Are substitution failures allowed outside of requires-expressions in concepts?

Consider this code:

#include <type_traits>                                           
#include <iostream>

template <class T> concept bool C1 = std::is_same<T, int>::value; 

template <class T> concept bool C2 =  
    C1<decltype(std::declval<T>() + std::declval<T>())>; 

struct A {};                   

int main() { 
  std::cout << C2<int>; 
  std::cout << C2<A>;                                                 
  return 0;                                                           
}

GCC compiles this fine and prints 10.

But section 14.10.1.2 of N4553 says

A predicate constraint is a constraint that evaluates a constant expression E (5.19).

and then

After substitution, E shall have type bool.

Since C1<decltype(std::declval<A>() + std::declval<A>())> is a substitution failure, rather than having type bool, does that mean this program should be ill-formed?