Because built-in operators take only operands with non-class type,
and operator overload resolution occurs only when an operand expression
originally has class or enumeration type,
operator overload resolution can resolve to a built-in operator only
when an operand has a class type that has a user-defined conversion to
a non-class type appropriate for the operator, or when an operand has
an enumeration type that can be converted to a type appropriate
for the operator.
Also note that some of the candidate operator functions given in this subclause are
more permissive than the built-in operators themselves.
described in [over.match.oper]
, after a built-in operator is selected
by overload resolution the expression is subject to the requirements for
the built-in operator given in [expr]
, and therefore to any
additional semantic constraints given there.
If there is a user-written
candidate with the same name and parameter types as a built-in
candidate operator function, the built-in operator function
is hidden and is not included in the set of candidate functions.
— end note