Fix PR c++/41863

gcc/cp/ChangeLog:

	PR c++/41863
	* pt.c (iterative_hash_template_arg): articifial parms
	don't have DECL_PARM_INDEX set. Do not hash it.

gcc/testsuite/ChangeLog:

	PR c++/41863
	* g++.dg/template/sizeof12.C: New test.

From-SVN: r153735