[PATCH] libstdc++: Implement LWG 3481 change to ranges::viewable_range

[Patrick Palka via Gcc-patches]

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

libstdc++-v3/ChangeLog:

        * include/bits/ranges_base.h (viewable_range): Adjust as per
        LWG 3481.
        * testsuite/std/ranges/adaptors/all.cc (test07): New test.
---
 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 7801b2fd023..43b0b9f7bf3 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -688,7 +688,8 @@ 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 9a6a31e6cb4..ce1384c2890 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
@@ -159,6 +159,20 @@ test06()
   static_assert(!noexcept(views::all(BorrowedRange<false, false>(x))));
 }
 
+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()
 {
@@ -168,4 +182,5 @@ main()
   static_assert(test04());
   test05();
   test06();
+  test07();
 }
-- 
2.33.1.711.g9d530dc002