PR feedback and another test

src/compiler/checker.ts

@@ -3574,7 +3574,8 @@ module ts {
 
         // Since removeSubtypes checks the subtype relation, and the subtype relation on a union
         // may attempt to reduce a union, it is possible that removeSubtypes could be called
-        // recursively on the same set of types.
+        // recursively on the same set of types. The removeSubtypesStack is used to track which
+        // sets of types are currently undergoing subtype reduction.
         let removeSubtypesStack: string[] = [];
         function removeSubtypes(types: Type[]) {
             let typeListId = getTypeListId(types);

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction2.errors.txt

@@ -0,0 +1,45 @@
+tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts(19,1): error TS2322: Type 'Property' is not assignable to type 'Class'.
+  Types of property 'parent' are incompatible.
+    Type 'Module | Class' is not assignable to type 'Namespace'.
+      Type 'Class' is not assignable to type 'Namespace'.
+        Property 'members' is missing in type 'Class'.
+tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts(20,1): error TS2322: Type 'Class' is not assignable to type 'Property'.
+  Types of property 'parent' are incompatible.
+    Type 'Namespace' is not assignable to type 'Module | Class'.
+      Type 'Namespace' is not assignable to type 'Class'.
+        Property 'parent' is missing in type 'Namespace'.
+
+
+==== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts (2 errors) ====
+    class Module {
+        public members: Class[];
+    }
+    
+    class Namespace {
+        public members: (Class | Property)[];
+    }
+    
+    class Class {
+        public parent: Namespace;
+    }
+    
+    class Property {
+        public parent: Module | Class;
+    }
+    
+    var c: Class;
+    var p: Property;
+    c = p;
+    ~
+!!! error TS2322: Type 'Property' is not assignable to type 'Class'.
+!!! error TS2322:   Types of property 'parent' are incompatible.
+!!! error TS2322:     Type 'Module | Class' is not assignable to type 'Namespace'.
+!!! error TS2322:       Type 'Class' is not assignable to type 'Namespace'.
+!!! error TS2322:         Property 'members' is missing in type 'Class'.
+    p = c;
+    ~
+!!! error TS2322: Type 'Class' is not assignable to type 'Property'.
+!!! error TS2322:   Types of property 'parent' are incompatible.
+!!! error TS2322:     Type 'Namespace' is not assignable to type 'Module | Class'.
+!!! error TS2322:       Type 'Namespace' is not assignable to type 'Class'.
+!!! error TS2322:         Property 'parent' is missing in type 'Namespace'.

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction2.js

@@ -0,0 +1,47 @@
+//// [unionTypeWithRecursiveSubtypeReduction2.ts]
+class Module {
+    public members: Class[];
+}
+
+class Namespace {
+    public members: (Class | Property)[];
+}
+
+class Class {
+    public parent: Namespace;
+}
+
+class Property {
+    public parent: Module | Class;
+}
+
+var c: Class;
+var p: Property;
+c = p;
+p = c;
+
+//// [unionTypeWithRecursiveSubtypeReduction2.js]
+var Module = (function () {
+    function Module() {
+    }
+    return Module;
+})();
+var Namespace = (function () {
+    function Namespace() {
+    }
+    return Namespace;
+})();
+var Class = (function () {
+    function Class() {
+    }
+    return Class;
+})();
+var Property = (function () {
+    function Property() {
+    }
+    return Property;
+})();
+var c;
+var p;
+c = p;
+p = c;

tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts

@@ -0,0 +1,20 @@
+class Module {
+    public members: Class[];
+}
+
+class Namespace {
+    public members: (Class | Property)[];
+}
+
+class Class {
+    public parent: Namespace;
+}
+
+class Property {
+    public parent: Module | Class;
+}
+
+var c: Class;
+var p: Property;
+c = p;
+p = c;