Consider:

template < class T >
struct Foo {
  T v;
  constexpr Foo() : v(){}
  static const Foo f;
};

template < class T >
constexpr const Foo<T> Foo<T>::f = Foo();

std::array<int, Foo<std::size_t>::f.v> a;

Clang rejects this, because it doesn't eagerly instantiate Foo<size_t>::f, presumably because the constexpr specifier isn't seen until the out-of-line definition is instantiated.