class A {
  friend std::ostream& operator<<(std::ostream &os, A &a);
};

A a() {
  return A{};
}

int main() {
  std::cout << a(); // error!
  //A aa = a(); std::cout << aa; // compiles just fine
}

error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘A’)
   std::cout << a();
             ^
...200 (!) more lines of stderr...