clang and gcc are not converging on constexpr

The differences relate to arrays. I think the standard leaves some flexibility in the specification, so there may not be a strict right or wrong - just different approaches. The challenge for creating portable code is knowing when it will work consistently across different compilers.
Sample
int main() {
     constexpr int a[] = {1, 2};
     static_assert(a[0] == 1);
}
works in clang but not in gcc