[patch] libstdc++/56383 Fix ambiguity with multiple enable_shared_from_this bases

[Jonathan Wakely <jwakely at redhat dot com>]

The friend function defined in enable_shared_from_this did not match
the declaration at namespace scope, so instead of defining the
previously declared function it added a new overload, which was always
a better match than the declared (but never defined) one.

That worked fine for a single base class, the friend function got
found by ADL, but with two enable_shared_from_this base classes the
two friend overloads were ambiguous.

This changes the friend to match the declaration, so that it can only
be called when an unambiguous enable_shared_from_this base can be
deduced, and so fails silently (as it is supposed to) when there is
not an unambiguous base class.

Tested powerpc64le-linux, committed to trunk.

This fix is simple enough that I'm going to backport it after 5.3 is
released.

commit 1fa7fd2d7699c41f147b42ae96f70bf7b9e8e2d6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 2 14:43:34 2015 +0000

    Fix ambiguity with multiple enable_shared_from_this bases
    
    	PR libstdc++/56383
    	* testsuite/20_util/enable_shared_from_this/56383.cc: New.
    	* include/bits/shared_ptr_base.h (__enable_shared_from_this): Make
    	friend declaration match previous declaration of
    	__enable_shared_from_this_helper.
    	* include/bits/shared_ptr.h (enable_shared_from_this): Likewise.

diff --git a/libstdc++-v3/include/bits/shared_ptr.h b/libstdc++-v3/include/bits/shared_ptr.h
index 2413b1b..26a0ad3 100644
--- a/libstdc++-v3/include/bits/shared_ptr.h
+++ b/libstdc++-v3/include/bits/shared_ptr.h
@@ -582,19 +582,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
 	{ _M_weak_this._M_assign(__p, __n); }
 
-      template<typename _Tp1>
+      template<typename _Tp1, typename _Tp2>
 	friend void
-	__enable_shared_from_this_helper(const __shared_count<>& __pn,
-					 const enable_shared_from_this* __pe,
-					 const _Tp1* __px) noexcept
-	{
-	  if (__pe != nullptr)
-	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
-	}
+	__enable_shared_from_this_helper(const __shared_count<>&,
+					 const enable_shared_from_this<_Tp1>*,
+					 const _Tp2*) noexcept;
 
       mutable weak_ptr<_Tp>  _M_weak_this;
     };
 
+  template<typename _Tp1, typename _Tp2>
+    inline void
+    __enable_shared_from_this_helper(const __shared_count<>& __pn,
+				     const enable_shared_from_this<_Tp1>*
+				     __pe, const _Tp2* __px) noexcept
+    {
+      if (__pe != nullptr)
+	__pe->_M_weak_assign(const_cast<_Tp2*>(__px), __pn);
+    }
+
   /**
    *  @brief  Create an object that is owned by a shared_ptr.
    *  @param  __a     An allocator.
diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 1a96b4c..f4f98e6 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -1546,19 +1546,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
 	{ _M_weak_this._M_assign(__p, __n); }
 
-      template<typename _Tp1>
+      template<_Lock_policy _Lp1, typename _Tp1, typename _Tp2>
 	friend void
-	__enable_shared_from_this_helper(const __shared_count<_Lp>& __pn,
-					 const __enable_shared_from_this* __pe,
-					 const _Tp1* __px) noexcept
-	{
-	  if (__pe != nullptr)
-	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
-	}
+	__enable_shared_from_this_helper(const __shared_count<_Lp1>&,
+					 const __enable_shared_from_this<_Tp1,
+					 _Lp1>*, const _Tp2*) noexcept;
 
       mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
     };
 
+  template<_Lock_policy _Lp1, typename _Tp1, typename _Tp2>
+    inline void
+    __enable_shared_from_this_helper(const __shared_count<_Lp1>& __pn,
+				     const __enable_shared_from_this<_Tp1,
+				     _Lp1>* __pe,
+				     const _Tp2* __px) noexcept
+    {
+      if (__pe != nullptr)
+	__pe->_M_weak_assign(const_cast<_Tp2*>(__px), __pn);
+    }
 
   template<typename _Tp, _Lock_policy _Lp, typename _Alloc, typename... _Args>
     inline __shared_ptr<_Tp, _Lp>
diff --git a/libstdc++-v3/testsuite/20_util/enable_shared_from_this/56383.cc b/libstdc++-v3/testsuite/20_util/enable_shared_from_this/56383.cc
new file mode 100644
index 0000000..ea0f28d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/enable_shared_from_this/56383.cc
@@ -0,0 +1,56 @@
+// 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 A : std::enable_shared_from_this<A>
+{
+    void* a() { return shared_from_this().get(); }
+};
+
+struct B : std::enable_shared_from_this<B>
+{
+};
+
+struct D : A, B
+{
+};
+
+void test01()
+{
+  bool test = false;
+
+  auto d = std::make_shared<D>();
+  try
+  {
+      d->a();
+  }
+  catch (const std::bad_weak_ptr&)
+  {
+    test = true;
+  }
+  VERIFY(test);
+}
+
+int
+main()
+{
+  test01();
+}