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