gcc/testsuite/g++.dg/cpp1z/static2.C

// { dg-do compile { target c++17 } }

template <class T>
struct A
{
  static constexpr auto x = T::x; // { dg-error "incomplete" }
};

struct B;
A<B> a;

struct B
{
  static constexpr auto x = 42;
};

auto x = a.x;