[PATCH] libstdc++: Implement LWG 3470 change to ranges::subrange

[Patrick Palka via Gcc-patches]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk and branches?

libstdc++-v3/ChangeLog:

        * include/bits/ranges_util.h
        (__detail::__uses_nonqualification_pointer_conversion): Define
        and use it ...
        (__convertible_to_nonslicing): ... here, as per LWG 3470.
        * testsuite/std/ranges/subrange/1.cc: New test.
---
 libstdc++-v3/include/bits/ranges_util.h       | 13 +++++++++----
 .../testsuite/std/ranges/subrange/1.cc        | 19 +++++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/subrange/1.cc

diff --git a/libstdc++-v3/include/bits/ranges_util.h 
b/libstdc++-v3/include/bits/ranges_util.h
index 7e7b958d274..765848e327d 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -184,11 +184,16 @@ namespace ranges
 
   namespace __detail
   {
-    template<class _From, class _To>
+    template<typename _From, typename _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<typename _From, typename _To>
       concept __convertible_to_non_slicing = convertible_to<_From, _To>
-       && !(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<typename _Tp>
       concept __pair_like
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/1.cc 
b/libstdc++-v3/testsuite/std/ranges/subrange/1.cc
new file mode 100644
index 00000000000..8a53261c78c
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/1.cc
@@ -0,0 +1,19 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+
+#include <ranges>
+
+void
+test01()
+{
+  // LWG 3470
+  int a[3] = {1,2,3};
+  int* b[3] = {&a[2], &a[0], &a[1]};
+  auto c = std::ranges::subrange<const int*const*>(b);
+}
+
+int
+main()
+{
+  test01();
+}
-- 
2.33.1.711.g9d530dc002