14.2.4 Binary operator overload resolution

An operation of the form x op y, where op is an overloadable binary
operator, x is an expression of type X,
and y is an expression of type Y, is processed as follows:
?The set of candidate user-defined operators provided by X and Y for the
operation operator op(x, y)
is determined. The set consists of the union of the candidate operators
provided by X and the candidate
operators provided by Y, each determined using the rules of ?4.2.5. If X
and Y are the same type, or if X
and Y are derived from a common base type, then shared candidate operators
only occur in the combined
set once.
?If the set of candidate user-defined operators is not empty, then this
becomes the set of candidate
operators for the operation. Otherwise, the predefined binary operator op
implementations become the
set of candidate operators for the operation. The predefined
implementations of a given operator are
specified in the description of the operator (?4.7 through ?4.13).
?The overload resolution rules of ?4.4.2 are applied to the set of
candidate operators to select the best
operator with respect to the argument list (x, y), and this operator
becomes the result of the overload
resolution process. If overload resolution fails to select a single best
operator, a compile-time error
occurs.