typing: fix a try_expand_once forgotten from ocaml#10170

Reported-by: Neven Villani <vanille@crans.org>

(cherry picked from commit 2d7053b)

Changes

@@ -39,6 +39,10 @@ OCaml 4.14 maintenance version
 - #12878: fix incorrect treatment of injectivity for private recursive types.
   (Jeremy Yallop, review by Gabriel Scherer and Jacques Garrigue)
 
+- #12971, #12974: fix an uncaught Ctype.Escape exception on some
+  invalid programs forming recursive types.
+  (Gabriel Scherer, review by Florian Angeletti, report by Neven Villani)
+
 - #12264, #12289: Fix compact_allocate to avoid a pathological case
   that causes very slow compaction.
   (Damien Doligez, report by Arseniy Alekseyev, review by Sadiq Jaffer)

typing/ctype.ml

@@ -1746,7 +1746,7 @@ let rec occur_rec env allow_recursive visited ty0 ty =
         let visited = TypeSet.add ty visited in
         iter_type_expr (occur_rec env allow_recursive visited ty0) ty
       with Occur -> try
-        let ty' = try_expand_head try_expand_once env ty in
+        let ty' = try_expand_head try_expand_safe env ty in
         (* This call used to be inlined, but there seems no reason for it.
            Message was referring to change in rev. 1.58 of the CVS repo. *)
         occur_rec env allow_recursive visited ty0 ty'