[libc++] [ranges] Fix LWG3470 "convertible-to-non-slicing seems to reject valid case"

[philnik]

libcxx/test/std/ranges/range.utility/range.subrange/lwg3470.pass.cpp

24–28

This should also work in constexpr context, right? Please test that case.

[• Quuxplusone]

Added constexpr test, and a couple more (arguably too tangential) things I thought to test.

[philnik]

I feel like the extra tests should be in it's own file, but I don't really care either way.

[Mordante]

LGTM!

libcxx/docs/Status/Cxx2bIssues.csv

 `3123 <https://wg21.link/LWG3123>`__,"``duration`` constructor from representation shouldn't be effectively non-throwing","October 2021","","","|chrono|"
 `3146 <https://wg21.link/LWG3146>`__,"Excessive unwrapping in ``std::ref/cref``","October 2021","|Complete|","14.0"
 `3152 <https://wg21.link/LWG3152>`__,"``common_type`` and ``common_reference`` have flaws in common","October 2021","",""
 `3293 <https://wg21.link/LWG3293>`__,"``move_iterator operator+()`` has incorrect constraints","October 2021","","","|ranges|"
 `3361 <https://wg21.link/LWG3361>`__,"``safe_range<SomeRange&>`` case","October 2021","","","|ranges|"
 `3392 <https://wg21.link/LWG3392>`__,"``ranges::distance()`` cannot be used on a move-only iterator with a sized sentinel","October 2021","","","|ranges|"
 `3407 <https://wg21.link/LWG3407>`__,"Some problems with the wording changes of P1739R4","October 2021","","","|ranges|"
 `3422 <https://wg21.link/LWG3422>`__,"Issues of ``seed_seq``'s constructors","October 2021","|Complete|","14.0"
-`3470 <https://wg21.link/LWG3470>`__,"``convertible-to-non-slicing`` seems to reject valid case","October 2021","","","|ranges|"
+`3470 <https://wg21.link/LWG3470>`__,"``convertible-to-non-slicing`` seems to reject valid case","October 2021","|Complete|","14.0","|ranges|"
 `3480 <https://wg21.link/LWG3480>`__,"``directory_iterator`` and ``recursive_directory_iterator`` are not C++20 ranges","October 2021","|Complete|","14.0","|ranges|"
 `3498 <https://wg21.link/LWG3498>`__,"Inconsistent ``noexcept``-specifiers for ``basic_syncbuf``","October 2021","",""
 `3535 <https://wg21.link/LWG3535>`__,"``join_view::iterator::iterator_category`` and ``::iterator_concept`` lie","October 2021","","","|ranges|"
 `3554 <https://wg21.link/LWG3554>`__,"``chrono::parse`` needs ``const charT*`` and ``basic_string_view<charT>`` overloads","October 2021","","","|chrono|"
 `3557 <https://wg21.link/LWG3557>`__,"The ``static_cast`` expression in ``convertible_to`` has the wrong operand","October 2021","",""
 `3559 <https://wg21.link/LWG3559>`__,"Semantic requirements of ``sized_range`` is circular","October 2021","","","|ranges|"
 `3560 <https://wg21.link/LWG3560>`__,"``ranges::equal`` and ``ranges::is_permutation`` should short-circuit for ``sized_ranges``","October 2021","","","|ranges|"
 `3561 <https://wg21.link/LWG3561>`__,"Issue with internal counter in ``discard_block_engine``","October 2021","",""

libcxx/include/__ranges/subrange.h

 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
 namespace ranges {
   template<class _From, class _To>
+  concept __uses_nonqualification_pointer_conversion =
+    is_pointer_v<_From> && is_pointer_v<_To> &&
+    !convertible_to<remove_pointer_t<_From>(*)[], remove_pointer_t<_To>(*)[]>;
+
+  template<class _From, class _To>
   concept __convertible_to_non_slicing =
     convertible_to<_From, _To> &&
-    // If they're both pointers, they must have the same element type.
-    !(is_pointer_v<decay_t<_From>> &&
-      is_pointer_v<decay_t<_To>> &&
-      __different_from<remove_pointer_t<decay_t<_From>>, remove_pointer_t<decay_t<_To>>>);
+    !__uses_nonqualification_pointer_conversion<decay_t<_From>, decay_t<_To>>;
 
   template<class _Tp>
   concept __pair_like =
     !is_reference_v<_Tp> && requires(_Tp __t) {
       typename tuple_size<_Tp>::type; // Ensures `tuple_size<T>` is complete.
       requires derived_from<tuple_size<_Tp>, integral_constant<size_t, 2>>;
       typename tuple_element_t<0, remove_const_t<_Tp>>;
       typename tuple_element_t<1, remove_const_t<_Tp>>;

libcxx/test/std/ranges/range.utility/range.subrange/lwg3470.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: libcpp-has-no-incomplete-ranges
+
+// class std::ranges::subrange;
+//   Test the example from LWG 3470,
+//   qualification conversions in __convertible_to_non_slicing
+
+#include <ranges>
+
+#include <cassert>
+
+#include "test_macros.h"
+
+constexpr bool test()
+{
+  // The example from LWG3470, using implicit conversion.
+  int a[3] = { 1, 2, 3 };
+  int* b[3] = { &a[2], &a[0], &a[1] };
+  std::ranges::subrange<const int* const*> c = b;
+  assert(c.begin() == b + 0);
+  assert(c.end() == b + 3);
+
+  // Also test CTAD and a subrange-to-subrange conversion.
+  std::same_as<std::ranges::subrange<int**>> auto d = std::ranges::subrange(b);
+  assert(d.begin() == b + 0);
+  assert(d.end() == b + 3);
+
+  std::ranges::subrange<const int* const*> e = d;
+  assert(e.begin() == b + 0);
+  assert(e.end() == b + 3);
+
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+  static_assert(test());
+
+  return 0;
+}