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Implement constructor type guard (#32774)
* Implement constructor type guard * Fix code review issues for constructor type guard. - Do not limit constructor expression to only identifiers - Fix `assumeTrue` and operator no-narrow check - Use better way to check that identifier type is a function - Loosen restriction on what expr is left of ".constructor" - Update typeGuardConstructorClassAndNumber test to include else cases * Fix grammar & spacing in `narrowTypeByConstructor` * fix bad merge * switch (back?) to crlf * update baselines Co-authored-by: Nathan Shively-Sanders <[email protected]>
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‎src/compiler/checker.ts‎

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@@ -20147,6 +20147,12 @@ namespace ts {
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if (right.kind === SyntaxKind.TypeOfExpression && isStringLiteralLike(left)) {
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return narrowTypeByTypeof(type, <TypeOfExpression>right, operator, left, assumeTrue);
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}
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if (isConstructorAccessExpression(left)) {
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return narrowTypeByConstructor(type, left, operator, right, assumeTrue);
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}
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if (isConstructorAccessExpression(right)) {
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return narrowTypeByConstructor(type, right, operator, left, assumeTrue);
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}
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if (isMatchingReference(reference, left)) {
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return narrowTypeByEquality(type, operator, right, assumeTrue);
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}
@@ -20432,6 +20438,59 @@ namespace ts {
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return getTypeWithFacts(mapType(type, narrowTypeForTypeofSwitch(impliedType)), switchFacts);
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}
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function narrowTypeByConstructor(type: Type, constructorAccessExpr: AccessExpression, operator: SyntaxKind, identifier: Expression, assumeTrue: boolean): Type {
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// Do not narrow when checking inequality.
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if (assumeTrue ? (operator !== SyntaxKind.EqualsEqualsToken && operator !== SyntaxKind.EqualsEqualsEqualsToken) : (operator !== SyntaxKind.ExclamationEqualsToken && operator !== SyntaxKind.ExclamationEqualsEqualsToken)) {
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return type;
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}
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// In the case of `x.y`, a `x.constructor === T` type guard resets the narrowed type of `y` to its declared type.
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if (!isMatchingReference(reference, constructorAccessExpr.expression)) {
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return declaredType;
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}
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// Get the type of the constructor identifier expression, if it is not a function then do not narrow.
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const identifierType = getTypeOfExpression(identifier);
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if (!isFunctionType(identifierType) && !isConstructorType(identifierType)) {
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return type;
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}
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// Get the prototype property of the type identifier so we can find out its type.
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const prototypeProperty = getPropertyOfType(identifierType, "prototype" as __String);
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if (!prototypeProperty) {
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return type;
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}
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// Get the type of the prototype, if it is undefined, or the global `Object` or `Function` types then do not narrow.
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const prototypeType = getTypeOfSymbol(prototypeProperty);
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const candidate = !isTypeAny(prototypeType) ? prototypeType : undefined;
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if (!candidate || candidate === globalObjectType || candidate === globalFunctionType) {
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return type;
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}
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// If the type that is being narrowed is `any` then just return the `candidate` type since every type is a subtype of `any`.
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if (isTypeAny(type)) {
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return candidate;
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}
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// Filter out types that are not considered to be "constructed by" the `candidate` type.
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return filterType(type, t => isConstructedBy(t, candidate));
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function isConstructedBy(source: Type, target: Type) {
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// If either the source or target type are a class type then we need to check that they are the same exact type.
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// This is because you may have a class `A` that defines some set of properties, and another class `B`
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// that defines the same set of properties as class `A`, in that case they are structurally the same
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// type, but when you do something like `instanceOfA.constructor === B` it will return false.
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if (source.flags & TypeFlags.Object && getObjectFlags(source) & ObjectFlags.Class ||
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target.flags & TypeFlags.Object && getObjectFlags(target) & ObjectFlags.Class) {
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return source.symbol === target.symbol;
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}
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// For all other types just check that the `source` type is a subtype of the `target` type.
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return isTypeSubtypeOf(source, target);
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}
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}
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function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
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const left = getReferenceCandidate(expr.left);
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if (!isMatchingReference(reference, left)) {

‎src/compiler/utilities.ts‎

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@@ -4426,6 +4426,13 @@ namespace ts {
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return isPropertyAccessExpression(node) && isEntityNameExpression(node.expression);
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}
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export function isConstructorAccessExpression(expr: Expression): expr is AccessExpression {
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return (
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isPropertyAccessExpression(expr) && idText(expr.name) === "constructor" ||
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isElementAccessExpression(expr) && isStringLiteralLike(expr.argumentExpression) && expr.argumentExpression.text === "constructor"
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);
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}
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export function tryGetPropertyAccessOrIdentifierToString(expr: Expression): string | undefined {
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if (isPropertyAccessExpression(expr)) {
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const baseStr = tryGetPropertyAccessOrIdentifierToString(expr.expression);

‎tests/baselines/reference/typeGuardConstructorClassAndNumber.errors.txt‎

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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(66,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(73,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(80,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(87,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(94,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(101,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(108,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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tests/cases/compiler/typeGuardConstructorClassAndNumber.ts(115,10): error TS2339: Property 'property1' does not exist on type 'number | C1'.
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Property 'property1' does not exist on type 'number'.
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==== tests/cases/compiler/typeGuardConstructorClassAndNumber.ts (8 errors) ====
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// Typical case
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class C1 {
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property1: string;
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}
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let var1: C1 | number;
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if (var1.constructor == C1) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (var1["constructor"] == C1) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (var1.constructor === C1) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (var1["constructor"] === C1) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (C1 == var1.constructor) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (C1 == var1["constructor"]) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (C1 === var1.constructor) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (C1 === var1["constructor"]) {
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var1; // C1
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var1.property1; // string
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}
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else {
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var1; // number | C1
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}
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if (var1.constructor != C1) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (var1["constructor"] != C1) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (var1.constructor !== C1) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (var1["constructor"] !== C1) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (C1 != var1.constructor) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (C1 != var1["constructor"]) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (C1 !== var1.constructor) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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if (C1 !== var1["constructor"]) {
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var1; // C1 | number
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var1.property1; // error
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~~~~~~~~~
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!!! error TS2339: Property 'property1' does not exist on type 'number | C1'.
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!!! error TS2339: Property 'property1' does not exist on type 'number'.
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}
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else {
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var1; // C1
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}
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