libstdc++: Implement LWG 3391 changes to move/counted_iterator::base()

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (move_iterator::base): Make the
	const& overload unconstrained and return a const reference as
	per LWG 3391.  Make unconditionally noexcept.
	(counted_iterator::base): Likewise.
	* testsuite/24_iterators/move_iterator/lwg3391.cc: New test.
	* testsuite/24_iterators/move_iterator/move_only.cc: Adjust
	has_member_base concept to decay-copy the result of base().

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

@@ -1409,11 +1409,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       base() const
       { return _M_current; }
 #else
-      constexpr iterator_type
-      base() const &
-#if __cpp_lib_concepts
-	requires copy_constructible<iterator_type>
-#endif
+      constexpr const iterator_type&
+      base() const & noexcept
       { return _M_current; }
 
       constexpr iterator_type
@@ -2141,10 +2138,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return *this;
 	}
 
-      constexpr _It
-      base() const &
-      noexcept(is_nothrow_copy_constructible_v<_It>)
-      requires copy_constructible<_It>
+      constexpr const _It&
+      base() const & noexcept
       { return _M_current; }
 
       constexpr _It

libstdc++-v3/testsuite/24_iterators/move_iterator/lwg3391.cc

@@ -0,0 +1,37 @@
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// Verify LWG 3391 changes.
+
+#include <iterator>
+#include <ranges>
+
+#include <testsuite_iterators.h>
+
+using __gnu_test::test_range;
+using __gnu_test::input_iterator_wrapper_nocopy;
+
+void
+test01()
+{
+  extern test_range<int, input_iterator_wrapper_nocopy> rx;
+  auto v = rx | std::views::take(5);
+  std::ranges::begin(v) != std::ranges::end(v);
+}

libstdc++-v3/testsuite/24_iterators/move_iterator/move_only.cc

@@ -43,7 +43,13 @@ template<> struct std::iterator_traits<move_only_iterator>
 static_assert(std::input_iterator<move_only_iterator>);
 
 template<typename T>
-  concept has_member_base = requires (T t) { std::forward<T>(t).base(); };
+  concept has_member_base = requires (T t) {
+    // LWG 3391 made the const& overload of move_iterator::base()
+    // unconstrained and return a const reference.  So rather than checking
+    // whether base() is valid (which is now trivially true in an unevaluated
+    // context), the below now checks whether decay-copying base() is valid.
+    [](auto){}(std::forward<T>(t).base());
+  };
 
 using move_only_move_iterator = std::move_iterator<move_only_iterator>;