[Bug c++/60304] Including <atomic> disables -Wconversion-null

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #16 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #15)
> Yes, that's what the patch I'm testing does.
> 
> IMHO we should only define it for -std=gnu++98 and not any other -std mode,
> but I'll be conservative and leave it defined for -std=c++98 as well.

Thus, the warning also needs fixing. Since the same behavior will occur if the
user directly or indirectly includes stdbool.h.

A testcase:

# 1 "false.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "false.c"
# 1 "sys.h" 1

# 2 "sys.h" 3
# 2 "false.c" 2
int * foo() {return 
# 2 "false.c" 3
                   false
# 2 "false.c"
                        ;}
# 1 "nonsys.h" 1
# 4 "false.c" 2
int * bar() {return false;}
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Subject: [Bug c++/60304] Including <atomic> disables -Wconversion-null
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #17 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Manuel López-Ibáñez from comment #16)
> A testcase:

==> sys.h <==
#pragma GCC system_header
#if defined false
#undef false
#endif
#define false false


==> nonsys.h <==
#if defined false
#undef false
#endif
#define false false


==> false.c <==
#include "sys.h"
int * foo() {return false;}
#include "nonsys.h"
int * bar() {return false;}
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Subject: [Bug c++/60304] Including <atomic> disables -Wconversion-null
Date: Fri, 24 Oct 2014 13:45:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id`304

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I assume that testcase is meant to be C++ (because it isn't valid C) but it's
not valid C++ either:

17.6.4.3.1 [macro.names]
"A translation unit that includes a header shall not contain any macros that
define names declared or defined in that header. Nor shall such a translation
unit define macros for names lexically identical to keywords."