[patch] Fix libstdc++/56112, inserting non-pairs into std::unordered_map

[Jonathan Wakely]

This is a much simpler fix than discussed in bugzilla, just forward to
_M_emplace() when inserting from a type that needs conversion to
value_type.  This also fixes the dangling reference problem as well as
the regression with calling insert() with convertible types.

       PR libstdc++/56112
       * include/bits/hashtable_policy.h (insert(_Pair&&)): Use _M_emplace
       to construct value_type explicitly before trying to extract the key.
       * testsuite/23_containers/unordered_map/cons/56112.cc: New.

Tested x86_64-linux, will commit to trunk tomorrow.

N.B. The dangling reference bug is present on the 4.6 and 4.7
branches, but not a regression.

commit ca741fccc5d052db08ba839bd45ece7564ac5004
Author: Jonathan Wakely <jwakely....@gmail.com>
Date:   Mon Jan 28 00:08:43 2013 +0000

        PR libstdc++/56112
        * include/bits/hashtable_policy.h (insert(_Pair&&)): Use _M_emplace
        to construct value_type explicitly before trying to extract the key.
        * testsuite/23_containers/unordered_map/cons/56112.cc: New.

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h 
b/libstdc++-v3/include/bits/hashtable_policy.h
index 023f46d..1ade3f9 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -836,7 +836,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        insert(_Pair&& __v)
        {
          __hashtable& __h = this->_M_conjure_hashtable();
-         return __h._M_insert(std::forward<_Pair>(__v), __unique_keys());
+         return __h._M_emplace(__unique_keys(), std::forward<_Pair>(__v));
        }
 
       template<typename _Pair, typename = _IFconsp<_Pair>>
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_map/cons/56112.cc 
b/libstdc++-v3/testsuite/23_containers/unordered_map/cons/56112.cc
new file mode 100644
index 0000000..c297ef7
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/cons/56112.cc
@@ -0,0 +1,49 @@
+// { dg-options "-std=gnu++0x" }
+
+// Copyright (C) 2013 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/>.
+
+#include <unordered_map>
+#include <utility>
+
+struct Key
+{
+  explicit Key(const int* p) : value(p) { }
+  ~Key() { value = nullptr; }
+
+  bool operator==(const Key& k) const { return *value == *k.value; }
+
+  const int* value;
+};
+
+struct hash
+{
+  std::size_t operator()(const Key& k) const noexcept { return *k.value; }
+};
+
+struct S
+{
+  int value;
+  operator std::pair<const Key, int>() const { return {Key(&value), value}; }
+};
+
+int main()
+{
+    S s[1] = { {2} };
+    std::unordered_map<Key, int, hash> m(s, s+1);
+    std::unordered_multimap<Key, int, hash> mm(s, s+1);
+}