[over.sub]

A subscripting operator function is a member function named operator[] with an arbitrary number of parameters.

It may have default arguments.

For an expression of the form

postfix-expression [ expression-list

{o p t]}_{o p t]}

the operator function is selected by overload resolution ([over.match.oper]).

If a member function is selected, the expression is interpreted as

postfix-expression . operator [] ( expression-list

{o p t)}_{o p t)}

[Example 1: struct X { Z operator[](std::initializer_list<int>); Z operator[](auto.); }; X x; x[{1,2,3}] = 7; / OK, meaning x.operator[]({1,2,3}) 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 a[1,2,3] = 7; / error: built-in subscript operator — end example]