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fix(checker): allow unknown[] as valid mixin constructor rest param type #63362

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fix(checker): allow unknown[] as valid mixin constructor rest param type #63362

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5 changes: 4 additions & 1 deletion src/compiler/checker.ts
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Expand Up @@ -13161,7 +13161,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const s = signatures[0];
if (!s.typeParameters && s.parameters.length === 1 && signatureHasRestParameter(s)) {
const paramType = getTypeOfParameter(s.parameters[0]);
return isTypeAny(paramType) || getElementTypeOfArrayType(paramType) === anyType;
const elementType = getElementTypeOfArrayType(paramType)
return isTypeAny(paramType)
|| elementType === anyType
|| elementType === unknownType
}
Comment on lines +13164 to 13168
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Copilot AI Apr 6, 2026

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isMixinConstructorType now unconditionally calls getElementTypeOfArrayType(paramType), whereas the previous expression only computed the element type when paramType wasn’t any. Since this check can run in class-checking hot paths, consider preserving the short-circuit (e.g. early-return when isTypeAny(paramType) is true). Also, this hunk introduces trailing whitespace and omits semicolons in a file that otherwise consistently uses them; please align formatting, and update the nearby comment that still says the rest parameter must be any[] now that unknown[] is accepted too.

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}
return false;
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30 changes: 30 additions & 0 deletions tests/baselines/reference/mixinWithUnknownRestParam.errors.txt
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@@ -0,0 +1,30 @@
mixinWithUnknownRestParam.ts(14,21): error TS2345: Argument of type 'typeof Base' is not assignable to parameter of type 'ClassConstructor'.
Types of construct signatures are incompatible.
Type 'new (x: number) => Base' is not assignable to type 'new (...args: unknown[]) => {}'.
Types of parameters 'x' and 'args' are incompatible.
Type 'unknown' is not assignable to type 'number'.
Comment on lines +1 to +5
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Copilot AI Apr 6, 2026

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This errors baseline asserts TS2345 about typeof Base not being assignable to ClassConstructor, which is unrelated to the PR’s stated goal (accepting unknown[] for mixin constructors) and will remain even after the checker change. If the intent is a positive conformance test for the mixin rule change, the test should be adjusted to compile without errors and the .errors.txt baseline removed/regenerated accordingly.

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==== mixinWithUnknownRestParam.ts (1 errors) ====
// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]

type ClassConstructor = new (...args: unknown[]) => {}

function mixin<C extends ClassConstructor>(Class: C) {
return class extends Class {}
}

class Base {
constructor(public x: number) {}
}

const Mixed = mixin(Base)
~~~~
!!! error TS2345: Argument of type 'typeof Base' is not assignable to parameter of type 'ClassConstructor'.
!!! error TS2345: Types of construct signatures are incompatible.
!!! error TS2345: Type 'new (x: number) => Base' is not assignable to type 'new (...args: unknown[]) => {}'.
!!! error TS2345: Types of parameters 'x' and 'args' are incompatible.
!!! error TS2345: Type 'unknown' is not assignable to type 'number'.
const instance = new Mixed(42)

36 changes: 36 additions & 0 deletions tests/baselines/reference/mixinWithUnknownRestParam.js
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//// [tests/cases/conformance/classes/mixinWithUnknownRestParam.ts] ////

//// [mixinWithUnknownRestParam.ts]
// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]

type ClassConstructor = new (...args: unknown[]) => {}

function mixin<C extends ClassConstructor>(Class: C) {
return class extends Class {}
}

class Base {
constructor(public x: number) {}
}

const Mixed = mixin(Base)
const instance = new Mixed(42)


//// [mixinWithUnknownRestParam.js]
"use strict";
// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]
function mixin(Class) {
return class extends Class {
};
}
class Base {
x;
constructor(x) {
this.x = x;
}
}
const Mixed = mixin(Base);
const instance = new Mixed(42);
37 changes: 37 additions & 0 deletions tests/baselines/reference/mixinWithUnknownRestParam.symbols
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//// [tests/cases/conformance/classes/mixinWithUnknownRestParam.ts] ////

=== mixinWithUnknownRestParam.ts ===
// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]

type ClassConstructor = new (...args: unknown[]) => {}
>ClassConstructor : Symbol(ClassConstructor, Decl(mixinWithUnknownRestParam.ts, 0, 0))
>args : Symbol(args, Decl(mixinWithUnknownRestParam.ts, 3, 29))

function mixin<C extends ClassConstructor>(Class: C) {
>mixin : Symbol(mixin, Decl(mixinWithUnknownRestParam.ts, 3, 54))
>C : Symbol(C, Decl(mixinWithUnknownRestParam.ts, 5, 15))
>ClassConstructor : Symbol(ClassConstructor, Decl(mixinWithUnknownRestParam.ts, 0, 0))
>Class : Symbol(Class, Decl(mixinWithUnknownRestParam.ts, 5, 43))
>C : Symbol(C, Decl(mixinWithUnknownRestParam.ts, 5, 15))

return class extends Class {}
>Class : Symbol(Class, Decl(mixinWithUnknownRestParam.ts, 5, 43))
}

class Base {
>Base : Symbol(Base, Decl(mixinWithUnknownRestParam.ts, 7, 1))

constructor(public x: number) {}
>x : Symbol(Base.x, Decl(mixinWithUnknownRestParam.ts, 10, 16))
}

const Mixed = mixin(Base)
>Mixed : Symbol(Mixed, Decl(mixinWithUnknownRestParam.ts, 13, 5))
>mixin : Symbol(mixin, Decl(mixinWithUnknownRestParam.ts, 3, 54))
>Base : Symbol(Base, Decl(mixinWithUnknownRestParam.ts, 7, 1))

const instance = new Mixed(42)
>instance : Symbol(instance, Decl(mixinWithUnknownRestParam.ts, 14, 5))
>Mixed : Symbol(Mixed, Decl(mixinWithUnknownRestParam.ts, 13, 5))

54 changes: 54 additions & 0 deletions tests/baselines/reference/mixinWithUnknownRestParam.types
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//// [tests/cases/conformance/classes/mixinWithUnknownRestParam.ts] ////

=== mixinWithUnknownRestParam.ts ===
// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]

type ClassConstructor = new (...args: unknown[]) => {}
>ClassConstructor : ClassConstructor
> : ^^^^^^^^^^^^^^^^
>args : unknown[]
> : ^^^^^^^^^

function mixin<C extends ClassConstructor>(Class: C) {
>mixin : <C extends ClassConstructor>(Class: C) => { new (...args: unknown[]): (Anonymous class); prototype: mixin<any>.(Anonymous class); } & C
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Class : C
> : ^

return class extends Class {}
>class extends Class {} : { new (...args: unknown[]): (Anonymous class); prototype: mixin<any>.(Anonymous class); } & C
> : ^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Class : {}
> : ^^
}

class Base {
>Base : Base
> : ^^^^

constructor(public x: number) {}
>x : number
> : ^^^^^^
}

const Mixed = mixin(Base)
>Mixed : { new (...args: unknown[]): mixin<ClassConstructor>.(Anonymous class); prototype: mixin<any>.(Anonymous class); } & ClassConstructor
> : ^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>mixin(Base) : { new (...args: unknown[]): mixin<ClassConstructor>.(Anonymous class); prototype: mixin<any>.(Anonymous class); } & ClassConstructor
> : ^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>mixin : <C extends ClassConstructor>(Class: C) => { new (...args: unknown[]): (Anonymous class); prototype: mixin<any>.(Anonymous class); } & C
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Base : typeof Base
> : ^^^^^^^^^^^

const instance = new Mixed(42)
>instance : mixin<ClassConstructor>.(Anonymous class)
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>new Mixed(42) : mixin<ClassConstructor>.(Anonymous class)
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Mixed : { new (...args: unknown[]): mixin<ClassConstructor>.(Anonymous class); prototype: mixin<any>.(Anonymous class); } & ClassConstructor
> : ^^^^^^^^^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>42 : 42
> : ^^

17 changes: 17 additions & 0 deletions tests/cases/conformance/classes/mixinWithUnknownRestParam.ts
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@@ -0,0 +1,17 @@
// @strict: true

// Repro for https://github.com/microsoft/TypeScript/issues/29707
// unknown[] should be a valid mixin constructor constraint, same as any[]

type ClassConstructor = new (...args: unknown[]) => {}

function mixin<C extends ClassConstructor>(Class: C) {
return class extends Class {}
}

class Base {
constructor(public x: number) {}
}

const Mixed = mixin(Base)
Comment on lines +12 to +16
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Copilot AI Apr 6, 2026

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The new test currently fails with TS2345 at the call site because Base (constructor (x: number)) is not assignable to new (...args: unknown[]) => {} under --strict. This introduces an unrelated error and prevents the test from cleanly validating the intended mixin/constructor behavior. Consider removing the mixin(Base) usage or changing Base to satisfy the unknown[] constructor constraint so the test exercises only the targeted regression.

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const instance = new Mixed(42)
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