17 Language support library [support]

The behavior of a program that adds specializations for the compare_three_way_result template defined in this subclause is undefined.

For the compare_three_way_result type trait applied to the types T and U, let t and u denote lvalues of types const remove_reference_t<T> and const remove_reference_t<U>, respectively.

If the expression t <=> u is well-formed when treated as an unevaluated operand ([expr.context]), the member typedef-name type denotes the type decltype(t <=> u).

Otherwise, there is no member type.