Microsoft/TypeScript

TS is overly picky when declaring a class constructor type

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#29,707 opened on Feb 3, 2019

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 (11 comments) (6 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: classesGood First IssueHelp Wanted

Description

TypeScript Version: 3.4.0-dev.20190202

Search Terms: class, extend, constructor, any[]

Code

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

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

Expected behavior:

I should be able to replace ...args: any[] with ...args: unknown[], or any other signature.

Actual behavior:

Error: Type 'C' is not a constructor function type. [2507]

Playground Link: https://www.typescriptlang.org/play/index.html#src=type%20ClassConstructor%20%3D%20new(...args%3A%20unknown%5B%5D)%20%3D%3E%20%7B%7D%0D%0A%0D%0Afunction%20mixin%3CC%20extends%20ClassConstructor%3E(Class%3A%20C)%20%7B%0D%0A%20%20return%20class%20extends%20Class%20%7B%7D%0D%0A%7D

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript fundamentalsgeneric constraintsclass constructor types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
Investigate the TypeScript compiler source code, particularly the `checker.ts` file where constructor type constraints are validated. Review the comments in the issue for potential solutions and test cases. The fix likely involves relaxing the constraint for `new(...args: unknown[]) => {}` to be accepted as a constructor type.
TS is overly picky when declaring a class constructor type · Microsoft/TypeScript#29707 | Good First Issue