libstdc++: decay in viewable_range should be remove_cvref (LWG 3375)

	* include/bits/stl_algobase.h (__iter_swap, __iter_swap<true>): Remove
	redundant _GLIBCXX20_CONSTEXPR.

libstdc++-v3/ChangeLog

@@ -3,6 +3,9 @@
 	* include/bits/stl_algobase.h (__iter_swap, __iter_swap<true>): Remove
 	redundant _GLIBCXX20_CONSTEXPR.
 
+	* include/std/ranges (viewable_range): Replace decay_t with
+	remove_cvref_t (LWG 3375).
+
 2020-02-05  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/iterator_concepts.h (iter_reference_t)

libstdc++-v3/include/std/ranges

@@ -92,7 +92,7 @@ namespace ranges
   /// A range which can be safely converted to a view.
   template<typename _Tp>
     concept viewable_range = range<_Tp>
-      && (safe_range<_Tp> || view<decay_t<_Tp>>);
+      && (safe_range<_Tp> || view<remove_cvref_t<_Tp>>);
 
   namespace __detail
   {