[Bug c++/123676] [13/14/15/16 Regression] ICE in tsubst_pack_expansion with lambda template parameter pack in trailing return type decltype of a concept since r12-7564

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123676

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:22f51c0f5e62a4e05bccbd0dfb23762ef2eeeff8

commit r16-7056-g22f51c0f5e62a4e05bccbd0dfb23762ef2eeeff8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jan 26 22:00:16 2026 -0500

    c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]

    Here since the expression within the decltype C<Ts...> is not instantiation
    dependent (we know its type is bool, and don't care about its value)
    finish_decltype_type instantiates it immediately via the usual tsubst_expr
    with NULL_TREE args.  During which however tsubst_pack_expansion isn't
    prepared to handle such a substitution due to an overly strict assert.
    This patch relaxes the assert accordingly.

            PR c++/123676

    gcc/cp/ChangeLog:

            * pt.cc (tsubst_pack_expansion): Relax unsubsituted_packs
            assert to allow !processing_template_decl when args is
            NULL_TREE.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-decltype5.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>