[Bug c++/123354] [14/15/16 Regression] ICE with nested structures and sizeof of constexpr variable since r14-8981

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123354

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:4652ca53fcbf01e236faa544688be25c63aefa06

commit r16-7034-g4652ca53fcbf01e236faa544688be25c63aefa06
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sat Jan 24 10:11:35 2026 +1100

    c++: Fix behaviour of nested maybe_push_to_top_level [PR123663]

    What happens in the linked PR is that when evaluating the concept we
    call 'push_to_top_level', we see cfun is non-null, and so call
    'push_function_context' which sets cfun to NULL and sets a flag so that
    we remember to pop it later.

    Then, when instantiating Foo's default constructor as part of the
    concept, we call 'maybe_push_to_top_level'.  Here we see that 'Foo' is
    function-local, so '!push_to_top', and we call 'push_function_context'.
    This allocates a new cfun for some reason, and pushes that empty cfun.

    Eventually we 'maybe_pop_from_top_level', and restore that newly
    allocated cfun (instead of a NULL cfun), which means that when we start
    trying to build the new-expression (which requires building a statement
    list) we try accessing the (uninitialized) cfun's x_stmt_tree rather
    than the scope_chain's x_stmt_tree, and so crash.

    This fixes the issue by also remembering whether we had a cfun when
    doing maybe_push_to_top_level so that we only do push_function_context
    if needed.

    This also seems to fix PR123354.

            PR c++/123663
            PR c++/123354

    gcc/cp/ChangeLog:

            * name-lookup.cc (struct local_state_t): New flag has_cfun.
            (local_state_t::save_and_clear): Set has_cfun, call
            push_function_context iff there's a cfun to save.
            (local_state_t::restore): call pop_function_context if
            has_cfun is set.
            (maybe_push_to_top_level): Delegte push_function_context to
            local_state_t::save_and_clear.
            (maybe_pop_from_top_level): Delegate pop_function_context to
            local_state_t::restore.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-pr123663.C: New test.
            * g++.dg/template/pr123354.C

    Reviewed-by: Jason Merrill <jason@redhat.com>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>