[Bug preprocessor/120778] [C++26] P2843R3 - Preprocessing is never undefined

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120778

--- Comment #12 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:2c5bd8e9fb16584cdd1d090b9a5a63dd9140b61b

commit r16-3060-g2c5bd8e9fb16584cdd1d090b9a5a63dd9140b61b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Aug 7 08:51:00 2025 +0200

    c++: Add testcase for CWG2576 [PR120778]

    Another part of the C++26 P2843R3, this time in
https://eel.is/c++draft/cpp.include#7
    changing former (compile time) undefined behavior to IFNDR.
    With IFNDR we wouldn't have to test anything I guess, but this still adds
    the cases from the 3.4 section of the paper plus related tests.
    The paper wonders about implementation divergence between most compilers
and
    gcc in the case of glueing tokens together with <> around from multiple
    macros, GCC doesn't add CPP_PADDING macros in that case (and even if it
    would, it would ignore them through get_token_no_padding).
    But I think this behavior is allowed by
    https://eel.is/c++draft/cpp.include#7.sentence-3
    and changing it now after 25+ years of such behavior might break real-world
    stuff, especially making it really hard to construct the <> includes from
    some name and adding < and > from some other macro around that.  We handle
      #define A <cstd def>
      #include A
    like clang++ and try to open 'cstd def' file.  But not adding spaces for
    all the padding means handling even stuff where nothing else can help,
    while for cstd def coming from different macros one can use token pasting
    to combine them together, e.g. < and following identifier or identifier
    followed by . or . followed by identifier or identifier followed by > can't
    be pasted together.

    2025-08-07  Jakub Jelinek  <jakub@redhat.com>

            PR preprocessor/120778
            * g++.dg/DRs/dr2576.C: New test.