[Bug 292960] Calling clang with -std=c11 (or any other standard <C23) exposes C23 _WIDTH macros

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292960

            Bug ID: 292960
           Summary: Calling clang with -std=c11 (or any other standard
                    <C23)  exposes C23 _WIDTH macros
           Product: Base System
           Version: 16.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: standards
          Assignee: [email protected]
          Reporter: [email protected]

Commit 7998a82d2f99a00086baf64b2a3343ba10f8806d added the C23 _WIDTH macros.
These macros are always exposed, regardless of the selected C standard, as long
as neither _POSIX_C_SOURCE nor one of the _SOURCE macros are defined.

Take this small example program:

----

#include <stdint.h>
#include <stdio.h>

int main() {
        printf("* __STDC_VERSION__: %li\n", __STDC_VERSION__);
        printf("* __ISO_C_VISIBLE:  %i\n", __ISO_C_VISIBLE);

#ifdef SIZE_WIDTH
        printf("* C23 macro SIZE_WIDTH is defined\n");
#else
        printf("* C23 macro SIZE_WIDTH is not defined\n");
#endif

        return 0;
}

----

C23 is correct:

% cc -o test -std=c23 test.c
% ./test
* __STDC_VERSION__: 202311
* __ISO_C_VISIBLE:  2023
* C23 macro SIZE_WIDTH is defined


C11 still sets __ISO_C_VISIBLE=2023 and thus exposes the SIZE_WIDTH macro:

% cc -o test -std=c11 test.c
* __STDC_VERSION__: 201112
* __ISO_C_VISIBLE:  2023
* C23 macro SIZE_WIDTH is defined


Defining _POSIX_C_SOURCE sets __ISO_C_VISIBLE=2011:

% cc -D_POSIX_C_SOURCE -o test -std=c11 test.c
% ./test
* __STDC_VERSION__: 201112
* __ISO_C_VISIBLE:  2011
* C23 macro SIZE_WIDTH is not defined


Defining _C11_SOURCE also sets __ISO_C_VISIBLE=2011:

% cc -D_C11_SOURCE -o test -std=c11 test.c
% ./test
* __STDC_VERSION__: 201112
* __ISO_C_VISIBLE:  2011
* C23 macro SIZE_WIDTH is not defined


This is caused by clang setting __STDC_VERSION__ to the requested C standard,
but _visible.h only evaluating __STDC_VERSION__ when _POSIX_C_SOURCE is also
set. When _POSIX_C_SOURCE is not set a _SOURCE macro must be defined, otherwise
the default case (which exposes everything) is selected.

I don't know if this this really bug. However it's not the the anticipated
behavior that requesting a C standard by -std= requires some other macros to be
set for the C23 (and maybe other?) macros don't get defined.

I have at least one real world example where a C11 code base breaks by the
exposed SIZE_WIDTH macro.

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