Why doesnt this operator usage in the trailing return type compile?

I was trying to reuse the return type of an operator in my trailing return type of another function, but unfortunately clang did not accept it

struct A {
  int operator[](int);
  auto at(int i) -> decltype((*this)[i]);
};

Clang says that my class does not have a operator[]. Gcc did accept my code. Is my code actually invalid?