Constrain __bind functor constructor such that it won't accidentally get used as a copy constructor from a non-const lvalue. Fixes <rdar://problem/11359080>.

llvm-svn: 156182

Author: Howard Hinnant

libcxx/include/functional

@@ -1707,7 +1707,11 @@ public:
 
 #endif  // _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 
-    template <class _Gp, class ..._BA>
+    template <class _Gp, class ..._BA,
+              class = typename enable_if
+                               <
+                                  is_constructible<_Fd, _Gp>::value
+                               >::type>
       _LIBCPP_INLINE_VISIBILITY
       explicit __bind(_Gp&& __f, _BA&& ...__bound_args)
         : __f_(_VSTD::forward<_Gp>(__f)),