libstdc++: make common_iterator<I, S> require copyable<I> (LWG 3385)

	* include/bits/stl_iterator.h (common_iterator): Add copyable<I>
	requirement (LWG 3385).
	* testsuite/24_iterators/headers/iterator/synopsis_c++20.cc: Adjust
	expected declaration.

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_iterator.h (common_iterator): Add copyable<I>
+	requirement (LWG 3385).
+	* testsuite/24_iterators/headers/iterator/synopsis_c++20.cc: Adjust
+	expected declaration.
+
 	* include/std/ranges (take_while_view, drop_view, drop_while_view)
 	(elements_view:_Iterator): Initialize data members (LWG 3364).
 

libstdc++-v3/include/bits/stl_iterator.h

@@ -1426,7 +1426,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// An iterator/sentinel adaptor for representing a non-common range.
   template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
-    requires (!same_as<_It, _Sent>)
+    requires (!same_as<_It, _Sent>) && copyable<_It>
   class common_iterator
   {
     template<typename _Tp, typename _Up>

libstdc++-v3/testsuite/24_iterators/headers/iterator/synopsis_c++20.cc

@@ -56,7 +56,7 @@ namespace std
   template<semiregular S> class move_sentinel;
 
   template<input_or_output_iterator I, sentinel_for<I> S>
-    requires (!same_as<I, S>)
+    requires (!same_as<I, S>) && copyable<I>
     class common_iterator;
 
   template<class I, class S>