Question

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?

Answer 1

I would say that clang is correct, due to 13.3.1.2p3 (1st bullet).

For a unary operator @ with an operand of a type whose cv-unqualified version is T1, and for a binary
operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type
whose cv-unqualified version is T2, three sets of candidate functions, designated member candidates, nonmember
candidates and built-in candidates, are constructed as follows:

• If T1 is a complete class type, the set of member candidates is the result of the qualified lookup of
  T1::operator@ (13.3.1.1.1); otherwise, the set of member candidates is empty.

(emphasis added by @sehe)