Consider:

  template <typename Ty>
  struct S {
    Ty i;
    consteval S() = default;
  };

  template <typename Ty>
  struct T {
    Ty i;
    consteval T() {}
  };

  S<int> one; // only Clang rejects
  T<int> two; // Clang, GCC, ICC, MSVC reject

  void locals() {
    S<int> three; // only Clang rejects
    T<int> four;  // Clang, GCC, ICC, MSVC reject
  }

A consteval function should always be evaluated at compile time and never fall back to runtime, thus all four cases should be rejected. Issue 2558 is related.