re PR c++/84843 (C++ ICE on builtin redefinition since r258391)

	PR c++/84843
	* decl.c (duplicate_decls): For redefinition of built-in, use error
	and return error_mark_node.  For redeclaration, return error_mark_node
	rather than olddecl if !flag_permissive.

	* g++.dg/ext/pr84843-1.C: New test.
	* g++.dg/ext/pr84843-2.C: New test.

From-SVN: r258503

gcc/cp/ChangeLog

+2018-03-13  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/84843
+	* decl.c (duplicate_decls): For redefinition of built-in, use error
+	and return error_mark_node.  For redeclaration, return error_mark_node
+	rather than olddecl if !flag_permissive.
+
 2018-03-13  Jason Merrill  <jason@redhat.com>
 
 	PR c++/82565 - ICE with concepts and generic lambda.

gcc/cp/decl.c

@@ -1583,13 +1583,20 @@ next_arg:;
 			  || memcmp (name + len - strlen ("_chk"),
 				     "_chk", strlen ("_chk") + 1) != 0))
 		    {
+		      if (DECL_INITIAL (newdecl))
+			{
+			  error_at (DECL_SOURCE_LOCATION (newdecl),
+				    "definition of %q#D ambiguates built-in "
+				    "declaration %q#D", newdecl, olddecl);
+			  return error_mark_node;
+			}
 		      if (permerror (DECL_SOURCE_LOCATION (newdecl),
 				     "new declaration %q#D ambiguates built-in"
 				     " declaration %q#D", newdecl, olddecl)
 			  && flag_permissive)
 			inform (DECL_SOURCE_LOCATION (newdecl),
 				"ignoring the %q#D declaration", newdecl);
-		      return olddecl;
+		      return flag_permissive ? olddecl : error_mark_node;
 		    }
 		}
 

gcc/testsuite/ChangeLog

+2018-03-13  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/84843
+	* g++.dg/ext/pr84843-1.C: New test.
+	* g++.dg/ext/pr84843-2.C: New test.
+
 2018-03-13  David Pagan  <dave.pagan@oracle.com>
 
 	PR c/46921

gcc/testsuite/g++.dg/ext/pr84843-1.C

+// PR c++/84843
+// { dg-do compile }
+// { dg-options "-fpermissive" }
+
+extern "C" int
+__atomic_compare_exchange (int x, int y)	// { dg-error "ambiguates built-in declaration" }
+{
+  return x + y;
+}

gcc/testsuite/g++.dg/ext/pr84843-2.C

+// PR c++/84843
+// { dg-do compile }
+// { dg-options "" }
+
+extern "C" int
+__atomic_compare_exchange (int x, int y)	// { dg-error "ambiguates built-in declaration" }
+{
+  return x + y;
+}