Fix infinite recursion in union type reduction

src/compiler/checker.ts

@@ -88,6 +88,7 @@ module ts {
         let undefinedType = createIntrinsicType(TypeFlags.Undefined | TypeFlags.ContainsUndefinedOrNull, "undefined");
         let nullType = createIntrinsicType(TypeFlags.Null | TypeFlags.ContainsUndefinedOrNull, "null");
         let unknownType = createIntrinsicType(TypeFlags.Any, "unknown");
+        let circularType = createIntrinsicType(TypeFlags.Any, "__circular__");
 
         let emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
         let anyFunctionType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
@@ -3566,28 +3567,14 @@ module ts {
             return false;
         }
 
-        // 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. 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);
-            if (removeSubtypesStack.lastIndexOf(typeListId) >= 0) {
-                return;
-            }
-
-            removeSubtypesStack.push(typeListId);
-
             let i = types.length;
             while (i > 0) {
                 i--;
                 if (isSubtypeOfAny(types[i], types)) {
                     types.splice(i, 1);
                 }
             }
-
-            removeSubtypesStack.pop();
         }
 
         function containsAnyType(types: Type[]) {
@@ -3642,10 +3629,20 @@ module ts {
             return type;
         }
 
+        // Subtype reduction is basically an optimization we do to avoid excessively large union types, which take longer
+        // to process and look strange in quick info and error messages. Semantically there is no difference between the
+        // reduced type and the type itself. So, when we detect a circularity we simply say that the reduced type is the
+        // type itself.
         function getReducedTypeOfUnionType(type: UnionType): Type {
-            // If union type was created without subtype reduction, perform the deferred reduction now
             if (!type.reducedType) {
-                type.reducedType = getUnionType(type.types, /*noSubtypeReduction*/ false);
+                type.reducedType = circularType;
+                let reducedType = getUnionType(type.types, /*noSubtypeReduction*/ false);
+                if (type.reducedType === circularType) {
+                    type.reducedType = reducedType;
+                }
+            }
+            else if (type.reducedType === circularType) {
+                type.reducedType = type;
             }
             return type.reducedType;
         }

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction.js → tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction1.js

@@ -1,4 +1,4 @@
-//// [unionTypeWithRecursiveSubtypeReduction.ts]
+//// [unionTypeWithRecursiveSubtypeReduction1.ts]
 class Module {
     public members: Class[];
 }
@@ -16,9 +16,10 @@ class Property {
 }
 
 var t: Class | Property;
-t.parent;
+t.parent;
+
 
-//// [unionTypeWithRecursiveSubtypeReduction.js]
+//// [unionTypeWithRecursiveSubtypeReduction1.js]
 var Module = (function () {
     function Module() {
     }

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction.symbols → tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction1.symbols

@@ -1,45 +1,45 @@
-=== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction.ts ===
+=== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction1.ts ===
 class Module {
->Module : Symbol(Module, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 0, 0))
+>Module : Symbol(Module, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 0, 0))
 
     public members: Class[];
->members : Symbol(members, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 0, 14))
->Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 6, 1))
+>members : Symbol(members, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 0, 14))
+>Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 6, 1))
 }
 
 class Namespace {
->Namespace : Symbol(Namespace, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 2, 1))
+>Namespace : Symbol(Namespace, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 2, 1))
 
     public members: (Class | Property)[];
->members : Symbol(members, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 4, 17))
->Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 6, 1))
->Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 10, 1))
+>members : Symbol(members, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 4, 17))
+>Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 6, 1))
+>Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 10, 1))
 }
 
 class Class {
->Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 6, 1))
+>Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 6, 1))
 
     public parent: Namespace;
->parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 8, 13))
->Namespace : Symbol(Namespace, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 2, 1))
+>parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 8, 13))
+>Namespace : Symbol(Namespace, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 2, 1))
 }
 
 class Property {
->Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 10, 1))
+>Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 10, 1))
 
     public parent: Module | Class;
->parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 12, 16))
->Module : Symbol(Module, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 0, 0))
->Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 6, 1))
+>parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 12, 16))
+>Module : Symbol(Module, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 0, 0))
+>Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 6, 1))
 }
 
 var t: Class | Property;
->t : Symbol(t, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 16, 3))
->Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 6, 1))
->Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 10, 1))
+>t : Symbol(t, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 16, 3))
+>Class : Symbol(Class, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 6, 1))
+>Property : Symbol(Property, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 10, 1))
 
 t.parent;
->t.parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 8, 13), Decl(unionTypeWithRecursiveSubtypeReduction.ts, 12, 16))
->t : Symbol(t, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 16, 3))
->parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction.ts, 8, 13), Decl(unionTypeWithRecursiveSubtypeReduction.ts, 12, 16))
+>t.parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 8, 13), Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 12, 16))
+>t : Symbol(t, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 16, 3))
+>parent : Symbol(parent, Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 8, 13), Decl(unionTypeWithRecursiveSubtypeReduction1.ts, 12, 16))
 

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction.types → tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction1.types

@@ -1,4 +1,4 @@
-=== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction.ts ===
+=== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction1.ts ===
 class Module {
 >Module : Module
 

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction2.errors.txt

@@ -42,4 +42,5 @@ tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts(20,1): error TS2
 !!! 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'.
+!!! error TS2322:         Property 'parent' is missing in type 'Namespace'.
+

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction2.js

@@ -18,7 +18,8 @@ class Property {
 var c: Class;
 var p: Property;
 c = p;
-p = c;
+p = c;
+
 
 //// [unionTypeWithRecursiveSubtypeReduction2.js]
 var Module = (function () {

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction3.errors.txt

@@ -0,0 +1,14 @@
+tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction3.ts(5,5): error TS2322: Type '{ prop: number; } | { prop: { prop: number; } | any; }' is not assignable to type 'string'.
+  Type '{ prop: number; }' is not assignable to type 'string'.
+
+
+==== tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction3.ts (1 errors) ====
+    var a27: { prop: number } | { prop: T27 };
+    type T27 = typeof a27;
+
+    var b: T27;
+    var s: string = b;
+        ~
+!!! error TS2322: Type '{ prop: number; } | { prop: { prop: number; } | any; }' is not assignable to type 'string'.
+!!! error TS2322:   Type '{ prop: number; }' is not assignable to type 'string'.
+

tests/baselines/reference/unionTypeWithRecursiveSubtypeReduction3.js

@@ -0,0 +1,12 @@
+//// [unionTypeWithRecursiveSubtypeReduction3.ts]
+var a27: { prop: number } | { prop: T27 };
+type T27 = typeof a27;
+
+var b: T27;
+var s: string = b;
+
+
+//// [unionTypeWithRecursiveSubtypeReduction3.js]
+var a27;
+var b;
+var s = b;

tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction.ts → tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction1.ts

@@ -15,4 +15,4 @@ class Property {
 }
 
 var t: Class | Property;
-t.parent;
+t.parent;

tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction2.ts

@@ -17,4 +17,4 @@ class Property {
 var c: Class;
 var p: Property;
 c = p;
-p = c;
+p = c;

tests/cases/compiler/unionTypeWithRecursiveSubtypeReduction3.ts

@@ -0,0 +1,5 @@
+var a27: { prop: number } | { prop: T27 };
+type T27 = typeof a27;
+
+var b: T27;
+var s: string = b;