Fix PR c++/42225, take 2

Fix PR c++/42225, take 2

gcc/cp/ChangeLog:
	PR c++/42225
	* pt.c (push_template_decl_real): Set DECL_CONTEXT of template type
	parms to their containing template decl.
	* typeck.c (comp_template_parms_position): Split this from
	structural_comptypes.
	(incompatible_template_type_parms_p): Renamed
	incompatible_dependent_typedefs_p into this. Change the function to
	handle comparison between TEMPLATE_TYPE_PARMs only.
	(structural_comptypes): Use comp_template_parms_position in
	TEMPLATE_TEMPLATE_PARM and BOUND_TEMPLATE_TEMPLATE_PARM cases.
	Use incompatible_template_type_parms_p in TEMPLATE_TYPE_PARM case.
	* mangle.c (decl_mangling_context): Template type parms don't have
	a mangling context.
	* tree.c (cp_set_underlying_type): Set type structural equality
	only for TEMPLATE_TYPE_PARMs.

gcc/testsuite/ChangeLog:
	PR c++/42225
	* g++.dg/template/typedef26.C: New test.

From-SVN: r155363