From ba63ad9b3bc4991a42a05b21dcc78534c273aa35 Mon Sep 17 00:00:00 2001
From: Richard Eisenberg <reisenberg@janestreet.com>
Date: Wed, 4 Jan 2023 17:01:35 -0500
Subject: [PATCH] Make subst restore the level even on error

---
 typing/ctype.ml | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/typing/ctype.ml b/typing/ctype.ml
index 306980c802..77c5c721cc 100644
--- a/typing/ctype.ml
+++ b/typing/ctype.ml
@@ -152,7 +152,8 @@ let global_level = s_ref 1
 let saved_level = s_ref []
 
 type levels =
-    { current_level: int; nongen_level: int; global_level: int;
+    { current_level: int; nongen_level: int;
+      global_level: int; (* level assigned to a fresh 'a in user code *)
       saved_level: (int * int) list; }
 let save_levels () =
   { current_level = !current_level;
@@ -1592,7 +1593,7 @@ let subst env level priv abbrev oty params args body =
     current_level := old_level;
     umode := old_umode;
     body'
-  with Unify _ ->
+  with Unify _ | Cannot_subst | Escape _ ->
     current_level := old_level;
     umode := old_umode;
     undo_abbrev ();