[Bug c++/100592] Bogus "qualifiers cannot be applied" error with reference type produced by dependent alias template

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100592

--- 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:d999d9b7e53b9a9cd2004a19e84c637e5e5013f5

commit r12-1183-gd999d9b7e53b9a9cd2004a19e84c637e5e5013f5
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 3 09:39:13 2021 -0400

    c++: cv-qualified dependent name of alias tmpl [PR100592]

    Here, the dependent template name in the return type of f() resolves to
    an alias of int& after substitution, and we end up complaining about
    qualifying this reference type with 'const' from cp_build_qualified_type
    rather than just silently dropping the qualification as per [dcl.ref]/1.

    The problem is ultimately that make_typename_type ignores the
    tf_keep_type_decl flag when the dependent name is a template-id.  This
    in turn causes the TYPE_DECL check within tsubst <case TYPENAME_TYPE>
    to fail, and so we end up not passing tf_ignore_bad_quals to
    cp_build_qualified_type.  This patch fixes this by making
    make_typename_type respect the tf_keep_type_decl flag in this situation.

            PR c++/100592

    gcc/cp/ChangeLog:

            * decl.c (make_typename_type): After calling
            lookup_template_class, adjust the result to its TYPE_NAME and
            then consider the tf_keep_type_decl flag.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/alias-decl-71.C: New test.