Consider:

  struct A {
    int n;
    constexpr A() {}
  };
  constexpr A a; // implementations reject

Paper P1331R2 (Permitting trivial default initialization in constexpr contexts) dropped the restriction that immediate invocations cannot yield results with some subobjects left uninitialized. It is unclear whether that change was intentional or accidental.

This issue is closely related to issue 2536.