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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100032] [8/9/10/11 Regression] renaming alias template that
 also adds cv-qualifiers is deemed equivalent to underlying template
Date: Sun, 11 Apr 2021 13:31:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100032

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |8.5
                 CC|                            |jason at gcc dot gnu.org
            Summary|renaming alias template     |[8/9/10/11 Regression]
                   |that also adds              |renaming alias template
                   |cv-qualifiers is deemed     |that also adds
                   |equivalent to underlying    |cv-qualifiers is deemed
                   |template                    |equivalent to underlying
                   |                            |template

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
We started accepting the testcase after r208152.

I tried making get_underlying_template look through an alias only if the
underlying type is unqualified:

--- a/gcc/cp/pt.c=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
+++ b/gcc/cp/pt.c=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
@@ -6574,6 +6574,8 @@ get_underlying_template (tree tmpl)=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
       /* The underlying type may have been ill-formed. Don't proceed.  */=
=20=20=20=20=20=20
       if (!orig_type)=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
        break;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
+      if (TYPE_QUALS (orig_type) !=3D TYPE_UNQUALIFIED)=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
+       break;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
       tree tinfo =3D TYPE_TEMPLATE_INFO_MAYBE_ALIAS (orig_type);=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20
       if (!tinfo)=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
        break;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20

but this also makes us (incorrectly?) reject

template <template<class> class> struct X { };
template <class> struct Y { };
template <class T> using Z =3D const Y<T>;
template <class T> using W =3D Z<T>;
using U =3D X<Z>;
using U =3D X<W>;

because the underlying type of W for some reason already has the const
qualifier, so get_underlying_template(W) yields W and we deem W not equival=
ent
to Z.=