std::ranges::viewable_range(3)

NAME
std::ranges::viewable_range - std::ranges::viewable_range

Synopsis
   Defined in header <ranges>
   template< class T >
   concept viewable_range =
       ranges::range<T> &&
       ((ranges::view<std::remove_cvref_t<T>> &&
         std::constructible_from<std::remove_cvref_t<T>, T>) || (since C++20)
        (!ranges::view<std::remove_cvref_t<T>> &&
         (std::is_lvalue_reference_v<T> ||
          (std::movable<std::remove_reference_t<T>> &&
   !/*is-initializer-list*/<T>))));
   The viewable_range concept is a refinement of range that describes a range that can
   be converted into a view through views::all.
   The constant /*is-initializer-list*/<T> is true if and only if
   std::remove_cvref_t<T> is a specialization of std::initializer_list.

Example
// Run this code
 #include <ranges>
 #include <string>
 #include <vector>
 struct valid_result {};
 struct invalid_result {};
 template <typename T>
 concept valid_viewable_range = std::same_as<T, valid_result>;
 template <typename T>
 concept invalid_viewable_range = std::same_as<T, invalid_result>;
 auto test_viewable_range(std::ranges::viewable_range auto &&) -> valid_result;
 auto test_viewable_range(auto&&) -> invalid_result;
 int main()
 {
     auto il = {1, 2, 3};
     int arr []{1, 2, 3};
     std::vector vec{1, 2, 3};
     std::ranges::ref_view r{arr};
     std::ranges::owning_view o{std::string("Hello")};
     static_assert(requires {
         { test_viewable_range(il) } -> valid_viewable_range;
         { test_viewable_range(std::move(il)) } -> invalid_viewable_range;
         { test_viewable_range(arr) } -> valid_viewable_range;
         { test_viewable_range(std::move(arr)) } -> invalid_viewable_range;
         { test_viewable_range(vec) } -> valid_viewable_range;
         { test_viewable_range(std::move(vec)) } -> valid_viewable_range;
         { test_viewable_range(r) } -> valid_viewable_range;
         { test_viewable_range(std::move(r)) } -> valid_viewable_range;
         { test_viewable_range(o) } -> invalid_viewable_range;
         { test_viewable_range(std::move(o)) } -> valid_viewable_range;
         { test_viewable_range(std::ranges::ref_view(o)) } -> valid_viewable_range;
     });
 }
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3481 C++20 viewable_range accepted an lvalue of a rejects
                       move-only view
   P2415R2 C++20 viewable_range only accepted non-view rvalues accepts more types
                       that are borrowed_range

See also
   views::all_t a view that includes all elements of a range
   views::all (alias template) (range adaptor object)
   (C++20)