c++: tf_partial and alias_ctad_tweaks [PR122621]HEADtrunkmaster

Like in r15-6740-g27d620d6769715 for instantiate_template with dependent
arguments, we also need to set tf_partial during the alias_ctad_tweaks
transformation mainly for benefit of properly handling extra-args trees.

In this testcase during alias_ctad_tweaks we substitute the dependent
ElemTs={Tuple<Us...>}, Ts={Ts...} into the requires-clause, which
tsubst_pack_expansion decides to defer via extra-args, and for the
subsequent add_extra_args (during guide overload resolution) to merge
the deferred dependent arguments correctly, tf_partial has to have been
set.

	PR c++/122621

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_expansion): Use tf_partial instead of
	tf_none, except when substituting outer (non-dependent) template
	arguments.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias26.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

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