<sys/cdefs.h>: Decay expression passed to fallback

version of __generic()

This ensures that __generic() more closely matches _Generic() when
using the fallback version when _Generic() is not available (such as
GCC).

Co-authored by:	jrtc27
Reviewed by:	jrtc27
Differential Revision:	https://reviews.freebsd.org/D38215

newlib/libc/include/sys/cdefs.h

@@ -328,6 +328,9 @@
  * __generic().  Unlike _Generic(), this macro can only distinguish
  * between a single type, so it requires nested invocations to
  * distinguish multiple cases.
+ *
+ * Note that the comma operator is used to force expr to decay in
+ * order to match _Generic().
  */
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) || \
@@ -337,7 +340,7 @@
 #elif __GNUC_PREREQ__(3, 1) && !defined(__cplusplus)
 #define	__generic(expr, t, yes, no)					\
 	__builtin_choose_expr(						\
-	    __builtin_types_compatible_p(__typeof(expr), t), yes, no)
+	    __builtin_types_compatible_p(__typeof((0, (expr))), t), yes, no)
 #endif
 
 /*