Issue #2255 & #1240 - Simplify and enhance Maybe and Some(). https://bugzilla.mozilla.org/show_bug.cgi?id=1325351 This is a prerequisite for our BigInt V8 fast forward and potential #2255 fix.

js/public/Proxy.h

@@ -588,6 +588,15 @@ class JS_FRIEND_API(AutoEnterPolicy)
     inline void recordLeave() {}
 #endif
 
+  private:
+    // This operator needs to be deleted explicitly, otherwise Visual C++ will
+    // create it automatically when it is part of the export JS API. In that
+    // case, compile would fail because HandleId is not allowed to be assigned
+    // and consequently instantiation of assign operator of mozilla::Maybe
+    // would fail. See bug 1325351 comment 16. Copy constructor is removed at
+    // the same time for consistency.
+    AutoEnterPolicy(const AutoEnterPolicy&) = delete;
+    AutoEnterPolicy& operator=(const AutoEnterPolicy&) = delete;
 };
 
 #ifdef JS_DEBUG

mfbt/Maybe.h

@@ -102,16 +102,11 @@ public:
   }
 
   /**
-   * Maybe<T*> can be copy-constructed from a Maybe<U*> if U* and T* are
-   * compatible, or from Maybe<decltype(nullptr)>.
+   * Maybe<T> can be copy-constructed from a Maybe<U> if U is convertible to T.
    */
   template<typename U,
            typename =
-             typename std::enable_if<std::is_pointer<T>::value &&
-                                     (std::is_same<U, decltype(nullptr)>::value ||
-                                      (std::is_pointer<U>::value &&
-                                       std::is_base_of<typename std::remove_pointer<T>::type,
-                                                       typename std::remove_pointer<U>::type>::value))>::type>
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
   MOZ_IMPLICIT
   Maybe(const Maybe<U>& aOther)
     : mIsSome(false)
@@ -131,16 +126,11 @@ public:
   }
 
   /**
-   * Maybe<T*> can be move-constructed from a Maybe<U*> if U* and T* are
-   * compatible, or from Maybe<decltype(nullptr)>.
+   * Maybe<T> can be move-constructed from a Maybe<U> if U is convertible to T.
    */
   template<typename U,
            typename =
-             typename std::enable_if<std::is_pointer<T>::value &&
-                                     (std::is_same<U, decltype(nullptr)>::value ||
-                                      (std::is_pointer<U>::value &&
-                                       std::is_base_of<typename std::remove_pointer<T>::type,
-                                                       typename std::remove_pointer<U>::type>::value))>::type>
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
   MOZ_IMPLICIT
   Maybe(Maybe<U>&& aOther)
     : mIsSome(false)
@@ -156,13 +146,7 @@ public:
     if (&aOther != this) {
       if (aOther.mIsSome) {
         if (mIsSome) {
-          // XXX(seth): The correct code for this branch, below, can't be used
-          // due to a bug in Visual Studio 2010. See bug 1052940.
-          /*
           ref() = aOther.ref();
-          */
-          reset();
-          emplace(*aOther);
         } else {
           emplace(*aOther);
         }
@@ -173,6 +157,23 @@ public:
     return *this;
   }
 
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
+  Maybe& operator=(const Maybe<U>& aOther)
+  {
+    if (aOther.isSome()) {
+      if (mIsSome) {
+        ref() = aOther.ref();
+      } else {
+        emplace(*aOther);
+      }
+    } else {
+      reset();
+    }
+    return *this;
+  }
+
   Maybe& operator=(Maybe&& aOther)
   {
     MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
@@ -191,6 +192,25 @@ public:
     return *this;
   }
 
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
+  Maybe& operator=(Maybe<U>&& aOther)
+  {
+    if (aOther.isSome()) {
+      if (mIsSome) {
+        ref() = Move(aOther.ref());
+      } else {
+        emplace(Move(*aOther));
+      }
+      aOther.reset();
+    } else {
+      reset();
+    }
+
+    return *this;
+  }
+
   /* Methods that check whether this Maybe contains a value */
   explicit operator bool() const { return isSome(); }
   bool isSome() const { return mIsSome; }
@@ -443,11 +463,12 @@ public:
  * if you need to construct a Maybe value that holds a const, volatile, or
  * reference value, you need to use emplace() instead.
  */
-template<typename T>
-Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
+template<typename T,
+         typename U = typename std::remove_cv<
+           typename std::remove_reference<T>::type>::type>
+Maybe<U>
 Some(T&& aValue)
 {
-  typedef typename RemoveCV<typename RemoveReference<T>::Type>::Type U;
   Maybe<U> value;
   value.emplace(Forward<T>(aValue));
   return value;