If either operand is of scoped enumeration type, no conversions
are performed; if the other operand does not have the same type, the expression is
ill-formed.
Otherwise, if the floating-point conversion ranks ([conv.rank]) of
the types of the operands are ordered but not equal,
then the operand of the type with the lesser floating-point conversion rank
is converted to the type of the other operand.
Otherwise, if the floating-point conversion ranks of the types of
the operands are equal,
then the operand with the lesser floating-point conversion subrank ([conv.rank])
is converted to the type of the other operand.
Otherwise, if both operands have signed integer types or both have
unsigned integer types, the operand with the type of lesser integer
conversion rank is converted to the type of the operand with
greater rank.
Otherwise, if the operand that has unsigned integer type has rank
greater than or equal to the rank of the type of the other operand, the
operand with signed integer type is converted to the type of the
operand with unsigned integer type.
Otherwise, if the type of the operand with signed integer type can
represent all of the values of the type of the operand with unsigned
integer type, the operand with unsigned integer type is converted
to the type of the operand with signed integer type.
If one operand is of enumeration type
and the other operand is of
a different enumeration type or
a floating-point type,
this behavior is deprecated ([depr.arith.conv.enum]).