typing: fix a try_expand_once forgotten from ocaml#10170

On OCaml versions 5.1 and older, this caused a Ctype.Escape(_)
uncaught exception on the included test case.

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

testsuite/tests/typing-misc-bugs/pr12971.compilers.reference

@@ -0,0 +1,9 @@
+type 'a t = T of 'a
+module Seq :
+  sig
+    type 'a t = unit -> 'a node
+    and 'a node
+    val empty : 'a t
+    val cons : 'a -> 'a t -> 'a t
+  end
+val to_seq : ('a Seq.t as 'a) Seq.t t -> 'a Seq.t Seq.t

testsuite/tests/typing-misc-bugs/pr12971.ml

@@ -0,0 +1,27 @@
+(* TEST_BELOW *)
+type 'a t = T of 'a
+
+module Seq : sig
+  type 'a t = unit -> 'a node
+  and 'a node
+
+  val empty : 'a t
+  val cons : 'a -> 'a t -> 'a t
+end = struct
+  type 'a t = unit -> 'a node
+  and 'a node = unit
+
+  let empty () = ()
+  let cons x xs () = ()
+end
+
+let to_seq (xt : 'a t) : 'a Seq.t =
+  let T x = xt in
+  Seq.cons Seq.empty x
+
+(* TEST
+ ocamlc_flags = "-i";
+ setup-ocamlc.byte-build-env;
+ ocamlc.byte;
+ check-ocamlc.byte-output;
+*)

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'