An ambiguity can occur with a requires-clause that ends in an operator-function-id:

  template<typename T> requires T::operator int
  const // part of operator-type-id or return type?
  unsigned f();

  template<typename T> requires T::operator int
  [[attr]] // appertains to type int or to declaration of g?
  void g();

Such cases are always ill-formed, because they involve an atomic constraint of non-bool type.