fix unification loop for a chain of recursive typedef and abstract (closes #9428)

src/core/tUnification.ml

@@ -90,6 +90,11 @@ let fast_eq_check type_param_check a b =
 
 let rec fast_eq a b = fast_eq_check fast_eq a b
 
+let rec fast_eq_unbound_mono a b =
+	match a, b with
+	| TMono { tm_type = None }, TMono { tm_type = None } -> true
+	| _ -> fast_eq_check fast_eq_unbound_mono a b
+
 let rec fast_eq_mono ml a b =
 	if fast_eq_check (fast_eq_mono ml) a b then
 		true
@@ -432,12 +437,12 @@ let rec unify (uctx : unification_context) a b =
 		| Some t -> unify uctx a t)
 	| TType (t,tl) , _ ->
 		rec_stack unify_stack (a,b)
-			(fun(a2,b2) -> fast_eq a a2 && fast_eq b b2)
+			(fun(a2,b2) -> fast_eq_unbound_mono a a2 && fast_eq b b2)
 			(fun() -> try_apply_params_rec t.t_params tl t.t_type (fun a -> unify uctx a b))
 			(fun l -> error (cannot_unify a b :: l))
 	| _ , TType (t,tl) ->
 		rec_stack unify_stack (a,b)
-			(fun(a2,b2) -> fast_eq a a2 && fast_eq b b2)
+			(fun(a2,b2) -> fast_eq a a2 && fast_eq_unbound_mono b b2)
 			(fun() -> try_apply_params_rec t.t_params tl t.t_type (unify uctx a))
 			(fun l -> error (cannot_unify a b :: l))
 	| TEnum (ea,tl1) , TEnum (eb,tl2) ->

tests/misc/compiler_loops/projects/Issue9428/TypedefAbstract.hx

@@ -0,0 +1,15 @@
+typedef Rec1<T> = {
+	function then<TResult>():Rec1<TResult>;
+}
+
+abstract Rec2<T>(Rec2Struct<T>) from Rec2Struct<T> {}
+
+typedef Rec2Struct<T> = {
+	function then<TOut>():Rec2<TOut>;
+}
+
+class TypedefAbstract {
+	static function main() {
+		var r:Rec2<Void> = (null:Rec1<Void>);
+	}
+}

tests/misc/compiler_loops/projects/Issue9428/typedefAbstract.hxml

@@ -0,0 +1 @@
+-main TypedefAbstract