C++ PATCH to handling of duplicate typedefs

[Jason Merrill <jason@redhat.com>]

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Another GC problem I noticed while looking at something else: when we 
freed a duplicate typedef, we were leaving its type in the variants 
list, with its TYPE_NAME still pointing to the now-freed TYPE_DECL, 
leading to a crash.

Tested x86_64-pc-linux-gnu, applying to trunk.

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commit e36d7607f157b5c90b56afe22786a2a0ff1711c8
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Nov 11 15:17:42 2015 -0500

    	* decl.c (duplicate_decls): When combining typedefs, remove the
    	new type from the variants list.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 76cc1d1..383b47d 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -2014,7 +2014,22 @@ duplicate_decls (tree newdecl, tree olddecl, bool newdecl_is_friend)
       /* For typedefs use the old type, as the new type's DECL_NAME points
 	 at newdecl, which will be ggc_freed.  */
       if (TREE_CODE (newdecl) == TYPE_DECL)
-	newtype = oldtype;
+	{
+	  newtype = oldtype;
+
+	  /* And remove the new type from the variants list.  */
+	  if (TYPE_NAME (TREE_TYPE (newdecl)) == newdecl)
+	    {
+	      tree remove = TREE_TYPE (newdecl);
+	      for (tree t = TYPE_MAIN_VARIANT (remove); ;
+		   t = TYPE_NEXT_VARIANT (t))
+		if (TYPE_NEXT_VARIANT (t) == remove)
+		  {
+		    TYPE_NEXT_VARIANT (t) = TYPE_NEXT_VARIANT (remove);
+		    break;
+		  }
+	    }
+	}
       else
 	/* Merge the data types specified in the two decls.  */
 	newtype = merge_types (TREE_TYPE (newdecl), TREE_TYPE (olddecl));