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3360. three_way_comparable_with is inconsistent with similar concepts

Section: 17.11.4 [cmp.concept] Status: C++20 Submitter: Casey Carter Opened: 2019-12-18 Last modified: 2021-02-25

Priority: 0

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Discussion:

The concept three_way_comparable_with is defined in 17.11.4 [cmp.concept] as:

template<class T, class U, class Cat = partial_ordering>
  concept three_way_comparable_with =
    weakly-equality-comparable-with<T, U> &&
    partially-ordered-with<T, U> &&
    three_way_comparable<T, Cat> &&
    three_way_comparable<U, Cat> &&
    common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
    three_way_comparable<
      common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
    requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
      { t <=> u } -> compares-as<Cat>;
      { u <=> t } -> compares-as<Cat>;
    };

Which notably doesn't follow the requirement ordering:

1. same-type requirements on T

2. same-type requirements on U

3. common_reference_with requirement

4. same-type requirements on common_reference_t<T, U>

5. cross-type requirements on T and U

that the other cross-type comparison concepts (18.5.4 [concept.equalitycomparable], 18.5.5 [concept.totallyordered]) use. There were some motivating reasons for that ordering:

1. The existence of a common reference type is effectively an opt-in to cross-type concepts. Avoiding checking cross-type expressions when no common reference type exists can enable the concepts to work even in the presence of poorly-constrained "accidental" cross-type operator templates which could otherwise produce compile errors instead of dissatisfied concepts.

2. Putting the simpler same-type requirements first can help produce simpler error messages when applying the wrong concept to a pair of types, or the right concept to the wrong pair of types. "Frobnozzle <=> Frobnozzle is not a valid expression" is more easily deciphered than is "std::common_reference<int, FrobNozzle> has no member named type".

three_way_comparable_with should be made consistent with equality_comparable_with and totally_ordered_with for the above reasons and to make it easier to reason about comparison concepts in general.

[2020-01 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 17.11.4 [cmp.concept] as indicated:

   template<class T, class U, class Cat = partial_ordering>
     concept three_way_comparable_with =
       weakly-equality-comparable-with<T, U> &&
       partially-ordered-with<T, U> &&
       three_way_comparable<T, Cat> &&
       three_way_comparable<U, Cat> &&
       common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
       three_way_comparable<
         common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
       weakly-equality-comparable-with<T, U> &&
       partially-ordered-with<T, U> &&
       requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
         { t <=> u } -> compares-as<Cat>;
         { u <=> t } -> compares-as<Cat>;
       };