Simplify std::rethrow_if_nested definition

diff --git a/libstdc++-v3/doc/xml/manual/intro.xml b/libstdc++-v3/doc/xml/manual/intro.xml
index 265ef67..22b792a 100644
--- a/libstdc++-v3/doc/xml/manual/intro.xml
+++ b/libstdc++-v3/doc/xml/manual/intro.xml
@@ -1078,6 +1078,14 @@  requirements of the license of GCC.
     <listitem><para>Divide by the object type.
     </para></listitem></varlistentry>
 
+    <varlistentry><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="../ext/lwg-defects.html#2484">2484</link>:
+       <emphasis><code>rethrow_if_nested()</code>  is doubly unimplementable
+       </emphasis>
+    </term>
+    <listitem><para>Avoid using <code>dynamic_cast</code> when it would be
+    ill-formed.
+    </para></listitem></varlistentry>
+
     <varlistentry><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="../ext/lwg-defects.html#2583">2583</link>:
        <emphasis>There is no way to supply an allocator for <code> basic_string(str, pos)</code>
        </emphasis>
diff --git a/libstdc++-v3/libsupc++/nested_exception.h b/libstdc++-v3/libsupc++/nested_exception.h
index 377b803..0c00d74 100644
--- a/libstdc++-v3/libsupc++/nested_exception.h
+++ b/libstdc++-v3/libsupc++/nested_exception.h
@@ -92,41 +92,21 @@  namespace std
       { }
     };
 
-  template<typename _Tp,
-	   bool __with_nested = !__is_base_of(nested_exception, _Tp)>
-    struct _Throw_with_nested_impl
+  // [except.nested]/8
+  // Throw an exception of unspecified type that is publicly derived from
+  // both remove_reference_t<_Tp> and nested_exception.
+  template<typename _Tp>
+    inline void
+    __throw_with_nested_impl(_Tp&& __t, true_type)
     {
-      template<typename _Up>
-	static void _S_throw(_Up&& __t)
-	{ throw _Nested_exception<_Tp>{static_cast<_Up&&>(__t)}; }
-    };
+      using _Up = typename remove_reference<_Tp>::type;
+      throw _Nested_exception<_Up>{std::forward<_Tp>(__t)};
+    }
 
   template<typename _Tp>
-    struct _Throw_with_nested_impl<_Tp, false>
-    {
-      template<typename _Up>
-	static void _S_throw(_Up&& __t)
-	{ throw static_cast<_Up&&>(__t); }
-    };
-
-  template<typename _Tp, bool = __is_class(_Tp) && !__is_final(_Tp)>
-    struct _Throw_with_nested_helper : _Throw_with_nested_impl<_Tp>
-    { };
-
-  template<typename _Tp>
-    struct _Throw_with_nested_helper<_Tp, false>
-    : _Throw_with_nested_impl<_Tp, false>
-    { };
-
-  template<typename _Tp>
-    struct _Throw_with_nested_helper<_Tp&, false>
-    : _Throw_with_nested_helper<_Tp>
-    { };
-
-  template<typename _Tp>
-    struct _Throw_with_nested_helper<_Tp&&, false>
-    : _Throw_with_nested_helper<_Tp>
-    { };
+    inline void
+    __throw_with_nested_impl(_Tp&& __t, false_type)
+    { throw std::forward<_Tp>(__t); }
 
   /// If @p __t is derived from nested_exception, throws @p __t.
   /// Else, throws an implementation-defined object derived from both.
@@ -135,33 +115,43 @@  namespace std
     inline void
     throw_with_nested(_Tp&& __t)
     {
-      _Throw_with_nested_helper<_Tp>::_S_throw(static_cast<_Tp&&>(__t));
+      using _Up = typename remove_reference<_Tp>::type;
+      using _CopyConstructible
+	= __and_<is_copy_constructible<_Up>, is_move_constructible<_Up>>;
+      static_assert(_CopyConstructible::value,
+	  "throw_with_nested argument must be CopyConstructible");
+      using __nest = __and_<is_class<_Up>, __bool_constant<!__is_final(_Up)>,
+			    __not_<is_base_of<nested_exception, _Up>>>;
+      return std::__throw_with_nested_impl(std::forward<_Tp>(__t), __nest{});
     }
 
-  template<typename _Tp, bool = __is_polymorphic(_Tp)>
-    struct _Rethrow_if_nested_impl
-    {
-      static void _S_rethrow(const _Tp& __t)
-      {
-	if (auto __tp =
-            dynamic_cast<const nested_exception*>(std::__addressof(__t)))
-	  __tp->rethrow_nested();
-      }
-    };
-
+  // Determine if dynamic_cast<const nested_exception&> would be well-formed.
   template<typename _Tp>
-    struct _Rethrow_if_nested_impl<_Tp, false>
+    using __rethrow_if_nested_cond = typename enable_if<
+      __and_<is_polymorphic<_Tp>,
+	     __or_<__not_<is_base_of<nested_exception, _Tp>>,
+		   is_convertible<_Tp*, nested_exception*>>>::value
+    >::type;
+
+  // Attempt dynamic_cast to nested_exception and call rethrow_nested().
+  template<typename _Ex>
+    inline __rethrow_if_nested_cond<_Ex>
+    __rethrow_if_nested_impl(const _Ex* __ptr)
     {
-      static void _S_rethrow(const _Tp&) { }
-    };
+      if (auto __ne_ptr = dynamic_cast<const nested_exception*>(__ptr))
+	__ne_ptr->rethrow_nested();
+    }
+
+  // Otherwise, no effects.
+  inline void
+  __rethrow_if_nested_impl(const void*)
+  { }
 
   /// If @p __ex is derived from nested_exception, @p __ex.rethrow_nested().
   template<typename _Ex>
     inline void
     rethrow_if_nested(const _Ex& __ex)
-    {
-      _Rethrow_if_nested_impl<_Ex>::_S_rethrow(__ex);
-    }
+    { std::__rethrow_if_nested_impl(std::__addressof(__ex)); }
 
   // @} group exceptions
 } // namespace std
diff --git a/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested.cc b/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested.cc
index 135679f..7751213 100644
--- a/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested.cc
+++ b/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested.cc
@@ -101,11 +101,30 @@  void test03()
   VERIFY( test );
 }
 
+void
+test04()
+{
+  // LWG 2484 requires that these cases are well-formed, but don't rethrow.
+
+  std::rethrow_if_nested(1);
+
+  struct S { } nonpolymorphic;
+  std::rethrow_if_nested(nonpolymorphic);
+
+  struct derived3 : derived, derived2 { };
+  derived3 ambiguous_base;
+  std::rethrow_if_nested(ambiguous_base);
+
+  struct derived4 : private std::nested_exception { };
+  derived4 private_base;
+  std::rethrow_if_nested(private_base);
+}
 
 int main()
 {
   test01();
   test02();
   test03();
+  test04();
   return 0;
 }