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DukeDeSouthcursoragent
DukeDeSouth
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cursoragent
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Lift computed property union literal types to union of object types
When a computed property name has a union literal type (e.g., key: 'a' | 'b'), the resulting object literal type is now a union of object types ({ a: V } | { b: V }) instead of an index signature ({ [x: string]: V }). This is sound because at runtime { [key]: value } creates exactly one property, not all possible properties. The previous behavior (index signature) was overly wide, and the unsound alternative ({ a: V; b: V }) was correctly rejected. Fixes #13948 Prior art: #21070 by @sandersn (2018) implemented the same union-type approach but was shelved due to baseline complexity. This implementation adapts the core idea to the modern checker architecture. Baseline changes: - declarationEmitSimpleComputedNames1: union literal computed properties now produce { f1 } | { f2 } instead of retained computed property name - declarationComputedPropertyNames (transpile): same — union instead of index signature for Math.random() > 0.5 ? "f1" : "f2" expression Co-authored-by: Cursor <cursoragent@cursor.com>
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‎src/compiler/checker.ts‎

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@@ -33581,6 +33581,52 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
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}
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}
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objectFlags |= getObjectFlags(type) & ObjectFlags.PropagatingFlags;
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// When a computed property name has a union literal type (e.g., key: 'a' | 'b'),
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// lift to a union of object types: { a: V } | { b: V }
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// This is sound because at runtime { [key]: value } creates exactly ONE property.
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// See: https://github.com/microsoft/TypeScript/issues/13948
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if (
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computedNameType &&
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(computedNameType.flags & TypeFlags.Union) &&
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every((computedNameType as UnionType).types, isTypeUsableAsPropertyName)
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) {
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// Flush any accumulated properties into the spread
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if (propertiesArray.length > 0) {
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spread = getSpreadType(spread, createObjectLiteralType(), node.symbol, objectFlags, inConstContext);
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propertiesArray = [];
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propertiesTable = createSymbolTable();
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hasComputedStringProperty = false;
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hasComputedNumberProperty = false;
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hasComputedSymbolProperty = false;
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}
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// Create one object type per union member, then union them
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const memberTypes: Type[] = [];
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for (const literalType of (computedNameType as UnionType).types) {
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const propName = getPropertyNameFromType(literalType as StringLiteralType | NumberLiteralType | UniqueESSymbolType);
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const prop = createSymbol(SymbolFlags.Property | member.flags, propName, checkFlags | CheckFlags.Late);
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prop.links.nameType = literalType;
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prop.declarations = member.declarations;
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prop.parent = member.parent;
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if (member.valueDeclaration) {
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prop.valueDeclaration = member.valueDeclaration;
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}
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prop.links.type = type;
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prop.links.target = member;
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const singlePropTable = createSymbolTable();
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singlePropTable.set(propName, prop);
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const singleObjType = createAnonymousType(node.symbol, singlePropTable, emptyArray, emptyArray, emptyArray);
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singleObjType.objectFlags |= objectFlags | ObjectFlags.ObjectLiteral | ObjectFlags.ContainsObjectOrArrayLiteral;
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memberTypes.push(singleObjType);
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}
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if (memberTypes.length > 0) {
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spread = getSpreadType(spread, getUnionType(memberTypes), node.symbol, objectFlags, inConstContext);
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}
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offset = propertiesArray.length;
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continue;
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}
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const nameType = computedNameType && isTypeUsableAsPropertyName(computedNameType) ? computedNameType : undefined;
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const prop = nameType ?
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createSymbol(SymbolFlags.Property | member.flags, getPropertyNameFromType(nameType), checkFlags | CheckFlags.Late) :

‎tests/baselines/reference/computedPropertyUnionLiftsToUnionType.js‎

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//// [tests/cases/conformance/es6/computedProperties/computedPropertyUnionLiftsToUnionType.ts] ////
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//// [computedPropertyUnionLiftsToUnionType.ts]
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declare var ab: 'a' | 'b';
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declare var cd: 'c' | 'd';
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declare var onetwo: 1 | 2;
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enum Alphabet {
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Aleph,
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Bet,
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}
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declare var alphabet: Alphabet;
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// Basic: union literal key lifts to union of object types
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const x: { a: string } | { b: string } = { [ab]: 'hi' }
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// Multiple unions create cross-product
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const y: { a: string, m: number, c: string }
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| { a: string, m: number, d: string }
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| { b: string, m: number, c: string }
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| { b: string, m: number, d: string } = { [ab]: 'hi', m: 1, [cd]: 'there' }
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// Union + spread
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const s: { a: string, c: string } | { b: string, c: string } = { [ab]: 'hi', ...{ c: 'no' }}
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// Number literal union
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const n: { "1": string } | { "2": string } = { [onetwo]: 'hi' }
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// Enum literal union
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const e: { "0": string } | { "1": string } = { [alphabet]: 'hi' }
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// Soundness check: accessing non-existent property should be error
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const obj = { [ab]: 1 }
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// obj should be { a: number } | { b: number }, not { a: number; b: number }
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// So accessing both .a and .b should require a type guard
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// Methods and getters alongside union computed property
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const m: { a: string, m(): number, p: number } | { b: string, m(): number, p: number } =
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{ [ab]: 'hi', m() { return 1 }, get p() { return 2 } }
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//// [computedPropertyUnionLiftsToUnionType.js]
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"use strict";
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var Alphabet;
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(function (Alphabet) {
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Alphabet[Alphabet["Aleph"] = 0] = "Aleph";
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Alphabet[Alphabet["Bet"] = 1] = "Bet";
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})(Alphabet || (Alphabet = {}));
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// Basic: union literal key lifts to union of object types
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const x = { [ab]: 'hi' };
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// Multiple unions create cross-product
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const y = { [ab]: 'hi', m: 1, [cd]: 'there' };
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// Union + spread
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const s = Object.assign({ [ab]: 'hi' }, { c: 'no' });
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// Number literal union
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const n = { [onetwo]: 'hi' };
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// Enum literal union
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const e = { [alphabet]: 'hi' };
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// Soundness check: accessing non-existent property should be error
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const obj = { [ab]: 1 };
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// obj should be { a: number } | { b: number }, not { a: number; b: number }
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// So accessing both .a and .b should require a type guard
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// Methods and getters alongside union computed property
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const m = { [ab]: 'hi', m() { return 1; }, get p() { return 2; } };
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//// [computedPropertyUnionLiftsToUnionType.d.ts]
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declare var ab: 'a' | 'b';
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declare var cd: 'c' | 'd';
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declare var onetwo: 1 | 2;
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declare enum Alphabet {
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Aleph = 0,
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Bet = 1
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}
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declare var alphabet: Alphabet;
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declare const x: {
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a: string;
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} | {
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b: string;
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};
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declare const y: {
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a: string;
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m: number;
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c: string;
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} | {
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a: string;
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m: number;
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d: string;
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} | {
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b: string;
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m: number;
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c: string;
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} | {
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b: string;
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m: number;
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d: string;
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};
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declare const s: {
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a: string;
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c: string;
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} | {
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b: string;
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c: string;
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};
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declare const n: {
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"1": string;
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} | {
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"2": string;
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};
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declare const e: {
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"0": string;
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} | {
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"1": string;
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};
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declare const obj: {
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a: number;
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} | {
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b: number;
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};
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declare const m: {
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a: string;
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m(): number;
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p: number;
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} | {
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b: string;
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m(): number;
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p: number;
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};

‎tests/baselines/reference/computedPropertyUnionLiftsToUnionType.symbols‎

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//// [tests/cases/conformance/es6/computedProperties/computedPropertyUnionLiftsToUnionType.ts] ////
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=== computedPropertyUnionLiftsToUnionType.ts ===
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declare var ab: 'a' | 'b';
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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declare var cd: 'c' | 'd';
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>cd : Symbol(cd, Decl(computedPropertyUnionLiftsToUnionType.ts, 1, 11))
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declare var onetwo: 1 | 2;
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>onetwo : Symbol(onetwo, Decl(computedPropertyUnionLiftsToUnionType.ts, 2, 11))
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enum Alphabet {
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>Alphabet : Symbol(Alphabet, Decl(computedPropertyUnionLiftsToUnionType.ts, 2, 26))
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Aleph,
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>Aleph : Symbol(Alphabet.Aleph, Decl(computedPropertyUnionLiftsToUnionType.ts, 3, 15))
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Bet,
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>Bet : Symbol(Alphabet.Bet, Decl(computedPropertyUnionLiftsToUnionType.ts, 4, 10))
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}
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declare var alphabet: Alphabet;
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>alphabet : Symbol(alphabet, Decl(computedPropertyUnionLiftsToUnionType.ts, 7, 11))
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>Alphabet : Symbol(Alphabet, Decl(computedPropertyUnionLiftsToUnionType.ts, 2, 26))
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// Basic: union literal key lifts to union of object types
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const x: { a: string } | { b: string } = { [ab]: 'hi' }
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>x : Symbol(x, Decl(computedPropertyUnionLiftsToUnionType.ts, 10, 5))
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>a : Symbol(a, Decl(computedPropertyUnionLiftsToUnionType.ts, 10, 10))
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>b : Symbol(b, Decl(computedPropertyUnionLiftsToUnionType.ts, 10, 26))
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>[ab] : Symbol([ab], Decl(computedPropertyUnionLiftsToUnionType.ts, 10, 42))
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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// Multiple unions create cross-product
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const y: { a: string, m: number, c: string }
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>y : Symbol(y, Decl(computedPropertyUnionLiftsToUnionType.ts, 13, 5))
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>a : Symbol(a, Decl(computedPropertyUnionLiftsToUnionType.ts, 13, 10))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 13, 21))
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>c : Symbol(c, Decl(computedPropertyUnionLiftsToUnionType.ts, 13, 32))
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| { a: string, m: number, d: string }
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>a : Symbol(a, Decl(computedPropertyUnionLiftsToUnionType.ts, 14, 7))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 14, 18))
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>d : Symbol(d, Decl(computedPropertyUnionLiftsToUnionType.ts, 14, 29))
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| { b: string, m: number, c: string }
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>b : Symbol(b, Decl(computedPropertyUnionLiftsToUnionType.ts, 15, 7))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 15, 18))
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>c : Symbol(c, Decl(computedPropertyUnionLiftsToUnionType.ts, 15, 29))
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| { b: string, m: number, d: string } = { [ab]: 'hi', m: 1, [cd]: 'there' }
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>b : Symbol(b, Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 7))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 18))
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>d : Symbol(d, Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 29))
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>[ab] : Symbol([ab], Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 45))
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 57))
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>[cd] : Symbol([cd], Decl(computedPropertyUnionLiftsToUnionType.ts, 16, 63))
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>cd : Symbol(cd, Decl(computedPropertyUnionLiftsToUnionType.ts, 1, 11))
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// Union + spread
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const s: { a: string, c: string } | { b: string, c: string } = { [ab]: 'hi', ...{ c: 'no' }}
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>s : Symbol(s, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 5))
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>a : Symbol(a, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 10))
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>c : Symbol(c, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 21))
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>b : Symbol(b, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 37))
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>c : Symbol(c, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 48))
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>[ab] : Symbol([ab], Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 64))
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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>c : Symbol(c, Decl(computedPropertyUnionLiftsToUnionType.ts, 19, 81))
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// Number literal union
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const n: { "1": string } | { "2": string } = { [onetwo]: 'hi' }
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>n : Symbol(n, Decl(computedPropertyUnionLiftsToUnionType.ts, 22, 5))
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>"1" : Symbol("1", Decl(computedPropertyUnionLiftsToUnionType.ts, 22, 10))
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>"2" : Symbol("2", Decl(computedPropertyUnionLiftsToUnionType.ts, 22, 28))
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>[onetwo] : Symbol([onetwo], Decl(computedPropertyUnionLiftsToUnionType.ts, 22, 46))
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>onetwo : Symbol(onetwo, Decl(computedPropertyUnionLiftsToUnionType.ts, 2, 11))
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// Enum literal union
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const e: { "0": string } | { "1": string } = { [alphabet]: 'hi' }
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>e : Symbol(e, Decl(computedPropertyUnionLiftsToUnionType.ts, 25, 5))
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>"0" : Symbol("0", Decl(computedPropertyUnionLiftsToUnionType.ts, 25, 10))
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>"1" : Symbol("1", Decl(computedPropertyUnionLiftsToUnionType.ts, 25, 28))
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>[alphabet] : Symbol([alphabet], Decl(computedPropertyUnionLiftsToUnionType.ts, 25, 46))
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>alphabet : Symbol(alphabet, Decl(computedPropertyUnionLiftsToUnionType.ts, 7, 11))
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// Soundness check: accessing non-existent property should be error
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const obj = { [ab]: 1 }
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>obj : Symbol(obj, Decl(computedPropertyUnionLiftsToUnionType.ts, 28, 5))
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>[ab] : Symbol([ab], Decl(computedPropertyUnionLiftsToUnionType.ts, 28, 13))
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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// obj should be { a: number } | { b: number }, not { a: number; b: number }
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// So accessing both .a and .b should require a type guard
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// Methods and getters alongside union computed property
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const m: { a: string, m(): number, p: number } | { b: string, m(): number, p: number } =
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 5))
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>a : Symbol(a, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 10))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 21))
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>p : Symbol(p, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 34))
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>b : Symbol(b, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 50))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 61))
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>p : Symbol(p, Decl(computedPropertyUnionLiftsToUnionType.ts, 33, 74))
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{ [ab]: 'hi', m() { return 1 }, get p() { return 2 } }
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>[ab] : Symbol([ab], Decl(computedPropertyUnionLiftsToUnionType.ts, 34, 5))
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>ab : Symbol(ab, Decl(computedPropertyUnionLiftsToUnionType.ts, 0, 11))
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>m : Symbol(m, Decl(computedPropertyUnionLiftsToUnionType.ts, 34, 17))
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>p : Symbol(p, Decl(computedPropertyUnionLiftsToUnionType.ts, 34, 35))
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