[Bug c++/124978] Wrong type with decltype of auto template parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124978

--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:a5d39331b959364c25908669194377bac19316b8

commit r16-9144-ga5d39331b959364c25908669194377bac19316b8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Apr 27 17:56:18 2026 -0400

    c++: fix decltype(id) for pointer-to-data-member access expr [PR124978]

    Here after substitution into decltype(X), X is the expanded but not
    constant-evaluated pointer-to-data-member access expression

      *((const int *) *cw<Divide{42}>::value + (sizetype)
*cw<&Divide::value>::value)

    and finish_decltype_type wrongly strips the outermost INDIRECT_REF under
    the assumption that it's an implicit dereference of a reference, but here
    it's an explicit pointer dereference.  This causes the decltype to yield
    const int* instead of the expected int.

    This patch fixes this particular bug by checking REFERENCE_REF_P instead
    of INDIRECT_REF_P which additionally verifies the dereferenced thing
    actually has reference type.  The decltype now yields the correct type
    modulo an unnecessary const due to the separate bug PR115314.

            PR c++/124978
            PR c++/115314

    gcc/cp/ChangeLog:

            * semantics.cc (finish_decltype_type): Check REFERENCE_REF_P
            instead of INDIRECT_REF_P before stripping implicit dereferences.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/nontype-class74.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit c607c686100689e3e68487cd8097c2fbd3904168)