libstdc++: Fix regression in std::move algorithm (PR 93872)

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.

3 files changed, 52 insertions, 1 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 5da5c22..f14a6e2 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,11 @@
+2020-02-24  Jonathan Wakely  <jwakely@redhat.com>
+
+	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.
+
 2020-02-24  Patrick Palka  <ppalka@redhat.com>
 
 	PR libstdc++/93884
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index efda15f..c6b7148 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -95,7 +95,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  for(; __num > 0; --__num)
 	    {
 	      if constexpr (_IsMove)
-		*__dst = std::move(*__src);
+		// This const_cast looks unsafe, but we only use this function
+		// for trivially-copyable types, which means this assignment
+		// is trivial and so doesn't alter the source anyway.
+		// See PR 93872 for why it's needed.
+		*__dst = std::move(*const_cast<_Tp*>(__src));
 	      else
 		*__dst = *__src;
 	      ++__src;
diff --git a/libstdc++-v3/testsuite/25_algorithms/move/93872.cc b/libstdc++-v3/testsuite/25_algorithms/move/93872.cc
new file mode 100644
index 0000000..c4dd43d
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/move/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(std::begin(a), std::end(a), std::begin(b));
+}