new file mode 100644
--- /dev/null
+++ b/mfbt/MaybeOneOf.h
@@ -0,0 +1,131 @@
+/* -*- 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/. */
+
+#ifndef mozilla_MaybeOneOf_h
+#define mozilla_MaybeOneOf_h
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Move.h"
+#include "mozilla/TemplateLib.h"
+
+// For placement new
+#include <new>
+
+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.
+ */
+template<class T1, class T2>
+class MaybeOneOf
+{
+  AlignedStorage<tl::Max<sizeof(T1), sizeof(T2)>::value> 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 *(T*)storage.addr();
+  }
+
+  template <class T>
+  const T& as() const {
+    MOZ_ASSERT(state == Type2State<T>::result);
+    return *(T*)storage.addr();
+  }
+
+ public:
+  MaybeOneOf() : state(None) {}
+  ~MaybeOneOf() { destroyIfConstructed(); }
+
+  bool empty() const { return state == None; }
+
+  template <class T>
+  bool constructed() const { return state == Type2State<T>::result; }
+
+  template <class T>
+  void construct() {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (storage.addr()) T();
+  }
+
+  template <class T, class U>
+  void construct(U&& u) {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (storage.addr()) T(Move(u));
+  }
+
+  template <class T, class U1>
+  void construct(const U1& u1) {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (storage.addr()) T(u1);
+  }
+
+  template <class T, class U1, class U2>
+  void construct(const U1& u1, const U2& u2) {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (storage.addr()) T(u1, u2);
+  }
+
+  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();
+  }
+
+  private:
+    MaybeOneOf(const MaybeOneOf& other) MOZ_DELETE;
+    const MaybeOneOf& operator=(const MaybeOneOf& other) MOZ_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 */

--- a/mfbt/moz.build
+++ b/mfbt/moz.build
@@ -36,16 +36,17 @@ EXPORTS.mozilla = [
     'IntegerPrintfMacros.h',
     'IntegerTypeTraits.h',
     'Likely.h',
     'LinkedList.h',
     'MacroArgs.h',
     'MacroForEach.h',
     'MathAlgorithms.h',
     'Maybe.h',
+    'MaybeOneOf.h',
     'MemoryChecking.h',
     'MemoryReporting.h',
     'Move.h',
     'MSIntTypes.h',
     'NullPtr.h',
     'NumericLimits.h',
     'PodOperations.h',
     'Poison.h',