libstdc++: LWG 3325 Constrain return type of transformation function for transform_view

libstdc++-v3/ChangeLog:

	LWG 3325 Constrain return type of transformation function for
	transform_view
	* include/std/ranges (transform_view): Constrain the return type of the
	transformation function as per LWG 3325.
	* testsuite/std/ranges/adaptors/lwg3325_neg.cc: New test.

libstdc++-v3/ChangeLog

 2020-02-25  Patrick Palka  <ppalka@redhat.com>
 
+	LWG 3325 Constrain return type of transformation function for
+	transform_view
+	* include/std/ranges (transform_view): Constrain the return type of the
+	transformation function as per LWG 3325.
+	* testsuite/std/ranges/adaptors/lwg3325_neg.cc: New test.
+
 	LWG 3313 join_view::_Iterator::operator-- is incorrectly constrained
 	* include/std/ranges (join_view::_Iterator::operator--): Require that
 	range_reference_t<_Base> models common_range.

libstdc++-v3/include/std/ranges

@@ -1540,6 +1540,8 @@ namespace views
   template<input_range _Vp, copy_constructible _Fp>
     requires view<_Vp> && is_object_v<_Fp>
       && regular_invocable<_Fp&, range_reference_t<_Vp>>
+      && std::__detail::__can_reference<invoke_result_t<_Fp&,
+							range_reference_t<_Vp>>>
     class transform_view : public view_interface<transform_view<_Vp, _Fp>>
     {
     private:

libstdc++-v3/testsuite/std/ranges/adaptors/lwg3325_neg.cc

+// 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 <ranges>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+void
+test01()
+{
+  auto f = [] (int x) { };
+  int x[] = {1};
+  auto v = ranges::transform_view{x, f}; // { dg-error "deduction failed|no match" }
+}
+
+// { dg-prune-output "constraint failure" }
+// { dg-prune-output "in requirements" }