[Bug c++/126324] [13/14/15/16/17 Regression] Virtual function call with covariant return in constant evaluation causes ICE

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126324

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <[email protected]>:

https://gcc.gnu.org/g:eb54aa2d961ec016d15bd39ea62579af4ad25710

commit r17-3321-geb54aa2d961ec016d15bd39ea62579af4ad25710
Author: Odysseas Georgoudis <[email protected]>
Date:   Fri Jul 24 23:18:32 2026 +0100

    c++: Adjust constexpr covariant call results [PR126324]

    Constant evaluation of a virtual call can evaluate the final overrider
    directly.  For a zero-offset covariant override, the result then has the
    overrider's declared type rather than the static type of the call.  Storing
    that result can make cxx_eval_store_expression mistake the scalar type
    mismatch for an empty-base initialization and ICE.

    For a nonzero covariant adjustment, virtual lookup uses a result thunk.
    cxx_eval_thunk_call applied the fixed offset even to a null result and left
    the adjusted result with the overrider's return type, causing the same type
    mismatch.

    Adjust successful virtual-call results to the static call type after
    caching the underlying function result.  Make result-thunk evaluation
    preserve null pointers, evaluate the call only once, and give the result
    the thunk's static return type.

    gcc/cp/ChangeLog:

            PR c++/126324
            * constexpr.cc (cxx_eval_thunk_call): Preserve null pointer
            results and adjust the result to the thunk's return type.
            (cxx_eval_call_expression): Adjust covariant virtual-call results
            to the static call type.

    gcc/testsuite/ChangeLog:

            PR c++/126324
            * g++.dg/cpp2a/constexpr-virtual-pr126324.C: New test.

    Signed-off-by: Odysseas Georgoudis <[email protected]>