Re: [committed] libstdc++: Fix regression in std::move algorithm (PR 93872)

[Jonathan Wakely]

On 25/02/20 12:40 +0000, Jonathan Wakely wrote:

The std::move and std::move_backward algorithms dispatch to the
std::__memmove helper when appropriate. That function uses a
pointer-to-const for the source values, preventing them from being
moved. The two callers of that function have the same problem.

Rather than altering __memmove and its callers to work with const or
non-const source pointers, this takes a more conservative approach of
casting away the const at the point where we want to do a move
assignment. This relies on the fact that we only use __memmove when the
type is trivially copyable, so we know the move assignment doesn't alter
the source anyway.

        PR libstdc++/93872
        * include/bits/stl_algobase.h (__memmove): Cast away const before
        doing move assignment.
        * testsuite/25_algorithms/move/93872.cc: New test.
        * testsuite/25_algorithms/move_backward/93872.cc: New test.

Oops, I forgot to 'git add' one of the new tests.

Tested x86_64-linux, committed to master.


commit 6de946e65c9558515a2c31be76853ae7653d4fc9
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Feb 25 12:42:03 2020 +0000

    libstdc++: Add test accidentally left out of previous commit
    
            * testsuite/25_algorithms/move_backward/93872.cc: Add test left out of
            previous commit.

diff --git a/libstdc++-v3/testsuite/25_algorithms/move_backward/93872.cc b/libstdc++-v3/testsuite/25_algorithms/move_backward/93872.cc
new file mode 100644
index 00000000000..69774fa86c6
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/move_backward/93872.cc
@@ -0,0 +1,39 @@
+// 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 <algorithm>
+
+struct X
+{
+  X() = default;
+
+  X(const X&) = delete;
+  X& operator=(const X&) = delete;
+
+  X(X&&) = default;
+  X& operator=(X&&) = default;
+};
+
+void
+test01()
+{
+  X a[2], b[2];
+  std::move_backward(std::begin(a), std::end(a), std::begin(b));
+}