[Bug c++/100893] Template argument conversion fails for dependant constant function pointer template parameters

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100893

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:6f8c9691495ad5a307db98dc19c3296ee4e6de64

commit r12-1224-g6f8c9691495ad5a307db98dc19c3296ee4e6de64
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Jun 4 14:08:26 2021 -0400

    c++: top-level cv-quals on type of NTTP [PR100893]

    Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in
    param4.C below due to a spurious constness mismatch between the type of
    the template argument &f and the substituted type of the parm F (the
    latter has a top-level const).  Note that this mismatch doesn't occur
    with object pointers because in that case a call to
    perform_qualification_conversions from convert_nontype_argument
    implicitly adds a top-level const to the argument (via a cast) to match.

    This however seems to be a manifestation of a more general conformance
    issue: we're not dropping top-level cv-quals on the substituted
    type of an NTTP as per [temp.param]/6 (we only do so at parse time in
    process_template_parm).  So this patch makes convert_template_argument
    drop top-level cv-quals accordingly.

            PR c++/100893

    gcc/cp/ChangeLog:

            * pt.c (convert_template_argument): Strip top-level cv-quals
            on the substituted type of a non-type template parameter.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/param4.C: New test.
            * g++.dg/template/param5.C: New test.
            * g++.dg/cpp1z/nontype-auto19.C: New test.
            * g++.dg/cpp2a/concepts-decltype.C: Don't expect that the
            deduced type of a decltype(auto) NTTP has top-level cv-quals.