libstdc++: Make structured bindings always work for subranges [PR 97512]

The definition of ranges::subrange was moved to the new
<bits/ranges_util.h> header so that it could be used in <algorithm>
without including the whole of <ranges>. However, the tuple-like support
that enables subrange to be used with structured bindings was left in
<ranges>. This is arguably conforming (to use a subrange you should
include <ranges>) but it's inconvenient and probably confusing.

This change makes the tuple-like support available whenever subrange
itself is available.

libstdc++-v3/ChangeLog:

	PR libstdc++/97512
	* include/bits/ranges_util.h (tuple_size<subrange>)
	(tuple_element<I, cv subrange>): Move here from ...
	* include/std/ranges: ... here.
	* testsuite/std/ranges/subrange/97512.cc: New test.

3 files changed, 54 insertions, 21 deletions

diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index a98658f..cc50e2a 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -410,6 +410,27 @@ namespace ranges
 
   using ranges::get;
 
+  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
+    struct tuple_size<ranges::subrange<_Iter, _Sent, _Kind>>
+    : integral_constant<size_t, 2>
+    { };
+
+  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
+    struct tuple_element<0, ranges::subrange<_Iter, _Sent, _Kind>>
+    { using type = _Iter; };
+
+  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
+    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 std
 #endif // library concepts
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index b03ae39..4d3a494 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3320,27 +3320,6 @@ namespace views
 
   namespace views = ranges::views;
 
-  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
-    struct tuple_size<ranges::subrange<_Iter, _Sent, _Kind>>
-    : integral_constant<size_t, 2>
-    { };
-
-  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
-    struct tuple_element<0, ranges::subrange<_Iter, _Sent, _Kind>>
-    { using type = _Iter; };
-
-  template<typename _Iter, typename _Sent, ranges::subrange_kind _Kind>
-    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
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/97512.cc b/libstdc++-v3/testsuite/std/ranges/subrange/97512.cc
new file mode 100644
index 0000000..b0fd0e2
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/97512.cc
@@ -0,0 +1,33 @@
+// 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 } }
+
+// PR libstdc++/97512
+// Check that structured bindings work for subranges without <ranges>.
+#include <algorithm>
+
+constexpr bool
+test01()
+{
+  int r[] = { 1, 2, 2, 3, 3, 3 };
+  auto [first, last] = std::ranges::unique(r);
+  return first == std::ranges::begin(r) + 3 && last == std::ranges::end(r);
+}
+
+static_assert( test01() );