Proposed resolution:

This wording is relative to N4981.

1. Modify [range.concat.view] as indicated:

   -1- The exposition-only concat-indirectly-readable concept is equivalent to:

   template<class Ref, class CRef>
   concept concat-ref-compatible-with = is_invocable_r_v<CRef, Ref()>;  // exposition only
     
   template<class Ref, class RRef, class It>
   concept concat-indirectly-readable-impl =                         // exposition only
     requires (const It it) {
       { *it } -> convertible_toconcat-ref-compatible-with<Ref>;
       { ranges::iter_move(it) } -> convertible_toconcat-ref-compatible-with<RRef>;
     };
   
   template<class... Rs>
     concept concat-indirectly-readable =                              // exposition only
       common_reference_with<concat-reference-t<Rs...>&&,
                             concat-value-t<Rs...>&> &&
       common_reference_with<concat-reference-t<Rs...>&&,
                             concat-rvalue-reference-t<Rs...>&&> &&
       common_reference_with<concat-rvalue-reference-t<Rs...>&&,
                             concat-value-t<Rs...> const&> &&
       (concat-indirectly-readable-impl<concat-reference-t<Rs...>,
                                        concat-rvalue-reference-t<Rs...>,
                                        iterator_t<Rs>> && ...);
   

[ 2024-06-24; Reflector poll ]

Set priority to 4 after reflector poll. "Proposed resolution loses the existing requirement that the conversion is equality-preserving." "Don't care about rejecting non-movable reference types."

In order to prevent non-equality-preserving behavior of operator* and iter_move, concat_view introduces the concat-indirectly-readable concept, part of which is:

template<class Ref, class RRef, class It>
  concept concat-indirectly-readable-impl =                         // exposition only
    requires (const It it) {
      { *it } -> convertible_to<Ref>;
      { ranges::iter_move(it) } -> convertible_to<RRef>;
    };

This isn't quite right because convertible_to checks is_convertible_v which doesn't understand copy elision. This makes the current concat_view unable to work with ranges whose reference is non-movable prvalue:

auto r = std::views::iota(0, 5)
       | std::views::transform([](int) { return NonMovable{}; });
auto c1 = std::ranges::concat_view(r);    // ill-formed, concat_indirectly_readable not satisfied
auto c2 = std::ranges::concat_view(r, r); // ditto

Since std::visit<R> is used in the implementation to perform reference conversion for the underlying iterator, the more accurate one should be is_invocable_r which does understand guaranteed elision.

Note that join_with_view has the same issue because compatible-joinable-ranges requires that the value_type of the inner range and pattern range must satisfy common_with, which always fails for non-movable types. However, this can be automatically resolved by LWG 4074's resolution.