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diff --git a/mfbt/MaybeOneOf.h b/mfbt/MaybeOneOf.h
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+++ b/mfbt/MaybeOneOf.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 class storing one of two optional value types that supports in-place lazy
+ * construction.
+ */
+
+#ifndef mozilla_MaybeOneOf_h
+#define mozilla_MaybeOneOf_h
+
+#include <stddef.h>  // for size_t
+
+#include <new>  // for placement new
+#include <utility>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/TemplateLib.h"
+
+namespace mozilla {
+
+/*
+ * MaybeOneOf<T1, T2> is like Maybe, but it supports constructing either T1
+ * or T2. When a MaybeOneOf<T1, T2> is constructed, it is |empty()|, i.e.,
+ * no value has been constructed and no destructor will be called when the
+ * MaybeOneOf<T1, T2> is destroyed. Upon calling |construct<T1>()| or
+ * |construct<T2>()|, a T1 or T2 object will be constructed with the given
+ * arguments and that object will be destroyed when the owning MaybeOneOf is
+ * destroyed.
+ *
+ * Because MaybeOneOf must be aligned suitable to hold any value stored within
+ * it, and because |alignas| requirements don't affect platform ABI with respect
+ * to how parameters are laid out in memory, MaybeOneOf can't be used as the
+ * type of a function parameter.  Pass MaybeOneOf to functions by pointer or
+ * reference instead.
+ */
+template <class T1, class T2>
+class MOZ_NON_PARAM MaybeOneOf {
+  static constexpr size_t StorageAlignment =
+      tl::Max<alignof(T1), alignof(T2)>::value;
+  static constexpr size_t StorageSize = tl::Max<sizeof(T1), sizeof(T2)>::value;
+
+  alignas(StorageAlignment) unsigned char storage[StorageSize];
+
+  // GCC fails due to -Werror=strict-aliasing if |storage| is directly cast to
+  // T*.  Indirecting through these functions addresses the problem.
+  void* data() { return storage; }
+  const void* data() const { return storage; }
+
+  enum State { None, SomeT1, SomeT2 } state;
+  template <class T, class Ignored = void>
+  struct Type2State {};
+
+  template <class T>
+  T& as() {
+    MOZ_ASSERT(state == Type2State<T>::result);
+    return *static_cast<T*>(data());
+  }
+
+  template <class T>
+  const T& as() const {
+    MOZ_ASSERT(state == Type2State<T>::result);
+    return *static_cast<const T*>(data());
+  }
+
+ public:
+  MaybeOneOf() : state(None) {}
+  ~MaybeOneOf() { destroyIfConstructed(); }
+
+  MaybeOneOf(MaybeOneOf&& rhs) : state(None) {
+    if (!rhs.empty()) {
+      if (rhs.constructed<T1>()) {
+        construct<T1>(std::move(rhs.as<T1>()));
+        rhs.as<T1>().~T1();
+      } else {
+        construct<T2>(std::move(rhs.as<T2>()));
+        rhs.as<T2>().~T2();
+      }
+      rhs.state = None;
+    }
+  }
+
+  MaybeOneOf& operator=(MaybeOneOf&& rhs) {
+    MOZ_ASSERT(this != &rhs, "Self-move is prohibited");
+    this->~MaybeOneOf();
+    new (this) MaybeOneOf(std::move(rhs));
+    return *this;
+  }
+
+  bool empty() const { return state == None; }
+
+  template <class T>
+  bool constructed() const {
+    return state == Type2State<T>::result;
+  }
+
+  template <class T, class... Args>
+  void construct(Args&&... aArgs) {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (KnownNotNull, data()) T(std::forward<Args>(aArgs)...);
+  }
+
+  template <class T>
+  T& ref() {
+    return as<T>();
+  }
+
+  template <class T>
+  const T& ref() const {
+    return as<T>();
+  }
+
+  void destroy() {
+    MOZ_ASSERT(state == SomeT1 || state == SomeT2);
+    if (state == SomeT1) {
+      as<T1>().~T1();
+    } else if (state == SomeT2) {
+      as<T2>().~T2();
+    }
+    state = None;
+  }
+
+  void destroyIfConstructed() {
+    if (!empty()) {
+      destroy();
+    }
+  }
+
+  template <typename Func>
+  constexpr auto mapNonEmpty(Func&& aFunc) const {
+    MOZ_ASSERT(!empty());
+    if (state == SomeT1) {
+      return std::forward<Func>(aFunc)(as<T1>());
+    }
+    return std::forward<Func>(aFunc)(as<T2>());
+  }
+  template <typename Func>
+  constexpr auto mapNonEmpty(Func&& aFunc) {
+    MOZ_ASSERT(!empty());
+    if (state == SomeT1) {
+      return std::forward<Func>(aFunc)(as<T1>());
+    }
+    return std::forward<Func>(aFunc)(as<T2>());
+  }
+
+ private:
+  MaybeOneOf(const MaybeOneOf& aOther) = delete;
+  const MaybeOneOf& operator=(const MaybeOneOf& aOther) = delete;
+};
+
+template <class T1, class T2>
+template <class Ignored>
+struct MaybeOneOf<T1, T2>::Type2State<T1, Ignored> {
+  typedef MaybeOneOf<T1, T2> Enclosing;
+  static const typename Enclosing::State result = Enclosing::SomeT1;
+};
+
+template <class T1, class T2>
+template <class Ignored>
+struct MaybeOneOf<T1, T2>::Type2State<T2, Ignored> {
+  typedef MaybeOneOf<T1, T2> Enclosing;
+  static const typename Enclosing::State result = Enclosing::SomeT2;
+};
+
+}  // namespace mozilla
+
+#endif /* mozilla_MaybeOneOf_h */