libstdc++: Use appropriate feature test macro for std::byte

libstdc++-v3/ChangeLog:

	* include/bits/cpp_type_traits.h (__is_byte<byte>): Guard with
	__glibcxx_byte macro instead of checking __cplusplus.

diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
index 19bf1edf647e05dc3c45e19dc564a20bdd694560..060652afb183ca671f0b613cf27b3b411cbc4b1c 100644 (file)
--- a/libstdc++-v3/include/bits/cpp_type_traits.h
+++ b/libstdc++-v3/include/bits/cpp_type_traits.h
@@ -414,7 +414,7 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
       typedef __true_type __type;
     };
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_byte // C++ >= 17
   enum class byte : unsigned char;
 
   template<>