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.
• (4.3)
  Otherwise, the operands have floating-point type, and the result is of type std​::​partial_ordering.

  The expression a <=> b yields std​::​partial_ordering​::​less if a is less than b, std​::​partial_ordering​::​greater if a is greater than b, std​::​partial_ordering​::​equivalent if a is equivalent to b, and std​::​partial_ordering​::​unordered otherwise.

If both operands have the same enumeration type E, the operator yields the result of converting the operands to the underlying type of E and applying <=> to the converted operands.

If at least one of the operands is of object pointer type and the other operand is of object pointer or array type, array-to-pointer conversions ([conv.array]), pointer conversions ([conv.ptr]), and qualification conversions are performed on both operands to bring them to their composite pointer type ([expr.type]).

After the conversions, the operands shall have the same type.

In this case, p <=> q is of type std​::​strong_ordering and the result is defined by the following rules:

• (6.1)
  If two pointer operands p and q compare equal ([expr.eq]), p <=> q yields std​::​strong_ordering​::​equal;
• (6.2)
  otherwise, if p and q compare unequal, p <=> q yields std​::​strong_ordering​::​less if q compares greater than p and std​::​strong_ordering​::​greater if p compares greater than q ([expr.rel]);
• (6.3)
  otherwise, the result is unspecified.

Otherwise, the program is ill-formed.

The three comparison category types ([cmp.categories]) (the types std​::​strong_ordering, std​::​weak_ordering, and std​::​partial_ordering) are not predefined; if a standard library declaration ([compare.syn], [std.modules]) of such a class type does not precede ([basic.lookup.general]) a use of that type — even an implicit use in which the type is not named (e.g., via the auto specifier ([dcl.spec.auto]) in a defaulted three-way comparison ([class.spaceship]) or use of the built-in operator) — the program is ill-formed.