Document newest react safety fix

Sudha247 · Nov 2, 2021 · caa590f · caa590f
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diff --git a/CHANGES b/CHANGES
@@ -12,6 +12,9 @@
     the behavior of Lwt_io.with_temp_dir following symlinks to
     directories on Win32. (#883, Antonin Décimo)
 
+  * Lwt_react.S.l[2-6]_s used polymorphic equality which could cause errors when
+    handling functional values. (#893, Jérôme Vouillon)
+
 ====== Additions ======
 
   * Lwt_bytes.blit_from_string: string complement of Lwt_bytes.blit (#882, Hugo Heuzard).

diff --git a/src/react/lwt_react.cppo.ml b/src/react/lwt_react.cppo.ml
@@ -455,6 +455,8 @@ module S = struct
     map_s ?eq f s1
 
   let l2_s ?eq f s1 s2 =
+    (* Some details about the use of [fun _ _ -> false] on
+       https://github.com/ocsigen/lwt/pull/893#pullrequestreview-783083496 *)
     map_s ?eq (fun (x1, x2) -> f x1 x2) (l2 ~eq:(fun _ _ -> false) (fun x1 x2 -> (x1, x2)) s1 s2)
 
   let l3_s ?eq f s1 s2 s3 =