typing: fix a try_expand_once forgotten from ocaml#10170

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

Changes

@@ -2429,6 +2429,10 @@ 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)

testsuite/tests/typing-misc/occur_check.ml

@@ -67,8 +67,10 @@ val wrong_to_seq : ('a Seq.t as 'a) Seq.t t -> 'a Seq.t Seq.t = <fun>
 
 let strange x = Seq.[cons x empty; cons empty x];;
 [%%expect{|
-Uncaught exception: Ctype.Escape(_)
-
-|}, Principal{|
-val strange : 'a Seq.t Seq.t -> 'a Seq.t Seq.t Seq.t list = <fun>
+Line 1, characters 12-48:
+1 | let strange x = Seq.[cons x empty; cons empty x];;
+                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: This expression has type "('a Seq.t as 'a) Seq.t -> 'a Seq.t Seq.t list"
+       but an expression was expected of type
+         "('a Seq.t as 'a) Seq.t -> 'a Seq.t Seq.t list"
 |}];;

typing/ctype.ml

@@ -1841,7 +1841,7 @@ 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'