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3896. The definition of viewable_range is not quite right

Section: 25.4.5 [range.refinements] Status: New Submitter: Hewill Kang Opened: 2023-02-27 Last modified: 2023-03-22

Priority: 4

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Discussion:

The requirements of viewable_range for view type is view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>, that is, when the decayed type of T models view, it must be constructible from T.

This part of the constraint corresponds to first bullet of views::all (25.7.6.1 [range.all.general]), which returns decay-copy(E) if the decayed type of E models view.

However, decay-copy(E) constraints convertible_to<T, decay_t<T>> which is a stronger requirement than constructible_from, which is reflected in its rejection of types with explicit copy constructors.

This inconsistency is such that the following causes the range adapter to produce a hard error when invoked (online example):

#include <ranges>

struct View : std::ranges::view_base 
{
  View();
  explicit View(const View&); // explicit copy constructor
  View& operator=(const View&);
  View(View&&);
  
  int* begin();
  int* end();
};

int main() 
{
  View v;
  auto r = std::views::take(v, 5); // hard error
}

[2023-03-22; Reflector poll]

Set priority to 4 after reflector poll. "About as contrived as it gets." "Add generic front matter telling users the library doesn't support types with explicit copy constructors."

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.4.5 [range.refinements] as indicated:

   template<class T>
     concept viewable_range =
       range<T> &&
       ((view<remove_cvref_t<T>> && convertible_to<T, remove_cvref_t<T>>constructible_from<remove_cvref_t<T>, T>) ||
        (!view<remove_cvref_t<T>> &&
         (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));