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

[gonzalo.gadeschi at gmail dot com via Gcc-bugs]

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

--- Comment #3 from gnzlbg <gonzalo.gadeschi at gmail dot com> ---
>From 102444 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102444):

See http://eel.is/c++draft/class.member.lookup#6

  [class.member.lookup]/p6: "If it [the result of the search] differs from the
result of a search in T for N from immediately after the class-specifier of T,
the program is ill-formed, no diagnostic required."  A class definition is not
allowed to change the meaning of a name that was already used earlier in the
class definition.

While no diagnostic is required, I think a quality implementation like gcc
should produce a diagnostic here and reject this program since it is
ill-formed.

Example (found in the wild): https://gcc.godbolt.org/z/jhEj68Kq8

    struct plus {
      template<typename... Args>
      using invoke = void;
    };

    template <typename Fn, typename... Args>
    using invoke = typename Fn::template invoke<Args...>;

    template <typename Fn>
    struct compose
    {
      template <typename X, typename Y>
      using F = invoke<Fn, X, Y>;

      template <typename X>
      using invoke = invoke<Fn, X, X>;
    };

    using Q = compose<plus>::F<int, int>;

Is accepted by gcc.