Implement is_error_code_v and is_error_condition_v for c++17. Rework the tests for is_error_code and is_error_condition, since they were really lacking. Thanks to Alisdair for the heads-up that we were missing these.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@282331 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/diagnostics/syserr/is_error_code_enum.pass.cpp

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: c++03
+
+// <future>
+
+// template <> struct is_error_code_enum<> : public false_type {};
+
+#include <future>
+#include <string>
+#include "test_macros.h"
+
+template <bool Expected, class T>
+void
+test()
+{
+    static_assert((std::is_error_code_enum<T>::value == Expected), "");
+#if TEST_STD_VER > 14
+    static_assert((std::is_error_code_enum_v<T>      == Expected), "");
+#endif
+}
+
+class A {
+	A();
+	operator std::error_code () const { return std::error_code(); }
+};
+
+// Specialize the template for my class
+namespace std
+{
+  template <>
+  struct is_error_code_enum<A> : public std::true_type {};
+}
+
+	
+int main()
+{
+    test<false, void>();
+    test<false, int>();
+    test<false, std::nullptr_t>();
+    test<false, std::string>();
+
+    test<true, A>();
+}