[gcc r11-9818] libstdc++: Implement LWG 3481 change to ranges::viewable_range

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:60b4a814777456527b48f77ef1d0a6af26a83242

commit r11-9818-g60b4a814777456527b48f77ef1d0a6af26a83242
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Oct 20 21:34:18 2021 -0400

    libstdc++: Implement LWG 3481 change to ranges::viewable_range
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_base.h (viewable_range): Adjust as per
            LWG 3481.
            * testsuite/std/ranges/adaptors/all.cc (test07): New test.
    
    (cherry picked from commit a2c2dcc6ca205a8c5c76b04ef2eb4fb097dcb069)

Diff:
---
 libstdc++-v3/include/bits/ranges_base.h           |  3 ++-
 libstdc++-v3/testsuite/std/ranges/adaptors/all.cc | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 48a9d0d95bb..87283b84028 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -664,7 +664,8 @@ namespace ranges
   /// A range which can be safely converted to a view.
   template<typename _Tp>
     concept viewable_range = range<_Tp>
-      && (borrowed_range<_Tp> || view<remove_cvref_t<_Tp>>);
+      && ((view<remove_cvref_t<_Tp>> && constructible_from<remove_cvref_t<_Tp>, _Tp>)
+	  || (!view<remove_cvref_t<_Tp>> && borrowed_range<_Tp>));
 
   // [range.iter.ops] range iterator operations
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
index 42913ad38a3..5facaee7b37 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
@@ -130,6 +130,20 @@ test05()
   static_assert(!requires { 0 | all; });
 }
 
+void
+test07()
+{
+  // LWG 3481
+  struct view_t : ranges::empty_view<int> { // move-only view
+    view_t(const view_t&) = delete;
+    view_t(view_t&&) = default;
+    view_t& operator=(const view_t&) = delete;
+    view_t& operator=(view_t&&) = default;
+  };
+  static_assert(std::movable<view_t> && !std::copyable<view_t>);
+  static_assert(!ranges::viewable_range<view_t&>);
+}
+
 int
 main()
 {
@@ -138,4 +152,5 @@ main()
   static_assert(test03());
   static_assert(test04());
   test05();
+  test07();
 }