[Bug c++/126483] [13/14/15 Regression] Wrong active union member in constant evaluation of variable template initializers since r13-6422

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126483

--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:05c7b3654af94b42978447e951c7c07703cee496

commit r13-10506-g05c7b3654af94b42978447e951c7c07703cee496
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Aug 14 15:53:12 2026 -0400

    c++: is_const_eval init folding inside lambda scope [PR126483]

    Here when processing a's copy-initialization we overeagerly fold its
    initializer as if the lambda scope is not constexpr, and thus assume
    is_constant_evaluated is false.  Subsequent constant evaluation of the
    lambda gives the wrong answer due to this premature folding.

    The culprit cp_fully_fold_init call is guarded by DECL_DECLARED_CONSTEXPR_P
    but that can't be relied on for maybe-constexpr lambdas until the lambda is
    fully parsed and its body is checked as a whole for constexpr suitability.
    This patch makes us check maybe_constexpr_fn instead, which considers such
    lambdas.

            PR c++/126483

    gcc/cp/ChangeLog:

            * typeck2.cc (store_init_value): Check maybe_constexpr_fn
            instead of DECL_DECLARED_CONSTEXPR_P of the function scope.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/is-constant-evaluated16.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit b3bca9f73eb6b29bddf5fec92e929f32675b8d20)