Bug 1277377: Don't permit `.append((float*) ptr)` to a `Vector<int*>` 😟 (patch from Luke)

cast.patch

diff --git a/mfbt/Vector.h b/mfbt/Vector.h
--- a/mfbt/Vector.h
+++ b/mfbt/Vector.h
@@ -158,17 +158,22 @@ struct VectorImpl
  */
 template<typename T, size_t N, class AP>
 struct VectorImpl<T, N, AP, true>
 {
   template<typename... Args>
   MOZ_NONNULL(1)
   static inline void new_(T* aDst, Args&&... aArgs)
   {
-    *aDst = T(Forward<Args>(aArgs)...);
+    // Explicitly construct a local object instead of using a temporary since
+    // T(args...) will be treated like a C-style cast in the unary case and
+    // allow unsafe conversions. Both forms should be equivalent to an
+    // optimizing compiler.
+    T temp(Forward<Args>(aArgs)...);
+    *aDst = temp;
   }
 
   static inline void destroy(T*, T*) {}
 
   static inline void initialize(T* aBegin, T* aEnd)
   {
     /*
      * You would think that memset would be a big win (or even break even)