[llvm-bugs] [Bug 51934] New: class definition not allowed to change meaning of previously used name

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51934

            Bug ID: 51934
           Summary: class definition not allowed to change meaning of
                    previously used name
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalo.gadeschi at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

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 clang
should produce a diagnostic here and reject this program since it is
ill-formed.

Example: https://gcc.godbolt.org/z/zG7Mn1Ebx

    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 clang.

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