c++: CTAD inside alias template [PR91911, PR103672]

c++: CTAD inside alias template [PR91911, PR103672]

In the first testcase below, when processing the alias template
ConstSpanType, transparency of alias template specializations means we
replace SpanType<T> with its instantiated definition.  But this
instantiation lowers the level of the CTAD placeholder for span{T()} from
2 to 1, and so the later instantiation of ConstSpanType<int> erroneously
substitutes this CTAD placeholder with the template argument at level 1
index 0, i.e. with int, before we get a chance to perform the CTAD.

Although we represent CTAD placeholders as template parameters, we never
actually want to replace them via tsubst.  So this patch adjusts tsubst
to handle CTAD placeholders by simply substituting the template and
returning a new CTAD placeholder.  Moreover, this means that the level
of a CTAD placeholder doesn't matter, so we may as well give them all
the same level.  This patch gives them the special level 0.

The change in tsubst_decl removes a likely dead !CHECKING_P safeguard
added in 2017, which would otherwise now get triggered for variables
with CTAD placeholder types (since their level is 0).

	PR c++/91911
	PR c++/103672

gcc/cp/ChangeLog:

	* pt.cc (keep_template_parm): Punt on a level 0 template parm.
	(tsubst_decl) <case VAR_DECL>: Remove !CHECKING_P safeguard.
	(tsubst) <case TEMPLATE_TYPE_PARM>: Handle CTAD placeholders
	specially.
	(make_auto_1): Add defaulted 'level' parameter.
	(make_template_placeholder): Pass 0 as 'level' to make_auto_1.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction101.C: New test.
	* g++.dg/cpp1z/class-deduction101a.C: New test.
	* g++.dg/cpp1z/class-deduction101b.C: New test.
	* g++.dg/cpp1z/class-deduction102.C: New test.
	* g++.dg/cpp1z/class-deduction102a.C: New test.
	* g++.dg/cpp1z/class-deduction102b.C: New test.
	* g++.dg/cpp1z/class-deduction103.C: New test.