Improve instanceof with structurally identical types

src/compiler/checker.ts

@@ -5919,6 +5919,13 @@ namespace ts {
             return isTypeRelatedTo(source, target, assignableRelation);
         }
 
+        // A type S is considered to be an instance of a type T if S and T are the same type or if S is a
+        // subtype of T but not structurally identical to T. This specifically means that two distinct but
+        // structurally identical types (such as two classes) are not considered instances of each other.
+        function isTypeInstanceOf(source: Type, target: Type): boolean {
+            return source === target || isTypeSubtypeOf(source, target) && !isTypeIdenticalTo(source, target);
+        }
+
         /**
          * This is *not* a bi-directional relationship.
          * If one needs to check both directions for comparability, use a second call to this function or 'checkTypeComparableTo'.
@@ -8570,12 +8577,12 @@ namespace ts {
 
             function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean) {
                 if (!assumeTrue) {
-                    return filterType(type, t => !isTypeSubtypeOf(t, candidate));
+                    return filterType(type, t => !isTypeInstanceOf(t, candidate));
                 }
-                // If the current type is a union type, remove all constituents that aren't assignable to
+                // If the current type is a union type, remove all constituents that couldn't be instances of
                 // the candidate type. If one or more constituents remain, return a union of those.
                 if (type.flags & TypeFlags.Union) {
-                    const assignableConstituents = filter((<UnionType>type).types, t => isTypeAssignableTo(t, candidate));
+                    const assignableConstituents = filter((<UnionType>type).types, t => isTypeInstanceOf(t, candidate));
                     if (assignableConstituents.length) {
                         return getUnionType(assignableConstituents);
                     }

tests/baselines/reference/controlFlowBinaryOrExpression.symbols

@@ -64,19 +64,19 @@ if (isNodeList(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 }
 
 if (isHTMLCollection(sourceObj)) {
 >isHTMLCollection : Symbol(isHTMLCollection, Decl(controlFlowBinaryOrExpression.ts, 18, 67))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 }
 
 if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
@@ -86,8 +86,8 @@ if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
+>sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
+>length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 }
 

tests/baselines/reference/controlFlowBinaryOrExpression.types

@@ -80,7 +80,7 @@ if (isNodeList(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : NodeList
 >length : number
 }
 
@@ -91,7 +91,7 @@ if (isHTMLCollection(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : HTMLCollection
 >length : number
 }
 
@@ -102,11 +102,11 @@ if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 >sourceObj : EventTargetLike
 >isHTMLCollection(sourceObj) : boolean
 >isHTMLCollection : (sourceObj: any) => sourceObj is HTMLCollection
->sourceObj : { a: string; }
+>sourceObj : HTMLCollection | { a: string; }
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList
+>sourceObj : NodeList | HTMLCollection
 >length : number
 }
 

tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.js

@@ -0,0 +1,172 @@
+//// [instanceofWithStructurallyIdenticalTypes.ts]
+// Repro from #7271
+
+class C1 { item: string }
+class C2 { item: string[] }
+class C3 { item: string }
+
+function foo1(x: C1 | C2 | C3): string {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+
+function foo2(x: C1 | C2 | C3): string {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+
+// More tests
+
+class A { a: string }
+class A1 extends A { }
+class A2 { a: string }
+class B extends A { b: string }
+
+function goo(x: A) {
+    if (x instanceof A) {
+        x;  // A
+    }
+    else {
+        x;  // never
+    }
+    if (x instanceof A1) {
+        x;  // A1
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof A2) {
+        x;  // A2
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof B) {
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+}
+
+
+//// [instanceofWithStructurallyIdenticalTypes.js]
+// Repro from #7271
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var C1 = (function () {
+    function C1() {
+    }
+    return C1;
+}());
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+}());
+var C3 = (function () {
+    function C3() {
+    }
+    return C3;
+}());
+function foo1(x) {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+function isC1(c) { return c instanceof C1; }
+function isC2(c) { return c instanceof C2; }
+function isC3(c) { return c instanceof C3; }
+function foo2(x) {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+// More tests
+var A = (function () {
+    function A() {
+    }
+    return A;
+}());
+var A1 = (function (_super) {
+    __extends(A1, _super);
+    function A1() {
+        _super.apply(this, arguments);
+    }
+    return A1;
+}(A));
+var A2 = (function () {
+    function A2() {
+    }
+    return A2;
+}());
+var B = (function (_super) {
+    __extends(B, _super);
+    function B() {
+        _super.apply(this, arguments);
+    }
+    return B;
+}(A));
+function goo(x) {
+    if (x instanceof A) {
+        x; // A
+    }
+    else {
+        x; // never
+    }
+    if (x instanceof A1) {
+        x; // A1
+    }
+    else {
+        x; // A
+    }
+    if (x instanceof A2) {
+        x; // A2
+    }
+    else {
+        x; // A
+    }
+    if (x instanceof B) {
+        x; // B
+    }
+    else {
+        x; // A
+    }
+}