PR c++/28058
	* pt.c (register_specialization): Return error_mark_node for
	specialization-after-instantiation.
	* decl2.c (mark_used): Mark the main function used when one of its
	clones is used.
	
	PR c++/28058
	* g++.dg/template/spec31.C: New test.

From-SVN: r116491