test: add utilty header is_permissive.hpp

Author: X1aomu

tests/third_party/msvc/CMakeLists.txt

@@ -1,3 +1,5 @@
+include_directories(SYSTEM "include")
+
 add_subdirectory(msvc_stl_p0323r12_test)
 if (MSVC_TOOLSET_VERSION GREATER_EQUAL 143)
     # can only built with v143 and later

tests/third_party/msvc/include/is_permissive.hpp

@@ -0,0 +1,31 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#pragma once
+
+#include <yvals_core.h>
+
+namespace detail {
+    constexpr bool permissive() {
+        return false;
+    }
+
+    template <class>
+    struct PermissiveTestBase {
+        static constexpr bool permissive() {
+            return true;
+        }
+    };
+
+    template <class T>
+    struct PermissiveTest : PermissiveTestBase<T> {
+        static constexpr bool test() {
+            return permissive();
+        }
+    };
+} // namespace detail
+
+template <class T>
+constexpr bool is_permissive_v = detail::PermissiveTest<T>::test();
+
+_INLINE_VAR constexpr bool is_permissive = is_permissive_v<int>;