libstdc++: tuple_element_t is also wrong for const subrange (LWG 3398)

	* include/std/ranges (tuple_element<0, const subrange<I, S, K>>)
	(tuple_element<1, const subrange<I, S, K>>): Add partial
	specializations (LWG 3398).
	* testsuite/std/ranges/subrange/tuple_like.cc: New test.

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/ranges (tuple_element<0, const subrange<I, S, K>>)
+	(tuple_element<1, const subrange<I, S, K>>): Add partial
+	specializations (LWG 3398).
+	* testsuite/std/ranges/subrange/tuple_like.cc: New test.
+
 	* include/bits/ranges_algo.h (__find_fn, __find_first_of_fn)
 	(__adjacent_find_fn, __remove_if_fn, __remove_copy_if_fn)
 	(__unique_fn, __unique_copy_fn): Remove redundant conversions to bool.

libstdc++-v3/include/std/ranges

@@ -3193,7 +3193,6 @@ namespace views
 	typename tuple_size<_Tp>::type;
 	requires _Nm < tuple_size_v<_Tp>;
 	typename tuple_element_t<_Nm, _Tp>;
-	// XXX: we applied P3323 here
 	{ std::get<_Nm>(__t) }
 	  -> convertible_to<const tuple_element_t<_Nm, _Tp>&>;
       };
@@ -3451,6 +3450,14 @@ namespace views
     struct tuple_element<1, ranges::subrange<_Iter, _Sent, _Kind>>
     { using type = _Sent; };
 
+  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
+    struct tuple_element<0, const ranges::subrange<_Iter, _Sent, _Kind>>
+    { using type = _Iter; };
+
+  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
+    struct tuple_element<1, const ranges::subrange<_Iter, _Sent, _Kind>>
+    { using type = _Sent; };
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 #endif // library concepts

libstdc++-v3/testsuite/std/ranges/subrange/tuple_like.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>
+
+using S1 = std::ranges::subrange<int*>;
+using S2 = std::ranges::subrange<long*, void*>;
+
+static_assert( std::tuple_size_v<S1> == 2 );
+static_assert( std::tuple_size_v<S2> == 2 );
+
+static_assert( std::same_as<std::tuple_element_t<0, S1>, int*> );
+static_assert( std::same_as<std::tuple_element_t<1, S1>, int*> );
+// LWG 3398
+static_assert( std::same_as<std::tuple_element_t<0, const S1>, int*> );
+static_assert( std::same_as<std::tuple_element_t<1, const S1>, int*> );
+
+static_assert( std::same_as<std::tuple_element_t<0, S2>, long*> );
+static_assert( std::same_as<std::tuple_element_t<1, S2>, void*> );
+// LWG 3398
+static_assert( std::same_as<std::tuple_element_t<0, const S2>, long*> );
+static_assert( std::same_as<std::tuple_element_t<1, const S2>, void*> );
+
+S1 s1;
+static_assert( std::same_as<decltype(std::get<0>(s1)), int*> );
+static_assert( std::same_as<decltype(std::get<1>(s1)), int*> );
+const S1 c1;
+static_assert( std::same_as<decltype(std::get<0>(c1)), int*> );
+static_assert( std::same_as<decltype(std::get<1>(c1)), int*> );
+S2 s2;
+static_assert( std::same_as<decltype(std::get<0>(s2)), long*> );
+static_assert( std::same_as<decltype(std::get<1>(s2)), void*> );
+const S2 c2;
+static_assert( std::same_as<decltype(std::get<0>(c2)), long*> );
+static_assert( std::same_as<decltype(std::get<1>(c2)), void*> );