Static_assert inside if constexpr not discarded

Question

The following static_assert should be discarded being in the false branch of if constexpr, yet compilation fails due to assert failure:

#include <type_traits>                                                                                                                                                                              

template <class T>                                                                                                                                                                                  
constexpr bool f() {                                                                                                                                                                                
  if constexpr (std::is_same<T, int>::value) return true;                                                                                                                                                                                                                                                                    
  else static_assert(false, "message");                                                                                                                                                             
}                                                                                                                                                                                                   

int main () {                                                                                                                                                                                       
  if constexpr (f<int>()) return 1;                                                                                                                                                                 
  return 0;                                                                                                                                                                                         
} 

I'd expect that the discarded branch of if constexpr wouldn't be evaluated because f is instantiated with type int.

Compiled with Gcc 7.2 (-std=c++17)