[clang] [clang] Prevent duplicated instantiation of enumerators of unscoped member enumerations (PR #124407)

[André Brand via cfe-commits]

thebrandre wrote:

I almost forgot to mention that this is actually valid code.
[C++23 [class.mem] p6](https://wg21.link/class.mem#general-6)
> A member shall not be declared twice in the *member-specification* except that
> * a nested class or member class template can be declared and then later defined, and
> * an enumeration can be introduced with an *opaque-enum-declaration* and later redeclared with an *enum-specifier*.


https://github.com/llvm/llvm-project/pull/124407