These two examples are very similar, but the second one is ill-formed per the wording despite implementations accepting it due to user demand:

  template <typename T>
  concept C = requires (typename T::type x) {
    x + 1;
  };

  static_assert(!C<int>);

vs.

  template <typename T>
  constexpr bool b = requires (T::type x) { x + 1; };

  static_assert(!b<int>);  // de-jure ill-formed, but widely accepted by implementations

In order to make the second case well-formed, the user needs to write a nested requires that checks for the presence of T::type first.

See CWG2565 for details.