[Bug c++/124173] [16 Regression] template argument deduction/substitution failed with nullptr_t in C++11/14 since r16-7582

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124173

--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:37738f0422aa4d70ac5a71bd82b6c257a9309bde

commit r16-7630-g37738f0422aa4d70ac5a71bd82b6c257a9309bde
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sun Feb 22 22:09:13 2026 +0100

    c++: Fix up recent regression in convert_nontype_argument for C++1[14]
[PR124173]

    The following testcase is rejected since my recent convert_nontype_argument
    change, but only in C++11/14 modes.
    The problem is that C++17 says that all NTTPs should be converted constant
    expressions, but C++11 only said that for integral and enumeration ones and
    something that applied to the decltype(nullptr) case was
    - a constant expression that evaluates to a null pointer value
    The problem is that for NULLPTR_TYPE_P for C++11/14, we just never called
    maybe_constant_value, so obviously just making sure it is integer_zerop and
    without tf_error returning NULL_TREE otherwise changes behavior on valid
    programs, we really need to constant evaluate it first.

    The following patch fixes that.

    I'm afraid I actually don't know what exactly C++14 specifies, whether I've
    grabbed a pre-C++14 or post-C++14 draft, which looked like the C++17
    wording.   So, if C++14 needs something different, we'll need a further
    change, but this at least fixes the regression.

    2026-02-22  Jakub Jelinek  <jakub@redhat.com>

            PR c++/124173
            * pt.cc (convert_nontype_argument): For C++11/C++14 handle
            NULLPTR_TYPE_P non-type arguments like TYPE_PTR_P.

            * g++.dg/cpp0x/pr124173.C: New test.