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From: "language.lawyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96593] New: No "declaration changes meaning" diagnostic for
 alias templates
Date: Wed, 12 Aug 2020 17:48:06 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: diagnostic
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96593

            Bug ID: 96593
           Summary: No "declaration changes meaning" diagnostic for alias
                    templates
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: language.lawyer at gmail dot com
  Target Milestone: ---

"A name N used in a class S shall refer to the same declaration in its cont=
ext
and when re-evaluated in the completed scope of S. No diagnostic is required
for a violation of this rule."

The diagnostic for the rule is not required, but since GCC diagnoses some of
the cases, there is a bugreport about missing diagnostic cases (bug 46983),=
 and
the check might become mandatory (will it?), I'd like file this bug.

GCC accepts the code with alias templates:

struct S {};

template <typename>
struct T {};

struct U
{
        template<typename>
        using S =3D S;

        template<typename>
        using T =3D T<void>;
};

but produces the "declaration changes meaning" error for type aliases.
GCC also diagnoses the case if the "outer" declaration is used before the
corresponding alias template:

struct S {};

struct U
{
        S s;

        template<typename>
        using S =3D S; // error: declaration of ... changes meaning of 'S'
};

(From https://stackoverflow.com/q/63369264/)=