7 Expressions [expr]

The three-way comparison operator groups left-to-right.

The expression p <=> q is a prvalue indicating whether p is less than, equal to, greater than, or incomparable with q.

If one of the operands is of type bool and the other is not, the program is ill-formed.

If both operands have arithmetic types, or one operand has integral type and the other operand has unscoped enumeration type, the usual arithmetic conversions are applied to the operands.

Then:

• (4.1)
  If a narrowing conversion is required, other than from an integral type to a floating-point type, the program is ill-formed.
• (4.2)
  Otherwise, if the operands have integral type, the result is of type std​::​strong_ordering.

  The result is std​::​strong_ordering​::​equal if both operands are arithmetically equal, std​::​strong_ordering​::​less if the first operand is arithmetically less than the second operand, and std​::​strong_ordering​::​greater otherwise.