Add support for N4389 - std::bool_constant

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@237636 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/meta/meta.hel/bool_constant.pass.cpp

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// type_traits
+
+// bool_constant
+
+#include <type_traits>
+#include <cassert>
+
+int main()
+{
+#if __cplusplus > 201402L
+    typedef std::bool_constant<true> _t;
+    static_assert(_t::value, "");
+    static_assert((std::is_same<_t::value_type, bool>::value), "");
+    static_assert((std::is_same<_t::type, _t>::value), "");
+    static_assert((_t() == true), "");
+
+    typedef std::bool_constant<false> _f;
+    static_assert(!_f::value, "");
+    static_assert((std::is_same<_f::value_type, bool>::value), "");
+    static_assert((std::is_same<_f::type, _f>::value), "");
+    static_assert((_f() == false), "");
+#endif
+}