The example in _N2914_.14.11.1.1 [temp.req.sat] paragraph 6 reads,

    concept C<typename T> { }
    concept D<typename T> { }
    namespace N2 {
      template<C T> void f(T);                // #1
      template<C T> requires D<T> void f(T);  // #2
      template<C T> void g(T x) {
        f(x);
      }
    ...

The call f(x) is ill-formed without a constraint indicating that x can be passed by value.