[Bug c++/105425] [12/13 Regression] ambiguous template instantiation with hana::when

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105425

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:38bdf2dccf6a239598ef808ed11a904e5f2a186e

commit r12-8317-g38bdf2dccf6a239598ef808ed11a904e5f2a186e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Apr 28 13:10:56 2022 -0400

    c++: partial ordering and dependent operator expr [PR105425]

    Here ever since r12-6022-gbb2a7f80a98de3 we stopped deeming the partial
    specialization #2 to be more specialized than #1 ultimately because
    dependent operator expressions now have a DEPENDENT_OPERATOR_TYPE type
    instead of an empty type, and this made unify stop deducing T(2) == 1
    for K during partial ordering for #1 and #2.

    This minimal patch fixes this by making the relevant logic in unify
    treat DEPENDENT_OPERATOR_TYPE like an empty type.

            PR c++/105425

    gcc/cp/ChangeLog:

            * pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Treat
            DEPENDENT_OPERATOR_TYPE like an empty type.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/partial-specialization13.C: New test.

    (cherry picked from commit 509fd16da8528444dccc98cef57a18a295c3f1b4)