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3996. projected<I, identity> should just be I

Section: 24.3.6.4 [projected] Status: NAD Submitter: Hewill Kang Opened: 2023-10-12 Last modified: 2024-06-24

Priority: Not Prioritized

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Discussion:

Currently, projected is a wrapper of the implementation type regardless of whether Proj is identity.

Since identity always returns a reference, this prevents projected<I, identity> from fully emulating the properties of the original iterator when its reference is a prvalue.

Such non-equivalence may lead to unexpected behavior in some cases (demo):

#include <algorithm>
#include <ranges>
#include <iostream>

int main() {
  auto outer = std::views::iota(0, 5)
             | std::views::transform([](int i) {
                 return std::views::single(i) | std::views::filter([](int) { return true; });
               });
  
  for (auto&& inner : outer)
    for (auto&& elem : inner)
      std::cout << elem << " "; // 0 1 2 3 4 
  
  std::ranges::for_each(
    outer,
    [](auto&& inner) {
      // error: passing 'const filter_view' as 'this' argument discards qualifiers
      for (auto&& elem : inner)
        std::cout << elem << " ";
    });
}

In the above example, ranges::for_each requires indirect_unary_predicate<Pred, projected<I, identity>> which ultimately requires invocable<Pred&, iter_common_reference_t<projected<I, identity>>>.

According to the current wording, the reference and indirect value type of projected<I, identity> are filter_view&& and filter_view& respectively, which causes its common reference to be eventually calculated as const filter_view&. Since the former is not const-iterable, this results in a hard error during instantiation because const begin is called unexpectedly in an unconstrained lambda.

It seems like having projected<I, identity> just be I is a more appropriate choice, which makes the concept checking really specific to I rather than a potentially incomplete iterator wrapper.

[2023-11-03; Reflector poll]

NAD. P2997 solves this, and more. "Applying the projection does in fact materialize prvalues, so this is just lying unless we special-case identity everywhere."

[St. Louis 2024-06-24 Status changed: Tentatively NAD → NAD.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 24.3.6.4 [projected] as indicated:

   -1- Class template projected is used to constrain algorithms that accept callable objects and projections (3.44 [defns.projection]). It combines an indirectly_readable type I and a callable object type Proj into a new indirectly_readable type whose reference type is the result of applying Proj to the iter_reference_t of I.

   namespace std {
     template<class I, class Proj>
     struct projected-impl {                               // exposition only
       struct type {                                       // exposition only
         using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
         using difference_type = iter_difference_t<I>;     // present only if I
                                                           // models weakly_incrementable
         indirect_result_t<Proj&, I> operator*() const;    // not defined
       };
     };
   
     template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
       using projected =  projected-impl<I, Proj>::type;
   }