Implement an _is_allocator type trait for use in deduction guides.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322306 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -5597,6 +5597,16 @@ struct __temp_value {
     };
 #endif
 
+#if _LIBCPP_STD_VER > 14
+template<typename _Alloc, typename = void>
+struct __is_allocator : false_type {};
+
+template<typename _Alloc>
+struct __is_allocator<_Alloc,
+     void_t<typename _Alloc::value_type, decltype(_VSTD::declval<_Alloc&>().allocate(size_t{}))>>
+   : true_type {};
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS

test/libcxx/memory/is_allocator.pass.cpp

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+
+// <memory>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// template<typename _Alloc>
+// struct __is_allocator;
+
+// Is either true_type or false_type depending on if A is an allocator.
+
+#include <memory>
+#include <string>
+
+#include "test_macros.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+
+template <typename T>
+void test_allocators()
+{
+	static_assert( std::__is_allocator<std::allocator<T>>::value, "" );
+	static_assert( std::__is_allocator<test_allocator<T>>::value, "" );
+	static_assert( std::__is_allocator<min_allocator<T>>::value, "" );
+}
+
+
+int main()
+{
+//	test_allocators<void>();
+	test_allocators<char>();
+	test_allocators<int>();
+	test_allocators<std::string>();
+}