[Bug testsuite/64885] libstdc++ all_attributes failure

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64885

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's a bug in gthr-single.h

--- a/libgcc/gthr-single.h
+++ b/libgcc/gthr-single.h
@@ -38,7 +38,7 @@ typedef int __gthread_recursive_mutex_t;
 #define __GTHREAD_MUTEX_INIT_FUNCTION(mx)
 #define __GTHREAD_RECURSIVE_MUTEX_INIT 0

-#define UNUSED __attribute__((unused))
+#define UNUSED __attribute__((__unused__))

 #ifdef _LIBOBJC