12 Overloading [over]

12.6 Overloaded operators [over.oper]

12.6.4 Function call [over.call]

operator() shall be a non-static member function with an arbitrary number of parameters.
It can have default arguments.
It implements the function call syntax
postfix-expression ( expression-list )
where the postfix-expression evaluates to a class object and the possibly empty expression-list matches the parameter list of an operator() member function of the class.
Thus, a call x(arg1,...) is interpreted as x.operator()(arg1, ...) for a class object x of type T if T​::​operator()(T1, T2, T3) exists and if the operator is selected as the best match function by the overload resolution mechanism ([over.match.best]).