patch 8.2.2449: Vim9: flatten() always changes the list type
Problem: Vim9: flatten() always changes the list type.
Solution: Disallow using flatten() and add flattennew().
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 965a977..98e9a10 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -665,6 +665,7 @@
count() count number of times a value appears in a List
repeat() repeat a List multiple times
flatten() flatten a List
+ flattennew() flatten a copy of a List
Dictionary manipulation: *dict-functions*
get() get an entry without an error for a wrong key