Only set std::enable_shared_from_this member once.

	* include/bits/shared_ptr.h (__enable_shared_from_this_helper): Use
	nullptr.
	* include/bits/shared_ptr_base.h (weak_ptr::_M_assign): Don't assign
	if ownership is already shared with a shared_ptr object.
	(__enable_shared_from_this_helper): Use nullptr.
	* testsuite/20_util/enable_shared_from_this/members/const.cc: New.
	* testsuite/20_util/enable_shared_from_this/members/reinit.cc: New.
	* testsuite/20_util/enable_shared_from_this/requirements/
	explicit_instantiation.cc: Instantiate with const and incomplete types.

From-SVN: r227232

libstdc++-v3/ChangeLog

 2015-08-26  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/shared_ptr.h (__enable_shared_from_this_helper): Use
+	nullptr.
+	* include/bits/shared_ptr_base.h (weak_ptr::_M_assign): Don't assign
+	if ownership is already shared with a shared_ptr object.
+	(__enable_shared_from_this_helper): Use nullptr.
+	* testsuite/20_util/enable_shared_from_this/members/const.cc: New.
+	* testsuite/20_util/enable_shared_from_this/members/reinit.cc: New.
+	* testsuite/20_util/enable_shared_from_this/requirements/
+	explicit_instantiation.cc: Instantiate with const and incomplete types.
+
 	* include/debug/map.h (map::try_emplace, map::insert_or_assign):
 	Define.
 	* include/debug/unordered_map (unordered_map::try_emplace,

libstdc++-v3/include/bits/shared_ptr.h

@@ -588,7 +588,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					 const enable_shared_from_this* __pe,
 					 const _Tp1* __px) noexcept
 	{
-	  if (__pe != 0)
+	  if (__pe != nullptr)
 	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 	}
 

libstdc++-v3/include/bits/shared_ptr_base.h

@@ -1468,9 +1468,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
       {
+	if (use_count() == 0)
+	  {
 	    _M_ptr = __ptr;
 	    _M_refcount = __refcount;
 	  }
+      }
 
       template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
       template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
@@ -1549,7 +1552,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					 const __enable_shared_from_this* __pe,
 					 const _Tp1* __px) noexcept
 	{
-	  if (__pe != 0)
+	  if (__pe != nullptr)
 	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 	}
 

libstdc++-v3/testsuite/20_util/enable_shared_from_this/members/const.cc

+// Copyright (C) 2015 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++11" }
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+template<typename T, typename U>
+  bool
+  share_ownership(const std::shared_ptr<T>& p1, const std::shared_ptr<U>& p2)
+  {
+    return !p1.owner_before(p2) && !p2.owner_before(p1);
+  }
+
+void
+test01()
+{
+  struct X : public std::enable_shared_from_this<X> { };
+  using CX = const X;
+  std::shared_ptr<CX> p(new X);
+  VERIFY( share_ownership(p->shared_from_this(), p) );
+  p.reset(new CX);
+  VERIFY( share_ownership(p->shared_from_this(), p) );
+  auto p2 = std::const_pointer_cast<X>(p)->shared_from_this();
+  VERIFY( share_ownership(p2, p) );
+}
+
+void
+test02()
+{
+  struct X;
+  using CX = const X;
+  struct X : public std::enable_shared_from_this<CX> { };
+  std::shared_ptr<CX> p(new X);
+  VERIFY( share_ownership(p->shared_from_this(), p) );
+  p.reset(new CX);
+  VERIFY( share_ownership(p->shared_from_this(), p) );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/20_util/enable_shared_from_this/members/reinit.cc

+// Copyright (C) 2015 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++11" }
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+struct X : public std::enable_shared_from_this<X> { };
+
+bool
+share_ownership(const std::shared_ptr<X>& p1, const std::shared_ptr<X>& p2)
+{
+  return !p1.owner_before(p2) && !p2.owner_before(p1);
+}
+
+void
+test01()
+{
+  std::shared_ptr<X> p1(new X);
+  VERIFY( share_ownership( p1->shared_from_this(), p1 ) );
+  {
+    std::shared_ptr<X> p2(p1.get(), [](X*){});
+    // The weak_ptr member of the enable_shared_from_this base should not
+    // be reset by creating a second control block that owns the pointer.
+    VERIFY( share_ownership( p2->shared_from_this(), p1 ) );
+  }
+  VERIFY( share_ownership( p1->shared_from_this(), p1 ) );
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/20_util/enable_shared_from_this/requirements/explicit_instantiation.cc

@@ -21,3 +21,8 @@
 #include <memory>
 
 template class std::enable_shared_from_this<int>;
+template class std::enable_shared_from_this<const int>;
+
+class Incomplete;
+template class std::enable_shared_from_this<Incomplete>;
+template class std::enable_shared_from_this<const Incomplete>;