Fix PR#7397

testsuite/tests/typing-gadts/pr7397.ml

@@ -0,0 +1,25 @@
+type +'a t
+
+class type a = object
+ method b : b
+end
+
+and b = object
+ method a : a
+end
+
+type _ response =
+ | C : #a t response;;
+[%%expect{|
+type +'a t
+class type a = object method b : b end
+and b = object method a : a end
+type _ response = C : #a t response
+|}]
+
+let f (type a) (a : a response) =
+ match a with
+ | C -> 0;;
+[%%expect{|
+val f : 'a response -> int = <fun>
+|}]

typing/ctype.ml

@@ -1657,7 +1657,10 @@ let rec local_non_recursive_abbrev strict visited env p ty =
   end
 
 let local_non_recursive_abbrev env p ty =
-  try local_non_recursive_abbrev false [] env p ty; true
+  try (* PR#7397: need to check trace_gadt_instances *)
+    wrap_trace_gadt_instances env
+      (local_non_recursive_abbrev false [] env p) ty;
+    true
   with Occur -> false