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diff --git a/js/src/frontend/EitherParser.h b/js/src/frontend/EitherParser.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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 variant-like class abstracting operations on a Parser with a given
+ * ParseHandler but unspecified character type.
+ */
+
+#ifndef frontend_EitherParser_h
+#define frontend_EitherParser_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Tuple.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Variant.h"
+
+#include <type_traits>
+#include <utility>
+
+#include "frontend/BCEParserHandle.h"
+#include "frontend/Parser.h"
+#include "frontend/TokenStream.h"
+
+namespace js {
+
+namespace detail {
+
+template <template <class Parser> class GetThis,
+          template <class This> class MemberFunction, typename... Args>
+struct InvokeMemberFunction {
+  mozilla::Tuple<std::decay_t<Args>...> args;
+
+  template <class This, size_t... Indices>
+  auto matchInternal(This* obj, std::index_sequence<Indices...>) -> decltype(
+      ((*obj).*(MemberFunction<This>::get()))(mozilla::Get<Indices>(args)...)) {
+    return ((*obj).*
+            (MemberFunction<This>::get()))(mozilla::Get<Indices>(args)...);
+  }
+
+ public:
+  template <typename... ActualArgs>
+  explicit InvokeMemberFunction(ActualArgs&&... actualArgs)
+      : args{std::forward<ActualArgs>(actualArgs)...} {}
+
+  template <class Parser>
+  auto operator()(Parser* parser)
+      -> decltype(this->matchInternal(GetThis<Parser>::get(parser),
+                                      std::index_sequence_for<Args...>{})) {
+    return this->matchInternal(GetThis<Parser>::get(parser),
+                               std::index_sequence_for<Args...>{});
+  }
+};
+
+// |this|-computing templates.
+
+template <class Parser>
+struct GetParser {
+  static Parser* get(Parser* parser) { return parser; }
+};
+
+template <class Parser>
+struct GetTokenStream {
+  static auto get(Parser* parser) { return &parser->tokenStream; }
+};
+
+// Member function-computing templates.
+
+template <class Parser>
+struct ParserOptions {
+  static constexpr auto get() { return &Parser::options; }
+};
+
+template <class TokenStream>
+struct TokenStreamComputeLineAndColumn {
+  static constexpr auto get() { return &TokenStream::computeLineAndColumn; }
+};
+
+// Generic matchers.
+
+struct ParseHandlerMatcher {
+  template <class Parser>
+  frontend::FullParseHandler& operator()(Parser* parser) {
+    return parser->handler_;
+  }
+};
+
+struct ParserSharedBaseMatcher {
+  template <class Parser>
+  frontend::ParserSharedBase& operator()(Parser* parser) {
+    return *static_cast<frontend::ParserSharedBase*>(parser);
+  }
+};
+
+struct ErrorReporterMatcher {
+  template <class Parser>
+  frontend::ErrorReporter& operator()(Parser* parser) {
+    return parser->tokenStream;
+  }
+};
+
+}  // namespace detail
+
+namespace frontend {
+
+class EitherParser : public BCEParserHandle {
+  // Leave this as a variant, to promote good form until 8-bit parser
+  // integration.
+  mozilla::Variant<Parser<FullParseHandler, char16_t>* const,
+                   Parser<FullParseHandler, mozilla::Utf8Unit>* const>
+      parser;
+
+  using Node = typename FullParseHandler::Node;
+
+  template <template <class Parser> class GetThis,
+            template <class This> class GetMemberFunction,
+            typename... StoredArgs>
+  using InvokeMemberFunction =
+      detail::InvokeMemberFunction<GetThis, GetMemberFunction, StoredArgs...>;
+
+ public:
+  template <class Parser>
+  explicit EitherParser(Parser* parser) : parser(parser) {}
+
+  FullParseHandler& astGenerator() final {
+    return parser.match(detail::ParseHandlerMatcher());
+  }
+
+  ErrorReporter& errorReporter() final {
+    return parser.match(detail::ErrorReporterMatcher());
+  }
+  const ErrorReporter& errorReporter() const final {
+    return parser.match(detail::ErrorReporterMatcher());
+  }
+
+  const JS::ReadOnlyCompileOptions& options() const final {
+    InvokeMemberFunction<detail::GetParser, detail::ParserOptions>
+        optionsMatcher;
+    return parser.match(std::move(optionsMatcher));
+  }
+
+  void computeLineAndColumn(uint32_t offset, uint32_t* line,
+                            uint32_t* column) const {
+    InvokeMemberFunction<detail::GetTokenStream,
+                         detail::TokenStreamComputeLineAndColumn, uint32_t,
+                         uint32_t*, uint32_t*>
+        matcher{offset, line, column};
+    return parser.match(std::move(matcher));
+  }
+
+  JSAtom* liftParserAtomToJSAtom(const ParserAtom* parserAtom) {
+    ParserSharedBase& base = parser.match(detail::ParserSharedBaseMatcher());
+    return base.liftParserAtomToJSAtom(parserAtom);
+  }
+
+  CompilationStencil& getCompilationStencil() {
+    ParserSharedBase& base = parser.match(detail::ParserSharedBaseMatcher());
+    return base.getCompilationStencil();
+  }
+};
+
+} /* namespace frontend */
+} /* namespace js */
+
+#endif /* frontend_EitherParser_h */