Bug 766347 - Adjust Handle<T> constructors to only accept Rooted<S> or Handle<S> where S is convertible to T. r=luke

--HG--
extra : rebase_source : b0fd1627db78134185402fc1ab59112cc5ffb697

js/src/gc/Root.h

@@ -8,10 +8,15 @@
 #ifndef jsgc_root_h__
 #define jsgc_root_h__
 
+#ifdef __cplusplus
+
+#include "mozilla/TypeTraits.h"
+
 #include "jspubtd.h"
 
 #include "js/Utility.h"
 
+
 #ifdef __cplusplus
 
 namespace js {
@@ -79,9 +84,11 @@ template <typename T>
 class Handle
 {
   public:
-    /* Copy handles of different types, with implicit coercion. */
-    template <typename S> Handle(Handle<S> handle) {
-        testAssign<S>();
+    /* Creates a handle from a handle of a type convertible to T. */
+    template <typename S>
+    Handle(Handle<S> handle,
+           typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0)
+    {
         ptr = reinterpret_cast<const T *>(handle.address());
     }
 
@@ -102,7 +109,10 @@ class Handle
      * Construct a handle from an explicitly rooted location. This is the
      * normal way to create a handle, and normally happens implicitly.
      */
-    template <typename S> inline Handle(Rooted<S> &root);
+    template <typename S>
+    inline
+    Handle(Rooted<S> &root,
+           typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
 
     const T *address() const { return ptr; }
     T value() const { return *ptr; }
@@ -114,16 +124,6 @@ class Handle
     Handle() {}
 
     const T *ptr;
-
-    template <typename S>
-    void testAssign() {
-#ifdef DEBUG
-        T a = RootMethods<T>::initial();
-        S b = RootMethods<S>::initial();
-        a = b;
-        (void)a;
-#endif
-    }
 };
 
 typedef Handle<JSObject*>    HandleObject;
@@ -214,9 +214,9 @@ class Rooted
 
 template<typename T> template <typename S>
 inline
-Handle<T>::Handle(Rooted<S> &root)
+Handle<T>::Handle(Rooted<S> &root,
+                  typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
 {
-    testAssign<S>();
     ptr = reinterpret_cast<const T *>(root.address());
 }