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3895. Various relation concepts are missing default values of the second template parameters

Section: 18.3 [concepts.syn], 24.2 [iterator.synopsis] Status: New Submitter: blacktea hamburger Opened: 2023-02-25 Last modified: 2023-03-22

Priority: 3

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Discussion:

std::indirect_equivalence_relation and std::indirect_strict_weak_order have default values for the second template parameters:

template<class F, class I1, class I2 = I1>
  concept indirect_equivalence_relation = see below;

template<class F, class I1, class I2 = I1>
  concept indirect_strict_weak_order = see below;

But std::relation, std::equivalence_relation, std::strict_weak_order, and std::indirect_binary_predicate are missing such default values:

template<class R, class T, class U>
  concept relation = see below;

template<class R, class T, class U>
  concept equivalence_relation = see below;

template<class R, class T, class U>
  concept strict_weak_order = see below;
  
template<class F, class I1, class I2>
  concept indirect_binary_predicate = see below;

That makes them inconsistent and it should not be expected.

[2023-03-22; Reflector poll]

Set priority to 3 after reflector poll. "Borderline design change." "Should not change indirect_binary_predicate." "NAD, write a paper." "NAD, default argument would make typos compile. Explicit is good."

Proposed resolution:

This wording is relative to N4928.

1. Modify 18.3 [concepts.syn], header <concepts> synopsis, as indicated:

   // all freestanding
   namespace std {
     […]
     
     // 18.7.5 [concept.relation], concept relation
     template<class R, class T, class U = T>
       concept relation = see below;
     
     // 18.7.6 [concept.equiv], concept equivalence_relation
     template<class R, class T, class U = T>
       concept equivalence_relation = see below;
     
     // 18.7.7 [concept.strictweakorder], concept strict_weak_order
     template<class R, class T, class U = T>
       concept strict_weak_order = see below;
   }
   

2. Modify 18.7.5 [concept.relation] as indicated:

   template<class R, class T, class U = T>
     concept relation =
       predicate<R, T, T> && predicate<R, U, U> &&
       predicate<R, T, U> && predicate<R, U, T>;
   

3. Modify 18.7.6 [concept.equiv] as indicated:

   template<class R, class T, class U = T>
     concept equivalence_relation = relation<R, T, U>;
   

4. Modify 18.7.7 [concept.strictweakorder] as indicated:

   template<class R, class T, class U = T>
     concept strict_weak_order = relation<R, T, U>;
   

5. Modify 24.2 [iterator.synopsis], header <iterator> synopsis, as indicated:

   […]
   namespace std {
     […]
     // 24.3.6.3 [indirectcallable.indirectinvocable], indirect callables
     […]
     template<class F, class I1, class I2 = I1>
       concept indirect_binary_predicate = see below; // freestanding
   
     template<class F, class I1, class I2 = I1>
       concept indirect_equivalence_relation = see below; // freestanding
     […]
   }
   

6. Modify 24.3.6.3 [indirectcallable.indirectinvocable] as indicated:

   -1- The indirect callable concepts are used to constrain those algorithms that accept callable objects (22.10.4 [func.require]) as arguments.

   […]
   template<class F, class I1, class I2 = I1>
     concept indirect_binary_predicate =
       indirectly_readable<I1> && indirectly_readable<I2> &&
       copy_constructible<F> &&
       predicate<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
       predicate<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
       predicate<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
       predicate<F&, iter_reference_t<I1>, iter_reference_t<I2>> &&
       predicate<F&, iter_common_reference_t<I1>, iter_common_reference_t<I2>>;
   […]