clang9(or trunk) vs gcc9: member array size as non-type template parameter [duplicate]

I have the following mock code

#include <array>
template<int N>
struct S {
    std::array<int, N> m{};
};

template<int N>
auto copy (S<N> const& i) {
    return S<i.m.size()>{};
}

int main() {
    copy(S<1>{});
}

Is i.m.size() supposed to be a valid non-type template parameter?
clang9 sais no, gcc9 sais yes: https://godbolt.org/z/PoHjH8

I kind of "get" clang because i is not constexpr, but since the i.m.size() is still known at compile time, gcc's approach makes more sense to me.
Who is correct here?