers
+        identifier = IDLUnresolvedIdentifier(
+            self.getLocation(p, 3), p[3], allowForbidden=True
+        )
+
+        defaultValue = p[4]
+
+        # We can't test t.isAny() here and give it a default value as needed,
+        # since at this point t is not a fully resolved type yet (e.g. it might
+        # be a typedef).  We'll handle the 'any' case in IDLArgument.complete.
+
+        p[0] = IDLArgument(
+            self.getLocation(p, 3), identifier, t, True, defaultValue, False
+        )
+
+    def p_ArgumentRest(self, p):
+        """
+        ArgumentRest : Type Ellipsis ArgumentName
+        """
+        t = p[1]
+        assert isinstance(t, IDLType)
+        # Arg names can be reserved identifiers
+        identifier = IDLUnresolvedIdentifier(
+            self.getLocation(p, 3), p[3], allowForbidden=True
+        )
+
+        variadic = p[2]
+
+        # We can't test t.isAny() here and give it a default value as needed,
+        # since at this point t is not a fully resolved type yet (e.g. it might
+        # be a typedef).  We'll handle the 'any' case in IDLArgument.complete.
+
+        # variadic implies optional
+        # Any attributes that precede this may apply to the type, so
+        # we configure the argument to forward type attributes down instead of producing
+        # a parse error
+        p[0] = IDLArgument(
+            self.getLocation(p, 3),
+            identifier,
+            t,
+            variadic,
+            None,
+            variadic,
+            allowTypeAttributes=True,
+        )
+
+    def p_ArgumentName(self, p):
+        """
+        ArgumentName : IDENTIFIER
+                     | ArgumentNameKeyword
+        """
+        p[0] = p[1]
+
+    def p_ArgumentNameKeyword(self, p):
+        """
+        ArgumentNameKeyword : ASYNC
+                            | ATTRIBUTE
+                            | CALLBACK
+                            | CONST
+                            | CONSTRUCTOR
+                            | DELETER
+                            | DICTIONARY
+                            | ENUM
+                            | EXCEPTION
+                            | GETTER
+                            | INCLUDES
+                            | INHERIT
+                            | INTERFACE
+                            | ITERABLE
+                            | LEGACYCALLER
+                            | MAPLIKE
+                            | MIXIN
+                            | NAMESPACE
+                            | PARTIAL
+                            | READONLY
+                            | REQUIRED
+                            | SERIALIZER
+                            | SETLIKE
+                            | SETTER
+                            | STATIC
+                            | STRINGIFIER
+                            | TYPEDEF
+                            | UNRESTRICTED
+        """
+        p[0] = p[1]
+
+    def p_AttributeName(self, p):
+        """
+        AttributeName : IDENTIFIER
+                      | AttributeNameKeyword
+        """
+        p[0] = p[1]
+
+    def p_AttributeNameKeyword(self, p):
+        """
+        AttributeNameKeyword : ASYNC
+                             | REQUIRED
+        """
+        p[0] = p[1]
+
+    def p_Ellipsis(self, p):
+        """
+        Ellipsis : ELLIPSIS
+        """
+        p[0] = True
+
+    def p_EllipsisEmpty(self, p):
+        """
+        Ellipsis :
+        """
+        p[0] = False
+
+    def p_ExceptionMember(self, p):
+        """
+        ExceptionMember : Const
+                        | ExceptionField
+        """
+        pass
+
+    def p_ExceptionField(self, p):
+        """
+        ExceptionField : Type IDENTIFIER SEMICOLON
+        """
+        pass
+
+    def p_ExtendedAttributeList(self, p):
+        """
+        ExtendedAttributeList : LBRACKET ExtendedAttribute ExtendedAttributes RBRACKET
+        """
+        p[0] = [p[2]]
+        if p[3]:
+            p[0].extend(p[3])
+
+    def p_ExtendedAttributeListEmpty(self, p):
+        """
+        ExtendedAttributeList :
+        """
+        p[0] = []
+
+    def p_ExtendedAttribute(self, p):
+        """
+        ExtendedAttribute : ExtendedAttributeNoArgs
+                          | ExtendedAttributeArgList
+                          | ExtendedAttributeIdent
+                          | ExtendedAttributeWildcard
+                          | ExtendedAttributeNamedArgList
+                          | ExtendedAttributeIdentList
+        """
+        p[0] = IDLExtendedAttribute(self.getLocation(p, 1), p[1])
+
+    def p_ExtendedAttributeEmpty(self, p):
+        """
+        ExtendedAttribute :
+        """
+        pass
+
+    def p_ExtendedAttributes(self, p):
+        """
+        ExtendedAttributes : COMMA ExtendedAttribute ExtendedAttributes
+        """
+        p[0] = [p[2]] if p[2] else []
+        p[0].extend(p[3])
+
+    def p_ExtendedAttributesEmpty(self, p):
+        """
+        ExtendedAttributes :
+        """
+        p[0] = []
+
+    def p_Other(self, p):
+        """
+        Other : INTEGER
+              | FLOATLITERAL
+              | IDENTIFIER
+              | STRING
+              | OTHER
+              | ELLIPSIS
+              | COLON
+              | SCOPE
+              | SEMICOLON
+              | LT
+              | EQUALS
+              | GT
+              | QUESTIONMARK
+              | ASTERISK
+              | DOMSTRING
+              | BYTESTRING
+              | USVSTRING
+              | UTF8STRING
+              | JSSTRING
+              | PROMISE
+              | ANY
+              | BOOLEAN
+              | BYTE
+              | DOUBLE
+              | FALSE
+              | FLOAT
+              | LONG
+              | NULL
+              | OBJECT
+              | OCTET
+              | OR
+              | OPTIONAL
+              | RECORD
+              | SEQUENCE
+              | SHORT
+              | SYMBOL
+              | TRUE
+              | UNSIGNED
+              | UNDEFINED
+              | ArgumentNameKeyword
+        """
+        pass
+
+    def p_OtherOrComma(self, p):
+        """
+        OtherOrComma : Other
+                     | COMMA
+        """
+        pass
+
+    def p_TypeSingleType(self, p):
+        """
+        Type : SingleType
+        """
+        p[0] = p[1]
+
+    def p_TypeUnionType(self, p):
+        """
+        Type : UnionType Null
+        """
+        p[0] = self.handleNullable(p[1], p[2])
+
+    def p_TypeWithExtendedAttributes(self, p):
+        """
+        TypeWithExtendedAttributes : ExtendedAttributeList Type
+        """
+        p[0] = p[2].withExtendedAttributes(p[1])
+
+    def p_SingleTypeDistinguishableType(self, p):
+        """
+        SingleType : DistinguishableType
+        """
+        p[0] = p[1]
+
+    def p_SingleTypeAnyType(self, p):
+        """
+        SingleType : ANY
+        """
+        p[0] = BuiltinTypes[IDLBuiltinType.Types.any]
+
+    def p_SingleTypePromiseType(self, p):
+        """
+        SingleType : PROMISE LT Type GT
+        """
+        p[0] = IDLPromiseType(self.getLocation(p, 1), p[3])
+
+    def p_UnionType(self, p):
+        """
+        UnionType : LPAREN UnionMemberType OR UnionMemberType UnionMemberTypes RPAREN
+        """
+        types = [p[2], p[4]]
+        types.extend(p[5])
+        p[0] = IDLUnionType(self.getLocation(p, 1), types)
+
+    def p_UnionMemberTypeDistinguishableType(self, p):
+        """
+        UnionMemberType : ExtendedAttributeList DistinguishableType
+        """
+        p[0] = p[2].withExtendedAttributes(p[1])
+
+    def p_UnionMemberType(self, p):
+        """
+        UnionMemberType : UnionType Null
+        """
+        p[0] = self.handleNullable(p[1], p[2])
+
+    def p_UnionMemberTypes(self, p):
+        """
+        UnionMemberTypes : OR UnionMemberType UnionMemberTypes
+        """
+        p[0] = [p[2]]
+        p[0].extend(p[3])
+
+    def p_UnionMemberTypesEmpty(self, p):
+        """
+        UnionMemberTypes :
+        """
+        p[0] = []
+
+    def p_DistinguishableType(self, p):
+        """
+        DistinguishableType : PrimitiveType Null
+                            | ARRAYBUFFER Null
+                            | OBJECT Null
+                            | UNDEFINED Null
+        """
+        if p[1] == "object":
+            type = BuiltinTypes[IDLBuiltinType.Types.object]
+        elif p[1] == "ArrayBuffer":
+            type = BuiltinTypes[IDLBuiltinType.Types.ArrayBuffer]
+        elif p[1] == "undefined":
+            type = BuiltinTypes[IDLBuiltinType.Types.undefined]
+        else:
+            type = BuiltinTypes[p[1]]
+
+        p[0] = self.handleNullable(type, p[2])
+
+    def p_DistinguishableTypeStringType(self, p):
+        """
+        DistinguishableType : StringType Null
+        """
+        p[0] = self.handleNullable(p[1], p[2])
+
+    def p_DistinguishableTypeSequenceType(self, p):
+        """
+        DistinguishableType : SEQUENCE LT TypeWithExtendedAttributes GT Null
+        """
+        innerType = p[3]
+        type = IDLSequenceType(self.getLocation(p, 1), innerType)
+        p[0] = self.handleNullable(type, p[5])
+
+    def p_DistinguishableTypeRecordType(self, p):
+        """
+        DistinguishableType : RECORD LT StringType COMMA TypeWithExtendedAttributes GT Null
+        """
+        keyType = p[3]
+        valueType = p[5]
+        type = IDLRecordType(self.getLocation(p, 1), keyType, valueType)
+        p[0] = self.handleNullable(type, p[7])
+
+    def p_DistinguishableTypeObservableArrayType(self, p):
+        """
+        DistinguishableType : OBSERVABLEARRAY LT TypeWithExtendedAttributes GT Null
+        """
+        innerType = p[3]
+        type = IDLObservableArrayType(self.getLocation(p, 1), innerType)
+        p[0] = self.handleNullable(type, p[5])
+
+    def p_DistinguishableTypeScopedName(self, p):
+        """
+        DistinguishableType : ScopedName Null
+        """
+        assert isinstance(p[1], IDLUnresolvedIdentifier)
+
+        if p[1].name == "Promise":
+            raise WebIDLError(
+                "Promise used without saying what it's " "parametrized over",
+                [self.getLocation(p, 1)],
+            )
+
+        type = None
+
+        try:
+            if self.globalScope()._lookupIdentifier(p[1]):
+                obj = self.globalScope()._lookupIdentifier(p[1])
+                assert not obj.isType()
+                if obj.isTypedef():
+                    type = IDLTypedefType(
+                        self.getLocation(p, 1), obj.innerType, obj.identifier.name
+                    )
+                elif obj.isCallback() and not obj.isInterface():
+                    type = IDLCallbackType(self.getLocation(p, 1), obj)
+                else:
+                    type = IDLWrapperType(self.getLocation(p, 1), p[1])
+                p[0] = self.handleNullable(type, p[2])
+                return
+        except Exception:
+            pass
+
+        type = IDLUnresolvedType(self.getLocation(p, 1), p[1])
+        p[0] = self.handleNullable(type, p[2])
+
+    def p_ConstType(self, p):
+        """
+        ConstType : PrimitiveType
+        """
+        p[0] = BuiltinTypes[p[1]]
+
+    def p_ConstTypeIdentifier(self, p):
+        """
+        ConstType : IDENTIFIER
+        """
+        identifier = IDLUnresolvedIdentifier(self.getLocation(p, 1), p[1])
+
+        p[0] = IDLUnresolvedType(self.getLocation(p, 1), identifier)
+
+    def p_PrimitiveTypeUint(self, p):
+        """
+        PrimitiveType : UnsignedIntegerType
+        """
+        p[0] = p[1]
+
+    def p_PrimitiveTypeBoolean(self, p):
+        """
+        PrimitiveType : BOOLEAN
+        """
+        p[0] = IDLBuiltinType.Types.boolean
+
+    def p_PrimitiveTypeByte(self, p):
+        """
+        PrimitiveType : BYTE
+        """
+        p[0] = IDLBuiltinType.Types.byte
+
+    def p_PrimitiveTypeOctet(self, p):
+        """
+        PrimitiveType : OCTET
+        """
+        p[0] = IDLBuiltinType.Types.octet
+
+    def p_PrimitiveTypeFloat(self, p):
+        """
+        PrimitiveType : FLOAT
+        """
+        p[0] = IDLBuiltinType.Types.float
+
+    def p_PrimitiveTypeUnrestictedFloat(self, p):
+        """
+        PrimitiveType : UNRESTRICTED FLOAT
+        """
+        p[0] = IDLBuiltinType.Types.unrestricted_float
+
+    def p_PrimitiveTypeDouble(self, p):
+        """
+        PrimitiveType : DOUBLE
+        """
+        p[0] = IDLBuiltinType.Types.double
+
+    def p_PrimitiveTypeUnrestictedDouble(self, p):
+        """
+        PrimitiveType : UNRESTRICTED DOUBLE
+        """
+        p[0] = IDLBuiltinType.Types.unrestricted_double
+
+    def p_StringType(self, p):
+        """
+        StringType : BuiltinStringType
+        """
+        p[0] = BuiltinTypes[p[1]]
+
+    def p_BuiltinStringTypeDOMString(self, p):
+        """
+        BuiltinStringType : DOMSTRING
+        """
+        p[0] = IDLBuiltinType.Types.domstring
+
+    def p_BuiltinStringTypeBytestring(self, p):
+        """
+        BuiltinStringType : BYTESTRING
+        """
+        p[0] = IDLBuiltinType.Types.bytestring
+
+    def p_BuiltinStringTypeUSVString(self, p):
+        """
+        BuiltinStringType : USVSTRING
+        """
+        p[0] = IDLBuiltinType.Types.usvstring
+
+    def p_BuiltinStringTypeUTF8String(self, p):
+        """
+        BuiltinStringType : UTF8STRING
+        """
+        p[0] = IDLBuiltinType.Types.utf8string
+
+    def p_BuiltinStringTypeJSString(self, p):
+        """
+        BuiltinStringType : JSSTRING
+        """
+        p[0] = IDLBuiltinType.Types.jsstring
+
+    def p_UnsignedIntegerTypeUnsigned(self, p):
+        """
+        UnsignedIntegerType : UNSIGNED IntegerType
+        """
+        # Adding one to a given signed integer type gets you the unsigned type:
+        p[0] = p[2] + 1
+
+    def p_UnsignedIntegerType(self, p):
+        """
+        UnsignedIntegerType : IntegerType
+        """
+        p[0] = p[1]
+
+    def p_IntegerTypeShort(self, p):
+        """
+        IntegerType : SHORT
+        """
+        p[0] = IDLBuiltinType.Types.short
+
+    def p_IntegerTypeLong(self, p):
+        """
+        IntegerType : LONG OptionalLong
+        """
+        if p[2]:
+            p[0] = IDLBuiltinType.Types.long_long
+        else:
+            p[0] = IDLBuiltinType.Types.long
+
+    def p_OptionalLong(self, p):
+        """
+        OptionalLong : LONG
+        """
+        p[0] = True
+
+    def p_OptionalLongEmpty(self, p):
+        """
+        OptionalLong :
+        """
+        p[0] = False
+
+    def p_Null(self, p):
+        """
+        Null : QUESTIONMARK
+             |
+        """
+        if len(p) > 1:
+            p[0] = self.getLocation(p, 1)
+        else:
+            p[0] = None
+
+    def p_ScopedName(self, p):
+        """
+        ScopedName : AbsoluteScopedName
+                   | RelativeScopedName
+        """
+        p[0] = p[1]
+
+    def p_AbsoluteScopedName(self, p):
+        """
+        AbsoluteScopedName : SCOPE IDENTIFIER ScopedNameParts
+        """
+        assert False
+        pass
+
+    def p_RelativeScopedName(self, p):
+        """
+        RelativeScopedName : IDENTIFIER ScopedNameParts
+        """
+        assert not p[2]  # Not implemented!
+
+        p[0] = IDLUnresolvedIdentifier(self.getLocation(p, 1), p[1])
+
+    def p_ScopedNameParts(self, p):
+        """
+        ScopedNameParts : SCOPE IDENTIFIER ScopedNameParts
+        """
+        assert False
+        pass
+
+    def p_ScopedNamePartsEmpty(self, p):
+        """
+        ScopedNameParts :
+        """
+        p[0] = None
+
+    def p_ExtendedAttributeNoArgs(self, p):
+        """
+        ExtendedAttributeNoArgs : IDENTIFIER
+        """
+        p[0] = (p[1],)
+
+    def p_ExtendedAttributeArgList(self, p):
+        """
+        ExtendedAttributeArgList : IDENTIFIER LPAREN ArgumentList RPAREN
+        """
+        p[0] = (p[1], p[3])
+
+    def p_ExtendedAttributeIdent(self, p):
+        """
+        ExtendedAttributeIdent : IDENTIFIER EQUALS STRING
+                               | IDENTIFIER EQUALS IDENTIFIER
+        """
+        p[0] = (p[1], p[3])
+
+    def p_ExtendedAttributeWildcard(self, p):
+        """
+        ExtendedAttributeWildcard : IDENTIFIER EQUALS ASTERISK
+        """
+        p[0] = (p[1], p[3])
+
+    def p_ExtendedAttributeNamedArgList(self, p):
+        """
+        ExtendedAttributeNamedArgList : IDENTIFIER EQUALS IDENTIFIER LPAREN ArgumentList RPAREN
+        """
+        p[0] = (p[1], p[3], p[5])
+
+    def p_ExtendedAttributeIdentList(self, p):
+        """
+        ExtendedAttributeIdentList : IDENTIFIER EQUALS LPAREN IdentifierList RPAREN
+        """
+        p[0] = (p[1], p[4])
+
+    def p_IdentifierList(self, p):
+        """
+        IdentifierList : IDENTIFIER Identifiers
+        """
+        idents = list(p[2])
+        # This is only used for identifier-list-valued extended attributes, and if
+        # we're going to restrict to IDENTIFIER here we should at least allow
+        # escaping with leading '_' as usual for identifiers.
+        ident = p[1]
+        if ident[0] == "_":
+            ident = ident[1:]
+        idents.insert(0, ident)
+        p[0] = idents
+
+    def p_IdentifiersList(self, p):
+        """
+        Identifiers : COMMA IDENTIFIER Identifiers
+        """
+        idents = list(p[3])
+        # This is only used for identifier-list-valued extended attributes, and if
+        # we're going to restrict to IDENTIFIER here we should at least allow
+        # escaping with leading '_' as usual for identifiers.
+        ident = p[2]
+        if ident[0] == "_":
+            ident = ident[1:]
+        idents.insert(0, ident)
+        p[0] = idents
+
+    def p_IdentifiersEmpty(self, p):
+        """
+        Identifiers :
+        """
+        p[0] = []
+
+    def p_error(self, p):
+        if not p:
+            raise WebIDLError(
+                (
+                    "Syntax Error at end of file. Possibly due to "
+                    "missing semicolon(;), braces(}) or both"
+                ),
+                [self._filename],
+            )
+        else:
+            raise WebIDLError(
+                "invalid syntax",
+                [Location(self.lexer, p.lineno, p.lexpos, self._filename)],
+            )
+
+    def __init__(self, outputdir="", lexer=None):
+        Tokenizer.__init__(self, outputdir, lexer)
+
+        logger = SqueakyCleanLogger()
+        try:
+            self.parser = yacc.yacc(
+                module=self,
+                outputdir=outputdir,
+                errorlog=logger,
+                write_tables=False,
+                # Pickling the grammar is a speedup in
+                # some cases (older Python?) but a
+                # significant slowdown in others.
+                # We're not pickling for now, until it
+                # becomes a speedup again.
+                # , picklefile='WebIDLGrammar.pkl'
+            )
+        finally:
+            logger.reportGrammarErrors()
+
+        self._globalScope = IDLScope(BuiltinLocation("<Global Scope>"), None, None)
+        self._installBuiltins(self._globalScope)
+        self._productions = []
+
+        self._filename = "<builtin>"
+        self.lexer.input(Parser._builtins)
+        self._filename = None
+
+        self.parser.parse(lexer=self.lexer, tracking=True)
+
+    def _installBuiltins(self, scope):
+        assert isinstance(scope, IDLScope)
+
+        # range omits the last value.
+        for x in range(
+            IDLBuiltinType.Types.ArrayBuffer, IDLBuiltinType.Types.Float64Array + 1
+        ):
+            builtin = BuiltinTypes[x]
+            name = builtin.name
+            IDLTypedef(BuiltinLocation("<builtin type>"), scope, builtin, name)
+
+    @staticmethod
+    def handleNullable(type, questionMarkLocation):
+        if questionMarkLocation is not None:
+            type = IDLNullableType(questionMarkLocation, type)
+
+        return type
+
+    def parse(self, t, filename=None):
+        self.lexer.input(t)
+
+        # for tok in iter(self.lexer.token, None):
+        #    print tok
+
+        self._filename = filename
+        self._productions.extend(self.parser.parse(lexer=self.lexer, tracking=True))
+        self._filename = None
+
+    def finish(self):
+        # If we have interfaces that are iterable, create their
+        # iterator interfaces and add them to the productions array.
+        interfaceStatements = []
+        for p in self._productions:
+            if isinstance(p, IDLInterface):
+                interfaceStatements.append(p)
+
+        for iface in interfaceStatements:
+            iterable = None
+            # We haven't run finish() on the interface yet, so we don't know
+            # whether our interface is maplike/setlike/iterable or not. This
+            # means we have to loop through the members to see if we have an
+            # iterable member.
+            for m in iface.members:
+                if isinstance(m, (IDLIterable, IDLAsyncIterable)):
+                    iterable = m
+                    break
+            if iterable and (iterable.isPairIterator() or iterable.isAsyncIterable()):
+
+                def simpleExtendedAttr(str):
+                    return IDLExtendedAttribute(iface.location, (str,))
+
+                if isinstance(iterable, IDLAsyncIterable):
+                    nextReturnType = IDLPromiseType(
+                        iterable.location, BuiltinTypes[IDLBuiltinType.Types.any]
+                    )
+                else:
+                    nextReturnType = BuiltinTypes[IDLBuiltinType.Types.object]
+                nextMethod = IDLMethod(
+                    iterable.location,
+                    IDLUnresolvedIdentifier(iterable.location, "next"),
+                    nextReturnType,
+                    [],
+                )
+                nextMethod.addExtendedAttributes([simpleExtendedAttr("Throws")])
+
+                methods = [nextMethod]
+
+                if iterable.getExtendedAttribute("GenerateReturnMethod"):
+                    assert isinstance(iterable, IDLAsyncIterable)
+
+                    returnMethod = IDLMethod(
+                        iterable.location,
+                        IDLUnresolvedIdentifier(iterable.location, "return"),
+                        IDLPromiseType(
+                            iterable.location, BuiltinTypes[IDLBuiltinType.Types.any]
+                        ),
+                        [
+                            IDLArgument(
+                                iterable.location,
+                                IDLUnresolvedIdentifier(
+                                    BuiltinLocation("<auto-generated-identifier>"),
+                                    "value",
+                                ),
+                                BuiltinTypes[IDLBuiltinType.Types.any],
+                                optional=True,
+                            ),
+                        ],
+                    )
+                    returnMethod.addExtendedAttributes([simpleExtendedAttr("Throws")])
+                    methods.append(returnMethod)
+
+                if iterable.isIterable():
+                    itr_suffix = "Iterator"
+                else:
+                    itr_suffix = "AsyncIterator"
+                itr_ident = IDLUnresolvedIdentifier(
+                    iface.location, iface.identifier.name + itr_suffix
+                )
+                if iterable.isIterable():
+                    classNameOverride = iface.identifier.name + " Iterator"
+                elif iterable.isAsyncIterable():
+                    classNameOverride = iface.identifier.name + " AsyncIterator"
+                itr_iface = IDLInterface(
+                    iface.location,
+                    self.globalScope(),
+                    itr_ident,
+                    None,
+                    methods,
+                    isKnownNonPartial=True,
+                    classNameOverride=classNameOverride,
+                )
+                itr_iface.addExtendedAttributes(
+                    [simpleExtendedAttr("LegacyNoInterfaceObject")]
+                )
+                # Make sure the exposure set for the iterator interface is the
+                # same as the exposure set for the iterable interface, because
+                # we're going to generate methods on the iterable that return
+                # instances of the iterator.
+                itr_iface._exposureGlobalNames = set(iface._exposureGlobalNames)
+                # Always append generated iterable interfaces after the
+                # interface they're a member of, otherwise nativeType generation
+                # won't work correctly.
+                if iterable.isIterable():
+                    itr_iface.iterableInterface = iface
+                else:
+                    itr_iface.asyncIterableInterface = iface
+                self._productions.append(itr_iface)
+                iterable.iteratorType = IDLWrapperType(iface.location, itr_iface)
+
+        # Make sure we finish IDLIncludesStatements before we finish the
+        # IDLInterfaces.
+        # XXX khuey hates this bit and wants to nuke it from orbit.
+        includesStatements = [
+            p for p in self._productions if isinstance(p, IDLIncludesStatement)
+        ]
+        otherStatements = [
+            p for p in self._productions if not isinstance(p, IDLIncludesStatement)
+        ]
+        for production in includesStatements:
+            production.finish(self.globalScope())
+        for production in otherStatements:
+            production.finish(self.globalScope())
+
+        # Do any post-finish validation we need to do
+        for production in self._productions:
+            production.validate()
+
+        # De-duplicate self._productions, without modifying its order.
+        seen = set()
+        result = []
+        for p in self._productions:
+            if p not in seen:
+                seen.add(p)
+                result.append(p)
+        return result
+
+    def reset(self):
+        return Parser(lexer=self.lexer)
+
+    # Builtin IDL defined by WebIDL
+    _builtins = """
+        typedef (ArrayBufferView or ArrayBuffer) BufferSource;
+    """
+
+
+def main():
+    # Parse arguments.
+    from optparse import OptionParser
+
+    usageString = "usage: %prog [options] files"
+    o = OptionParser(usage=usageString)
+    o.add_option(
+        "--cachedir",
+        dest="cachedir",
+        default=None,
+        help="Directory in which to cache lex/parse tables.",
+    )
+    o.add_option(
+        "--verbose-errors",
+        action="store_true",
+        default=False,
+        help="When an error happens, display the Python traceback.",
+    )
+    (options, args) = o.parse_args()
+
+    if len(args) < 1:
+        o.error(usageString)
+
+    fileList = args
+    baseDir = os.getcwd()
+
+    # Parse the WebIDL.
+    parser = Parser(options.cachedir)
+    try:
+        for filename in fileList:
+            fullPath = os.path.normpath(os.path.join(baseDir, filename))
+            f = open(fullPath, "rb")
+            lines = f.readlines()
+            f.close()
+            print(fullPath)
+            parser.parse("".join(lines), fullPath)
+        parser.finish()
+    except WebIDLError as e:
+        if options.verbose_errors:
+            traceback.print_exc()
+        else:
+            print(e)
+
+
+if __name__ == "__main__":
+    main()