[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #8 from CVS 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:2499540e9abb55079b5f7b7ccdac97fbc63d9ab4

commit r14-619-g2499540e9abb55079b5f7b7ccdac97fbc63d9ab4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 9 16:05:22 2023 +0200

    c++: Reject pack expansion of assume attribute [PR109756]

    http://eel.is/c++draft/dcl.attr#grammar-4 says
    "In an attribute-list, an ellipsis may appear only if that attribute's
    specification permits it."
    and doesn't explicitly permit it on any standard attribute.
    The https://wg21.link/p1774r8 paper which introduced assume attribute says
    "We could therefore hypothetically permit the assume attribute to directly
    support pack expansion:
    template <int... args>
    void f() {
    [[assume(args >= 0)...]];
    }
    However, we do not propose this. It would require substantial additional
work
    for a very rare use case. Note that this can instead be expressed with a
fold
    expression, which is equivalent to the above and works out of the box
without
    any extra effort:
    template <int... args>
    void f() {
    [[assume(((args >= 0) && ...))]];
    }
    ", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
    ... if it contains packs.
    The following patch rejects those instead of ICE and for C++17 or later
    suggests using fold expressions instead (it doesn't make sense to suggest
    it for C++14 and earlier when we'd error on the fold expressions).

    2023-05-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/109756
            * cp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
            expansion of assume attribute.

            * g++.dg/cpp23/attr-assume11.C: New test.