[Bug 255290] _POSIX_C_SOURCE=200809 hides static_assert

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255290

Eldred Habert <freebsdbugs at eldred.fr> changed:

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                 CC|                            |freebsdbugs at eldred.fr

--- Comment #10 from Eldred Habert <freebsdbugs at eldred.fr> ---
(In reply to Warner Losh from comment #7)

Hey, upstream here. Thank you very much Tobik both for reporting the issue to
us and here ^^

I interpret the following:

> Additional symbols not required or explicitly permitted by IEEE Std 1003.1-2001 to be in that header shall not be made visible, except when enabled by another feature test macro.

... as permitting the use of C11 functionality, since our codebase specifies
the `-std=gnu11` flag (which should be c11, but we had to switch to fix the
build with DJGPP), itself defining __STDC_VERSION__ to request `static_assert`.

That point has been made already, and I disagree with the following reply to
it:

> I would interpret that as requesting undefined behavior. POSIX is C99 and C99 only. You requested POSIX, and then requested something else also. That behavior is not defined. Pick one or the other, but you cannot have both as there's no well-defined way to provide both.

I don't read the snippet copied in Comment #6 as "POSIX requires C99
functionality, neither more nor less", but rather "POSIX does not aim to be
https://xkcd.com/927/, thus definitions were aligned on C99 whenever possible".
As far as I'm aware, C11 does not make any modifications to the standard that
are incompatible with POSIX 2008, and thus there are no conflicts, and hence,
no undefined behavior.

I back this up by POSIX allowing other feature test macros to define symbols
outside of POSIX's scope; if defining any other such macros would result in UB,
why would the standard explicitly allow it?

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