[Bug c++/122752] [15/16 Regression] Compilation error about class not being a non-union class type for valid C++ code

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122752

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-15 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:bb061a3b815249f2a4f375c3f7c6ea88978b9804

commit r15-10611-gbb061a3b815249f2a4f375c3f7c6ea88978b9804
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Dec 15 14:30:53 2025 -0500

    c++: nested typename type resolving to wildcard type [PR122752]

    Here typename A<S>::VertexSet::size_type within the out-of-line
    declaration is initially parsed at namespace scope, so the LHS
    A<S>::VertexSet is treated as a dependent name and represented as a
    TYPENAME_TYPE with tag_type as class_type.[1]

    Once we realize we're parsing a member declarator we call
    maybe_update_decl_type to reprocess the TYPENAME_TYPE relative to the
    class template scope, during which make_typename_type succeeds in
    resolving the lookup and returns the member typedef VertexSet (to the
    TEMPLATE_TYPE_PARM S).  But then the caller tsubst complains that this
    result isn't a class type as per the tag_type.

    This patch just relaxes tsubst to allow TYPENAME_TYPE getting resolved
    to a wildcard type regardless of the tag_type.  This does mean we lose
    information about the tag_type during a subsequent tsubst, but that's
    probably harmless (famous last words).

    [1]: The tag_type should probably be scope_type.  Changing this seems
    to be a matter of changing cp_parser_qualifying_entity to pass
    scope_type instead of class_type, but I don't feel confident about that
    and it seems risky.  I then got confused as to why that function passes
    none_type in the !type_p case; to me it should use scope_type
    unconditionally, but doing so breaks things.  This approach seems safer
    to backport.

            PR c++/122752

    gcc/cp/ChangeLog:

            * pt.cc (tsubst) <case TYPENAME_TYPE>: Allow TYPENAME_TYPE
            resolving to another wildcard type.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/dependent-name19.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit 2bf057c53e97004034ed1528d850a214372305b8)