[PATCH] libstdc++: LWG 3313 join_view::iterator::operator-- is incorrectly constrained

[Patrick Palka]

libstdc++-v3/ChangeLog:

        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.
        * testsuite/std/ranges/adaptors/lwg3313_neg.cc: New test.
---
 libstdc++-v3/include/std/ranges               |  2 +
 .../std/ranges/adaptors/lwg3313_neg.cc        | 40 +++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/adaptors/lwg3313_neg.cc

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index aed90e9710e..ffdd3367b05 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2399,6 +2399,7 @@ namespace views
          operator--()
            requires _S_ref_is_glvalue && bidirectional_range<_Base>
              && bidirectional_range<range_reference_t<_Base>>
+             && common_range<range_reference_t<_Base>>
          {
            if (_M_outer == ranges::end(_M_parent->_M_base))
              _M_inner = ranges::end(*--_M_outer);
@@ -2412,6 +2413,7 @@ namespace views
          operator--(int)
            requires _S_ref_is_glvalue && bidirectional_range<_Base>
              && bidirectional_range<range_reference_t<_Base>>
+             && common_range<range_reference_t<_Base>>
          {
            auto __tmp = *this;
            --*this;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/lwg3313_neg.cc 
b/libstdc++-v3/testsuite/std/ranges/adaptors/lwg3313_neg.cc
new file mode 100644
index 00000000000..054df92554b
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/lwg3313_neg.cc
@@ -0,0 +1,40 @@
+// 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 <array>
+#include <iterator>
+#include <ranges>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+void
+test01()
+{
+  int x[] = {1};
+  auto rx = ranges::subrange{std::counted_iterator(x,1), 
std::default_sentinel};
+  std::array<decltype(rx), 5> y = {rx, rx, rx, rx, rx,};
+  auto v = y | views::join;
+  auto i = v.begin();
+  ++i;
+  ++i;
+  --i; // { dg-error "no match" }
+  i--; // { dg-error "" }
+}
-- 
2.25.1.291.ge68e29171c