Eager instantiation of static data member

Clang Frontend
1

I'm puzzled why Clang (up through recent trunk) rejects

$ cat test.cc
template<typename T> struct S { static const int n = T::n; };
S<int> s;

$ clang++ -std=c++17 -c test.cc
test.cc:1:54: error: type 'int' cannot be used prior to '::' because it has no members
template<typename T> struct S { static const int n = T::n; };
                                                     ^
test.cc:2:8: note: in instantiation of template class 'S<int>' requested here
S<int> s;
       ^
1 error generated.

when C++17 [temp.inst]/3 states that "the initialization (and any associated side effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist."

Is the initializer instantiated to check whether it is a constant expression (as required by [class.static.data]/3)? If yes, is the compiler indeed allowed/required to check that when instantiating S<int> here?

(A similar

template<typename T> struct S { static constexpr int n = T::n; };
S<int> s;

is accepted by Clang, FWIW.)

2

I'm puzzled why Clang (up through recent trunk) rejects

$ cat test.cc
template<typename T> struct S { static const int n = T::n; };
S<int> s;

$ clang++ -std=c++17 -c test.cc
test.cc:1:54: error: type 'int' cannot be used prior to '::' because it has no members
template<typename T> struct S { static const int n = T::n; };
^
test.cc:2:8: note: in instantiation of template class 'S<int>' requested here
S<int> s;
^
1 error generated.

when C++17 [temp.inst]/3 states that "the initialization (and any associated side effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist."

Is the initializer instantiated to check whether it is a constant expression (as required by [class.static.data]/3)? If yes, is the compiler indeed allowed/required to check that when instantiating S<int> here?

(A similar

template<typename T> struct S { static constexpr int n = T::n; };
S<int> s;

is accepted by Clang, FWIW.)

Upon closer inspection,

   static const int n = T::n;

is a declaration but not a definition, so C++17 [temp.inst]/2 "The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions [...] of the [...] static data members [...]" arguably doesn't kick in to avoid instantiation of the initializer. (Whereas the implicitly inline

   static constexpr int n = T::n;

/is/ a definition.)

While for the initializers of non-static data members the intent is apparently to not instantiate them eagerly (see <C++ Standard Core Language Active Issues; "Deferred instantiation and checking of non-static data member initializers", and

template<typename T> struct S { int n = T::n; };
S<int> s{0};

is indeed accepted by Clang), this appears to not carry over to initializers of (non-inline, in-class) static data member declarations.

The resulting question is whether that is a conscious decision or an oversight in the standard. (Note that at least recent GCC appears to not eagerly instantiate such initializers; it accepts the original test.cc.)