According to cppreference, both gcc and msvc have completed the implementation of C++20 feature using enum, which means we can using-declaration with an enum:

struct A { 
  enum e { /* ... */ }; 
};

struct S { 
  using enum A::e;
};

But when I apply it to the templates:

template <class T>
struct S { 
  using enum T::e;
};

gcc rejects it with:

<source>:7:14: error: 'using enum' of dependent type 'typename T::e'
    7 |   using enum T::e;
      |              ^~~~
<source>:7:17: note: declared here
    7 |   using enum T::e;
      |                 ^

msvc also rejects it with:

<source>(7): error C2868: 'e': ill-formed using-declaration; expected a qualified-name
<source>(8): note: see reference to class template instantiation 'S<T>' being compiled

I have no idea why this cannot work since it seems to be no different from non-templates.

Is this a compiler bug or this is just ill-formed?

If I put using-declaration inside a member function:

template <class T>
struct S { 
  void f() {
    using enum T::e;
  }
};

then msvc accepts it, but gcc still rejects it with the same error message. Is there a difference compared to the first?

If this is just a common bug, according to temp.variadic#5.2:

Pack expansions can occur in the following contexts:

• In a using-declaration ([namespace.udecl]); the pattern is a using-declarator.

Does this mean that the following case is well-formed even if gcc and msvc do not accept it currently?

template <class... Ts>
struct S {
  using enum Ts::e...;
};

Source: Windows Questions C++