[Bug c++/126483] [13/14/15/16/17 Regression] Wrong active union member in constant evaluation of variable template initializers since r13-6422

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

--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Patrick Palka
<[email protected]>:

https://gcc.gnu.org/g:62b3305d69025d25a5b8cbc531b14a6adb3d1cbc

commit r16-9538-g62b3305d69025d25a5b8cbc531b14a6adb3d1cbc
Author: Patrick Palka <[email protected]>
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.

    This however means we no longer fold a's copy-init even for the runtime
    version of the lambda body, hence the is_constant_evaluated3b.C xfail.
    Making maybe_constexpr_fn distinguish between fully parsed constexpr and
    non-constexpr lambdas doesn't help because we don't hit this code path
    again.  And cp_fold_function doesn't fold the copy-init because it's
    represented as an INIT_EXPR of an AGGR_INIT_EXPR which it doesn't yet
    handle.  The subsequent patch will enable folding of INIT_EXPR
    initializers to address this xfail.

            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/opt/is_constant_evaluated3b.C: xfail a1 initializer
            folding.
            * g++.dg/cpp2a/is-constant-evaluated16.C: New test.

    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit b3bca9f73eb6b29bddf5fec92e929f32675b8d20)