-- main.mcl --
include foo(struct{
	str0 => ["A", "B",],
})

class foo($st1) {
	$str1 = $st1->str0

	include bar(struct{
		str2 => $str1,
	})
}

class bar($st2) {
	# Tricky because LHS and optional arg are both strings! Make sure to
	# check that this type matches what's in the incoming struct in case
	# that field in the struct exists.
	#$str3 = _struct_lookup_optional($st2, "str2", "C")
	$str3 = $st2->str2 || "C" # should not unify

	test ["str: ${str3}",] {}
}
-- OUTPUT --
# err: errUnify: error setting type: func() { <built-in:_struct_lookup_optional> }, error: non-optional arg must match return type: base kind does not match (str != list)