add warning to Iterators.filter about assumptions on predicate (#50497

)

addresses #50440

base/iterators.jl

@@ -491,6 +491,15 @@ invocation of `filter`. Calls to `flt` will be made when iterating over the
 returned iterable object. These calls are not cached and repeated calls will be
 made when reiterating.
 
+!!! warning
+    Subsequent *lazy* transformations on the iterator returned from `filter`, such
+    as those performed by `Iterators.reverse` or `cycle`, will also delay calls to `flt`
+    until collecting or iterating over the returned iterable object. If the filter
+    predicate is nondeterministic or its return values depend on the order of iteration
+    over the elements of `itr`, composition with lazy transformations may result in
+    surprising behavior. If this is undesirable, either ensure that `flt` is a pure
+    function or collect intermediate `filter` iterators before further transformations.
+
 See [`Base.filter`](@ref) for an eager implementation of filtering for arrays.
 
 # Examples