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Use Partial<T> instead of Omit<T, keyof T> when rest spreading #62573

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52fd500
Use Partial<T> instead of Omit<T, keyof T> when rest spreading
RyanCavanaugh Oct 9, 2025
b1bd4cd
Initial plan
Copilot Oct 9, 2025
03d7fa1
Add compiler test for Partial<T> rest spreading with keyof T
Copilot Oct 9, 2025
034278d
Add contrast test for Omit cases (not using Partial)
Copilot Oct 9, 2025
57242ff
Merge pull request #26 from RyanCavanaugh/copilot/add-testcases-for-p…
RyanCavanaugh Oct 9, 2025
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28 changes: 28 additions & 0 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -2292,6 +2292,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
var deferredGlobalAsyncDisposableType: ObjectType | undefined;
var deferredGlobalExtractSymbol: Symbol | undefined;
var deferredGlobalOmitSymbol: Symbol | undefined;
var deferredGlobalPartialSymbol: Symbol | undefined;
var deferredGlobalAwaitedSymbol: Symbol | undefined;
var deferredGlobalBigIntType: ObjectType | undefined;
var deferredGlobalNaNSymbol: Symbol | undefined;
Expand Down Expand Up @@ -11577,6 +11578,17 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return source;
}

// In the case where we have
// const { [key]: _, ...rest } = obj;
// where key is keyof typeof obj, then the best type for rest is Partial<typeof obj>
if (isOrContainsKeyofT(omitKeyType, source)) {
const partialTypeAlias = getGlobalPartialSymbol();
if (!partialTypeAlias) {
return errorType;
}
return getTypeAliasInstantiation(partialTypeAlias, [source]);
}

const omitTypeAlias = getGlobalOmitSymbol();
if (!omitTypeAlias) {
return errorType;
Expand All @@ -11592,6 +11604,16 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return result;
}

function isOrContainsKeyofT(keyType: Type, sourceType: Type): boolean {
if (keyType.flags & TypeFlags.Union) {
return !!forEach((keyType as UnionType).types, t => isOrContainsKeyofT(t, sourceType));
}
if (keyType.flags & TypeFlags.Index && (keyType as IndexType).type === sourceType) {
return true;
}
return false;
}
Comment on lines +11607 to +11615
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I think this traversal could be just a call to someType.


function isGenericTypeWithUndefinedConstraint(type: Type) {
return !!(type.flags & TypeFlags.Instantiable) && maybeTypeOfKind(getBaseConstraintOfType(type) || unknownType, TypeFlags.Undefined);
}
Expand Down Expand Up @@ -17573,6 +17595,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return deferredGlobalOmitSymbol === unknownSymbol ? undefined : deferredGlobalOmitSymbol;
}

function getGlobalPartialSymbol(): Symbol | undefined {
// We always report an error, so cache a result in the event we could not resolve the symbol to prevent reporting it multiple times
deferredGlobalPartialSymbol ||= getGlobalTypeAliasSymbol("Partial" as __String, /*arity*/ 1, /*reportErrors*/ true) || unknownSymbol;
return deferredGlobalPartialSymbol === unknownSymbol ? undefined : deferredGlobalPartialSymbol;
}

function getGlobalAwaitedSymbol(reportErrors: boolean): Symbol | undefined {
// Only cache `unknownSymbol` if we are reporting errors so that we don't report the error more than once.
deferredGlobalAwaitedSymbol ||= getGlobalTypeAliasSymbol("Awaited" as __String, /*arity*/ 1, reportErrors) || (reportErrors ? unknownSymbol : undefined);
Expand Down
139 changes: 139 additions & 0 deletions tests/baselines/reference/restSpreadingWithKeyofT.js
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@@ -0,0 +1,139 @@
//// [tests/cases/compiler/restSpreadingWithKeyofT.ts] ////

//// [restSpreadingWithKeyofT.ts]
// Test case 1: Using keyof T directly in rest spreading
// Should result in Partial<T> instead of Omit<T, keyof T>
function f1<T>(obj: T, key: keyof T) {
const { [key]: removed, ...rest } = obj;
return rest;
}

// Test case 2: Union of keyof T
// Should result in Partial<T> since both k1 and k2 are keyof T
function f2<T>(obj: T, k1: keyof T, k2: keyof T) {
const { [k1]: removed1, [k2]: removed2, ...rest } = obj;
return rest;
}

// Test case 3: keyof T with additional literal
// Should still use Partial<T> since the union contains keyof T
function f3<T>(obj: T, key: keyof T | "extra") {
const { [key]: removed, ...rest } = obj;
return rest;
}

// Test case 4: Specific type with keyof
type MyObj = { a: string; b: number; c: boolean; };
function f4(obj: MyObj, key: keyof MyObj) {
const { [key]: removed, ...rest } = obj;
return rest;
}

// Test case 5: Constraint with keyof
function f5<T extends { a: string; b: number }>(obj: T, key: keyof T) {
const { [key]: removed, ...rest } = obj;
return rest;
}

// Test case 6: Multiple parameters with keyof in object literal
function f6<T>(obj: T, k1: keyof T, k2: keyof T, k3: keyof T) {
const { [k1]: r1, [k2]: r2, [k3]: r3, ...rest } = obj;
return rest;
}

// Test case 7: Nested rest with keyof
function f7<T>(obj: T, key: keyof T) {
const { [key]: val, ...rest } = obj;
const consumed = val;
return { consumed, rest };
}

// Test case 8: Array of keyof (not applicable but shows edge case)
function f8<T>(obj: T, keys: Array<keyof T>) {
// Can't destructure with array, but showing the type relationship
const key = keys[0];
const { [key]: removed, ...rest } = obj;
return rest;
}


//// [restSpreadingWithKeyofT.js]
"use strict";
var __rest = (this && this.__rest) || function (s, e) {
var t = {};
for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
t[p] = s[p];
if (s != null && typeof Object.getOwnPropertySymbols === "function")
for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) {
if (e.indexOf(p[i]) < 0 && Object.prototype.propertyIsEnumerable.call(s, p[i]))
t[p[i]] = s[p[i]];
}
return t;
};
// Test case 1: Using keyof T directly in rest spreading
// Should result in Partial<T> instead of Omit<T, keyof T>
function f1(obj, key) {
var _a = obj, _b = key, removed = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
return rest;
}
// Test case 2: Union of keyof T
// Should result in Partial<T> since both k1 and k2 are keyof T
function f2(obj, k1, k2) {
var _a = obj, _b = k1, removed1 = _a[_b], _c = k2, removed2 = _a[_c], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + "", typeof _c === "symbol" ? _c : _c + ""]);
return rest;
}
// Test case 3: keyof T with additional literal
// Should still use Partial<T> since the union contains keyof T
function f3(obj, key) {
var _a = obj, _b = key, removed = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
return rest;
}
function f4(obj, key) {
var _a = obj, _b = key, removed = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
return rest;
}
// Test case 5: Constraint with keyof
function f5(obj, key) {
var _a = obj, _b = key, removed = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
return rest;
}
// Test case 6: Multiple parameters with keyof in object literal
function f6(obj, k1, k2, k3) {
var _a = obj, _b = k1, r1 = _a[_b], _c = k2, r2 = _a[_c], _d = k3, r3 = _a[_d], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + "", typeof _c === "symbol" ? _c : _c + "", typeof _d === "symbol" ? _d : _d + ""]);
return rest;
}
// Test case 7: Nested rest with keyof
function f7(obj, key) {
var _a = obj, _b = key, val = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
var consumed = val;
return { consumed: consumed, rest: rest };
}
// Test case 8: Array of keyof (not applicable but shows edge case)
function f8(obj, keys) {
// Can't destructure with array, but showing the type relationship
var key = keys[0];
var _a = obj, _b = key, removed = _a[_b], rest = __rest(_a, [typeof _b === "symbol" ? _b : _b + ""]);
return rest;
}


//// [restSpreadingWithKeyofT.d.ts]
declare function f1<T>(obj: T, key: keyof T): Partial<T>;
declare function f2<T>(obj: T, k1: keyof T, k2: keyof T): Partial<T>;
declare function f3<T>(obj: T, key: keyof T | "extra"): Partial<T>;
type MyObj = {
a: string;
b: number;
c: boolean;
};
declare function f4(obj: MyObj, key: keyof MyObj): {};
declare function f5<T extends {
a: string;
b: number;
}>(obj: T, key: keyof T): Partial<T>;
declare function f6<T>(obj: T, k1: keyof T, k2: keyof T, k3: keyof T): Partial<T>;
declare function f7<T>(obj: T, key: keyof T): {
consumed: T[keyof T];
rest: Partial<T>;
};
declare function f8<T>(obj: T, keys: Array<keyof T>): Partial<T>;
188 changes: 188 additions & 0 deletions tests/baselines/reference/restSpreadingWithKeyofT.symbols
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@@ -0,0 +1,188 @@
//// [tests/cases/compiler/restSpreadingWithKeyofT.ts] ////

=== restSpreadingWithKeyofT.ts ===
// Test case 1: Using keyof T directly in rest spreading
// Should result in Partial<T> instead of Omit<T, keyof T>
function f1<T>(obj: T, key: keyof T) {
>f1 : Symbol(f1, Decl(restSpreadingWithKeyofT.ts, 0, 0))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 2, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 2, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 2, 12))
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 2, 22))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 2, 12))

const { [key]: removed, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 2, 22))
>removed : Symbol(removed, Decl(restSpreadingWithKeyofT.ts, 3, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 3, 27))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 2, 15))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 3, 27))
}

// Test case 2: Union of keyof T
// Should result in Partial<T> since both k1 and k2 are keyof T
function f2<T>(obj: T, k1: keyof T, k2: keyof T) {
>f2 : Symbol(f2, Decl(restSpreadingWithKeyofT.ts, 5, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 9, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 9, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 9, 12))
>k1 : Symbol(k1, Decl(restSpreadingWithKeyofT.ts, 9, 22))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 9, 12))
>k2 : Symbol(k2, Decl(restSpreadingWithKeyofT.ts, 9, 35))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 9, 12))

const { [k1]: removed1, [k2]: removed2, ...rest } = obj;
>k1 : Symbol(k1, Decl(restSpreadingWithKeyofT.ts, 9, 22))
>removed1 : Symbol(removed1, Decl(restSpreadingWithKeyofT.ts, 10, 11))
>k2 : Symbol(k2, Decl(restSpreadingWithKeyofT.ts, 9, 35))
>removed2 : Symbol(removed2, Decl(restSpreadingWithKeyofT.ts, 10, 27))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 10, 43))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 9, 15))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 10, 43))
}

// Test case 3: keyof T with additional literal
// Should still use Partial<T> since the union contains keyof T
function f3<T>(obj: T, key: keyof T | "extra") {
>f3 : Symbol(f3, Decl(restSpreadingWithKeyofT.ts, 12, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 16, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 16, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 16, 12))
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 16, 22))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 16, 12))

const { [key]: removed, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 16, 22))
>removed : Symbol(removed, Decl(restSpreadingWithKeyofT.ts, 17, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 17, 27))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 16, 15))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 17, 27))
}

// Test case 4: Specific type with keyof
type MyObj = { a: string; b: number; c: boolean; };
>MyObj : Symbol(MyObj, Decl(restSpreadingWithKeyofT.ts, 19, 1))
>a : Symbol(a, Decl(restSpreadingWithKeyofT.ts, 22, 14))
>b : Symbol(b, Decl(restSpreadingWithKeyofT.ts, 22, 25))
>c : Symbol(c, Decl(restSpreadingWithKeyofT.ts, 22, 36))

function f4(obj: MyObj, key: keyof MyObj) {
>f4 : Symbol(f4, Decl(restSpreadingWithKeyofT.ts, 22, 51))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 23, 12))
>MyObj : Symbol(MyObj, Decl(restSpreadingWithKeyofT.ts, 19, 1))
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 23, 23))
>MyObj : Symbol(MyObj, Decl(restSpreadingWithKeyofT.ts, 19, 1))

const { [key]: removed, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 23, 23))
>removed : Symbol(removed, Decl(restSpreadingWithKeyofT.ts, 24, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 24, 27))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 23, 12))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 24, 27))
}

// Test case 5: Constraint with keyof
function f5<T extends { a: string; b: number }>(obj: T, key: keyof T) {
>f5 : Symbol(f5, Decl(restSpreadingWithKeyofT.ts, 26, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 29, 12))
>a : Symbol(a, Decl(restSpreadingWithKeyofT.ts, 29, 23))
>b : Symbol(b, Decl(restSpreadingWithKeyofT.ts, 29, 34))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 29, 48))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 29, 12))
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 29, 55))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 29, 12))

const { [key]: removed, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 29, 55))
>removed : Symbol(removed, Decl(restSpreadingWithKeyofT.ts, 30, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 30, 27))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 29, 48))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 30, 27))
}

// Test case 6: Multiple parameters with keyof in object literal
function f6<T>(obj: T, k1: keyof T, k2: keyof T, k3: keyof T) {
>f6 : Symbol(f6, Decl(restSpreadingWithKeyofT.ts, 32, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 35, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 35, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 35, 12))
>k1 : Symbol(k1, Decl(restSpreadingWithKeyofT.ts, 35, 22))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 35, 12))
>k2 : Symbol(k2, Decl(restSpreadingWithKeyofT.ts, 35, 35))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 35, 12))
>k3 : Symbol(k3, Decl(restSpreadingWithKeyofT.ts, 35, 48))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 35, 12))

const { [k1]: r1, [k2]: r2, [k3]: r3, ...rest } = obj;
>k1 : Symbol(k1, Decl(restSpreadingWithKeyofT.ts, 35, 22))
>r1 : Symbol(r1, Decl(restSpreadingWithKeyofT.ts, 36, 11))
>k2 : Symbol(k2, Decl(restSpreadingWithKeyofT.ts, 35, 35))
>r2 : Symbol(r2, Decl(restSpreadingWithKeyofT.ts, 36, 21))
>k3 : Symbol(k3, Decl(restSpreadingWithKeyofT.ts, 35, 48))
>r3 : Symbol(r3, Decl(restSpreadingWithKeyofT.ts, 36, 31))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 36, 41))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 35, 15))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 36, 41))
}

// Test case 7: Nested rest with keyof
function f7<T>(obj: T, key: keyof T) {
>f7 : Symbol(f7, Decl(restSpreadingWithKeyofT.ts, 38, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 41, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 41, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 41, 12))
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 41, 22))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 41, 12))

const { [key]: val, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 41, 22))
>val : Symbol(val, Decl(restSpreadingWithKeyofT.ts, 42, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 42, 23))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 41, 15))

const consumed = val;
>consumed : Symbol(consumed, Decl(restSpreadingWithKeyofT.ts, 43, 9))
>val : Symbol(val, Decl(restSpreadingWithKeyofT.ts, 42, 11))

return { consumed, rest };
>consumed : Symbol(consumed, Decl(restSpreadingWithKeyofT.ts, 44, 12))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 44, 22))
}

// Test case 8: Array of keyof (not applicable but shows edge case)
function f8<T>(obj: T, keys: Array<keyof T>) {
>f8 : Symbol(f8, Decl(restSpreadingWithKeyofT.ts, 45, 1))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 48, 12))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 48, 15))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 48, 12))
>keys : Symbol(keys, Decl(restSpreadingWithKeyofT.ts, 48, 22))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
>T : Symbol(T, Decl(restSpreadingWithKeyofT.ts, 48, 12))

// Can't destructure with array, but showing the type relationship
const key = keys[0];
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 50, 9))
>keys : Symbol(keys, Decl(restSpreadingWithKeyofT.ts, 48, 22))

const { [key]: removed, ...rest } = obj;
>key : Symbol(key, Decl(restSpreadingWithKeyofT.ts, 50, 9))
>removed : Symbol(removed, Decl(restSpreadingWithKeyofT.ts, 51, 11))
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 51, 27))
>obj : Symbol(obj, Decl(restSpreadingWithKeyofT.ts, 48, 15))

return rest;
>rest : Symbol(rest, Decl(restSpreadingWithKeyofT.ts, 51, 27))
}

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