[Bug c++/63942] [5.0 Regression] constexpr conflicts with previous declaration

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63942

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
Reduced:

template <class T>
struct A
{
  A(const T&) {}
  A(const A&) {}
};

template class A<decltype(nullptr)>;