[Bug c/114780] Using C23 nullptr as sentinel gives missing sentinel warning

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114780

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:e802786436851b1f5efca21a14d4f41c83c83f4f

commit r13-8637-ge802786436851b1f5efca21a14d4f41c83c83f4f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Apr 20 00:12:36 2024 +0200

    c-family: Allow arguments with NULLPTR_TYPE as sentinels [PR114780]

    While in C++ the ellipsis argument conversions include
    "An argument that has type cv std::nullptr_t is converted to type void*"
    in C23 a nullptr_t argument is not promoted in any way, but va_arg
    description says:
    "the type of the next argument is nullptr_t and type is a pointer type that
has the same
    representation and alignment requirements as a pointer to a character
type."
    So, while in C++ check_function_sentinel will never see NULLPTR_TYPE, for
    C23 it can see that and currently we incorrectly warn about those.

    The only question is whether we should warn on any argument with
    nullptr_t type or just about nullptr (nullptr_t argument with integer_zerop
    value).  Through undefined behavior guess one could pass non-NULL pointer
    that way, say by union { void *p; nullptr_t q; } u; u.p = &whatever;
    and pass u.q to ..., but valid code should always pass something that will
    read as (char *) 0 when read using va_arg (ap, char *), so I think it is
    better not to warn rather than warn in those cases.

    Note, clang seems to pass (void *)0 rather than expression of nullptr_t
    type to ellipsis in C23 mode as if it did the C++ ellipsis argument
    conversions, in that case guess not warning about that would be even safer,
    but what GCC does I think follows the spec more closely, even when in a
    valid program one shouldn't be able to observe the difference.

    2024-04-20  Jakub Jelinek  <jakub@redhat.com>

            PR c/114780
            * c-common.cc (check_function_sentinel): Allow as sentinel any
            argument of NULLPTR_TYPE.

            * gcc.dg/format/sentinel-2.c: New test.

    (cherry picked from commit 2afdecccbaf5c5b1c7a235509b37092540906c02)