libstdc++: Reduce the size of a subrange with empty sentinel type

libstdc++-v3/ChangeLog:

	* include/bits/ranges_util.h (subrange::_M_end): Give it
	[[no_unique_address]].
	* testsuite/std/ranges/subrange/sizeof.cc: New test.

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

@@ -208,7 +208,7 @@ namespace ranges
 	= _Kind == subrange_kind::sized && !sized_sentinel_for<_Sent, _It>;
 
       _It _M_begin = _It();
-      _Sent _M_end = _Sent();
+      [[no_unique_address]] _Sent _M_end = _Sent();
 
       template<typename, bool = _S_store_size>
 	struct _Size

libstdc++-v3/testsuite/std/ranges/subrange/sizeof.cc

@@ -0,0 +1,28 @@
+// Copyright (C) 2020 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 } }
+
+#include <iterator>
+#include <ranges>
+
+// Verify we optimize away the 'end' data member of a subrange with an empty
+// sentinel type.
+static_assert(sizeof(std::ranges::subrange<std::counted_iterator<int*>,
+					   std::default_sentinel_t>)
+	      == sizeof(std::counted_iterator<int*>));