Четверг, 18 июля 2019, 18:55 +03:00 от Brian Bi <bbi5291@gmail.com>:

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

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@lists.isocpp.org> 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

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Brian Bi