Extended Description

Consider:

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

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

std::array<int, Foostd::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.