c++: parameter pack inside constexpr if [PR101764]

c++: parameter pack inside constexpr if [PR101764]

Here when partially instantiating the first pack expansion, substitution
into the condition of the constexpr if yields a still-dependent tree, so
tsubst_expr returns an IF_STMT with an unsubstituted IF_COND and with
IF_STMT_EXTRA_ARGS added to.  Hence after partial instantiation the pack
expansion pattern still refers to the unlowered parameter pack 'ts' of
level 2, and it's thusly recorded in the new PACK_EXPANSION_PARAMETER_PACKS.
During the subsequent final instantiation of the regenerated lambda we
crash in tsubst_pack_expansion because it can't find an argument pack
for this unlowered 'ts', due to the level mismatch.  (Likewise when the
constexpr if is replaced by a requires-expr, which also uses the extra
args mechanism for avoiding partial instantiation.)

So essentially, a pack expansion pattern that contains an "extra args"
tree doesn't play well with partial instantiation.  This patch fixes
this by forcing such pack expansions to use the extra args mechanism as
well.

	PR c++/101764

gcc/cp/ChangeLog:

	* cp-tree.h (PACK_EXPANSION_FORCE_EXTRA_ARGS_P): New accessor
	macro.
	* pt.c (has_extra_args_mechanism_p): New function.
	(find_parameter_pack_data::found_extra_args_tree_p): New data
	member.
	(find_parameter_packs_r): Set ppd->found_extra_args_tree_p
	appropriately.
	(make_pack_expansion): Set PACK_EXPANSION_FORCE_EXTRA_ARGS_P if
	ppd.found_extra_args_tree_p.
	(use_pack_expansion_extra_args_p): Return true if there were
	unsubstituted packs and PACK_EXPANSION_FORCE_EXTRA_ARGS_P.
	(tsubst_pack_expansion): Pass the pack expansion to
	use_pack_expansion_extra_args_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-if35.C: New test.