"definition of explicitly-defaulted" error with explicit template instantiation definition (Clang VS GCC)

Question

Consider the following code:

template <typename> struct S { S(); };
// extern template struct S<int>;
template <typename T> S<T>::S() = default;
template S<int>::S();

clang++ happily accepts this code.

g++ complains with:

error: definition of explicitly-defaulted 'S< <template-parameter-1-1> >::S() 
       [with <template-parameter-1-1> = int]'
    3 | template <typename T> S<T>::S() = default;
      |                       ^~~~

Uncommenting the extern template fixes compilation with g++ (and clang++ is still happy).

• live example on godbolt.org

Is g++ correct in rejecting the code?