c++: NTTP constraint depending on outer parms [PR109160]

c++: NTTP constraint depending on outer parms [PR109160]

Here we're crashing during satisfaction for the NTTP 'C<B> auto V'
ultimately because convert_template_argument / unify don't pass all
outer template arguments to do_auto_deduction, and during satisfaction
we need to know all arguments.  While these callers do pass some outer
arguments, they are only sufficient to properly substitute the
(level-lowered) 'auto' and are not necessarily the entire set.

Fortunately it seems these callers have access to the full set of outer
arguments via convert_template_argument's 'in_decl' parameter and
unify's 'tparms' parameter.  So this patch adds a new parameter to
do_auto_deduction, used only during adc_unify deduction, through which
these callers can pass the enclosing (partially instantiated) template
and from which do_auto_deduction can obtain _all_ outer template
arguments for sake of satisfaction.

This patch also ensures that the 'in_decl' argument passed to
coerce_template_parms is always a TEMPLATE_DECL, which in turn allows us
to pass it as-is to do_auto_deduction; the only coerce_template_parms
caller that needed adjustment was tsubst_decl it seems.

	PR c++/109160

gcc/cp/ChangeLog:

	* cp-tree.h (do_auto_deduction): Add defaulted tmpl parameter.
	* pt.cc (convert_template_argument): Pass 'in_decl' as 'tmpl' to
	do_auto_deduction.
	(tsubst_decl) <case VAR_/TYPE_DECL>: Pass 'tmpl' instead of 't' as
	'in_decl' to coerce_template_parms.
	(unify) <case TEMPLATE_PARM_INDEX>: Pass TPARMS_PRIMARY_TEMPLATE
	as 'tmpl' to do_auto_deduction.
	(do_auto_deduction): Document default arguments.  Rename local
	variable 'tmpl' to 'ctmpl'.  Use 'tmpl' to obtain a full set of
	template arguments for satisfaction in the adc_unify case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-placeholder12.C: New test.