[glibc] sys/cdefs.h: Do not require C++ compilers to define __STDC__

[glibc] sys/cdefs.h: Do not require C++ compilers to define __STDC__

[Jonathan Wakely]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=21244c70c24db4b3bd7a2169a7a48f637cad5930

commit 21244c70c24db4b3bd7a2169a7a48f637cad5930
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri May 13 14:16:34 2022 +0100

    sys/cdefs.h: Do not require C++ compilers to define __STDC__
    
    The check for an ISO C compiler assumes that anything GCC-like will
    define __STDC__, even if it's actually a C++ compiler. That's currently
    true for G++ and compilers like clang++ that also define __GNUC__, but
    it might not always be true.
    
    The C++ standard leaves it implementation-defined whether or not
    __STDC__ is defined by C++ compilers. And really the check should be
    "ISO C or ISO C++ conforming compiler" anyway. So only give an error if
    __GNUC__ is defined and neither __STDC__ nor __cplusplus is defined.
    
    Reviewed-by: Fangrui Song <maskray@google.com>

Diff:
---
 misc/sys/cdefs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index 1c2b044a0d..f525f67547 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -27,8 +27,8 @@
 /* The GNU libc does not support any K&R compilers or the traditional mode
    of ISO C compilers anymore.  Check for some of the combinations not
    supported anymore.  */
-#if defined __GNUC__ && !defined __STDC__
-# error "You need a ISO C conforming compiler to use the glibc headers"
+#if defined __GNUC__ && !defined __STDC__ && !defined __cplusplus
+# error "You need a ISO C or C++ conforming compiler to use the glibc headers"
 #endif
 
 /* Some user header file might have defined this before.  */