PR c++/18652
	* name-lookup.c (pushtag): Change return type to tree.
	* cp-tree.h (pushtag): Adjust declaration.
	* decl.c (xref_tag, start_enum): Use return value of pushtag.
	* pt.c (push_template_decl_real): Return immediately if
	pushdecl_namespace_level returns error_mark_node.

	* g++.dg/lookup/crash6.C: New test.

From-SVN: r91470