Intro TEST_MAYBE_UNUSED macro for tests

Author: frederick-vs-ja

libcxx/test/support/test_macros.h

@@ -508,6 +508,16 @@ inline Tp const& DoNotOptimize(Tp const& value) {
 #define TEST_NO_UNIQUE_ADDRESS
 #endif
 
+#if TEST_STD_VER >= 17 && __has_cpp_attribute(maybe_unused)
+#  define TEST_MAYBE_UNUSED [[maybe_unused]]
+#elif __has_cpp_attribute(__maybe_unused__)
+#  define TEST_MAYBE_UNUSED [[__maybe_unused__]]
+#elif __has_cpp_attribute(gnu::unused)
+#  define TEST_MAYBE_UNUSED [[gnu::unused]]
+#else
+#  define TEST_MAYBE_UNUSED
+#endif
+
 #ifdef _LIBCPP_SHORT_WCHAR
 #  define TEST_SHORT_WCHAR
 #endif