[libcxx][ranges] makes `ranges::subrange` a borrowed range

[cjdb]

uglifies names

[zoecarver]

Looks good to me. This seems like a pretty much obviously correct patch, so I wouldn't be opposed to just landing it.

[cjdb]

fixes test

[ldionne]

Thanks for fixing this! Can you update the comment in the test?

libcxx/test/std/ranges/range.utility/range.subrange/enable_borrowed_range.compile.pass.cpp

13

This comment is wrong.

libcxx/include/__ranges/subrange.h

namespace ranges {
   template<size_t _Index, class _Iter, class _Sent, subrange_kind _Kind>
     requires (_Index < 2)
   constexpr auto get(subrange<_Iter, _Sent, _Kind>&& __subrange) {
     if constexpr (_Index == 0)
       return __subrange.begin();
     else
       return __subrange.end();
   }
+
+  template<class _Ip, class _Sp, subrange_kind _Kp>
+  inline constexpr bool enable_borrowed_range<subrange<_Ip, _Sp, _Kp>> = true;
 } // namespace ranges
 
 using ranges::get;
 
 // clang-format off
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___RANGES_SUBRANGE_H

libcxx/include/ranges

namespace std::ranges {
   template<class T>
   concept viewable_range = see below;
 
   // [view.interface], class template view_interface
   template<class D>
     requires is_class_v<D> && same_as<D, remove_cv_t<D>>
   class view_interface;
 
+  // [range.subrange], sub-ranges
+  enum class subrange_kind : bool { unsized, sized };
+
+  template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = see below>
+    requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
+  class subrange;
+
+  template<class I, class S, subrange_kind K>
+    inline constexpr bool enable_borrowed_range<subrange<I, S, K>> = true;
+
   // [range.empty], empty view
   template<class T>
     requires is_object_v<T>
   class empty_view;
 
   // [range.all], all view
   namespace views {
     inline constexpr unspecified all = unspecified;

libcxx/test/std/ranges/range.utility/range.subrange/enable_borrowed_range.compile.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// class std::ranges::subrange;
+
+#include <ranges>
+
+#include "test_iterators.h"
+
+namespace ranges = std::ranges;
+
+static_assert(ranges::borrowed_range<ranges::subrange<int*>>);
+static_assert(ranges::borrowed_range<ranges::subrange<int*, int const*>>);
+static_assert(ranges::borrowed_range<ranges::subrange<int*, sentinel_wrapper<int*>, ranges::subrange_kind::unsized>>);