[Bug c++/96593] New: No "declaration changes meaning" diagnostic for alias templates

["language.lawyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96593

            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 context
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 = S;

        template<typename>
        using T = 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 = S; // error: declaration of ... changes meaning of 'S'
};

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