By "C++20" I assume you mean the working draft?

It seems your copy-paste somehow omitted some template arguments. See the
correct version below, and also here
<http://eel.is/c++draft/temp.expl.spec#7>

template<class T> struct A {
  enum E : T;
  enum class S : T;
};
template<> enum A<int>::E : int { eint }; // OK
template<> enum class A<int>::S : int { sint }; // OK
template<class T> enum A<T>::E : T { eT };
template<class T> enum class A<T>::S : T { sT };
template<> enum A<char>::E : char { echar }; // ill-formed,
A<char>::E was instantiated
                                                    // when A<char> was
instantiated
template<> enum class A<char>::S : char { schar };

On Thu, Jul 18, 2019 at 10:49 AM Vladimir Grigoriev via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> In the p.#7 of the section 13.8.3 Explicit specialization of the C++ 20
> there is the following example
>
> template <class T> struct A {
> enum E : T;
> enum class S : T;
> };
>
> template<> enum A::E : int { eint }; // OK
> template<> enum class A::S : int { sint }; // OK
> template<class T> enum A<T>::E : T { eT };
> template<class T> enum class A<T>::S : T { sT };
> template<> enum A::E : char { echar }; // ill-formed, A::E was
> instantiated // when A was instantiated
> template<> enum class A::S : char { schar }; // OK
>
> It is unclear why
>
> template<> enum A::E : char { echar };
>
> was already instantiated.
>
> Is it because
>
> template<> enum A::E : int { eint }; // OK
>
> was already instantiated?
>
> With best regards
> (Vlad from Moscow)
>
> You can meet me at http://cpp.forum24.ru/ or www.stackoverflow.com or
> http://ru.stackoverflow.com
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> http://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>

-- 
*Brian Bi*