c++: naming a dependently-scoped template for CTAD [PR104641]

c++: naming a dependently-scoped template for CTAD [PR104641]

In order to be able to perform CTAD for a dependently-scoped template
(such as A<T>::B in the testcase below), we need to permit a
typename-specifier to resolve to a template as per [dcl.type.simple]/3,
at least when it appears in a CTAD-enabled context.

This patch implements this using a new tsubst flag tf_tst_ok to control
when a TYPENAME_TYPE is allowed to name a template, and sets this flag
when substituting into the type of a CAST_EXPR, CONSTRUCTOR or VAR_DECL
(each of which is a CTAD-enabled context).

	PR c++/104641

gcc/cp/ChangeLog:

	* cp-tree.h (tsubst_flags::tf_tst_ok): New flag.
	* decl.cc (make_typename_type): Allow a typename-specifier to
	resolve to a template when tf_tst_ok, in which case return
	a CTAD placeholder for the template.
	* pt.cc (tsubst_decl) <case VAR_DECL>: Set tf_tst_ok when
	substituting the type.
	(tsubst): Clear tf_tst_ok and remember if it was set.
	<case TYPENAME_TYPE>: Pass tf_tst_ok to make_typename_type
	appropriately.
	(tsubst_copy) <case CAST_EXPR>: Set tf_tst_ok when substituting
	the type.
	(tsubst_copy_and_build) <case CAST_EXPR>: Likewise.
	<case CONSTRUCTOR>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction107.C: New test.