[libcxx] [test] In test/support/test_allocator.h, fix construct() to avoid moving immovable types.

Summary:
In test/support/test_allocator.h, fix construct() to avoid moving immovable types.

This improves the allocator's conformance, and fixes compiler errors with MSVC's STL. The scenario is when the allocator is asked to construct an object of type X that's immovable (deleted copy/move ctors), but implicitly constructible from an argument type A. When perfectly forwarded, X can be (explicitly) constructed from A, and everything is fine. That's std::allocator's behavior, and the Standard's default when a user allocator's construct() doesn't exist. The previous implementation of construct() here mishandled this scenario. Passing A to this construct() would implicitly construct an X temporary, bound to (non-templated) T&&. Then construct() would attempt to move-construct X from that X temporary, but X is immovable, boom.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21094

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272747 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/test_allocator.h

@@ -80,14 +80,14 @@ public:
         {assert(data_ >= 0); --alloc_count; ::operator delete((void*)p);}
     size_type max_size() const throw()
         {return UINT_MAX / sizeof(T);}
+#if TEST_STD_VER < 11
     void construct(pointer p, const T& val)
-        {::new(p) T(val);}
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    void construct(pointer p, T&& val)
-        {::new(p) T(std::move(val));}
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
+        {::new(static_cast<void*>(p)) T(val);}
+#else
+    template <class U> void construct(pointer p, U&& val)
+        {::new(static_cast<void*>(p)) T(std::forward<U>(val));}
+#endif
     void destroy(pointer p) {p->~T();}
-
     friend bool operator==(const test_allocator& x, const test_allocator& y)
         {return x.data_ == y.data_;}
     friend bool operator!=(const test_allocator& x, const test_allocator& y)
@@ -140,12 +140,13 @@ public:
         {assert(data_ >= 0); --alloc_count; ::operator delete((void*)p); }
     size_type max_size() const throw()
         {return UINT_MAX / sizeof(T);}
+#if TEST_STD_VER < 11
     void construct(pointer p, const T& val)
-        {::new(p) T(val);}
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    void construct(pointer p, T&& val)
-        {::new(p) T(std::move(val));}
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
+        {::new(static_cast<void*>(p)) T(val);}
+#else
+    template <class U> void construct(pointer p, U&& val)
+        {::new(static_cast<void*>(p)) T(std::forward<U>(val));}
+#endif
     void destroy(pointer p) {p->~T();}
 
     friend bool operator==(const non_default_test_allocator& x, const non_default_test_allocator& y)