postfix-expression [ expr-or-braced-init-list ]Thus, a subscripting expression x[y] is interpreted as x.operator[](y) for a class object x of type T if T::operator[](T1) exists and if the operator is selected as the best match function by the overload resolution mechanism ([over.match.best]).
struct X { Z operator[](std::initializer_list<int>); }; X x; x[{1,2,3}] = 7; // OK: meaning x.operator[]({1,2,3}) int a[10]; a[{1,2,3}] = 7; // error: built-in subscript operator