Lib: args rest type should always be array of any

[antongolub]

TypeScript Version: 3.8.0-dev.20200119

Search Terms: rest, ConstructorParameters, ReturnType
Code

/**
 * Obtain the parameters of a constructor function type in a tuple
 */
type ConstructorParameters<T extends new (...args: any) => any> = T extends new (...args: infer P) => any ? P : never;

Expected behavior:
...args rest param should extend any[]

Actual behavior:
...args has any type

Playground Link:

Related Issues:

[dragomirtitian]

Why is this an issue ? any is assignable to anything, including to any[] and can be the type of a rest parameter. Is this causing any problems anywhere? I can't think of any problems this would cause. I don't think changing this for the sake of change is worth it.

[antongolub]

I experimented with mixin factories and got signature mismatch of my IConstructable type when I unfolded the inheritance chain and tried to check the type through InstanceType<typeof MixinResultClass>

type IConstructable<T= {}, A extends any[] = any[]> = new (...args: A) => T
<T extends new (...args: any) => any>

As I understand any[] extends any, but any does not extend any[]

[dragomirtitian]

@antongolub Maybe post a self contained example reproducing the issue?

[antongolub]

Well, once I tried to localize problem point, I've found accidental iface and variable names clash.

interface D {
}
// ~200 lines of code
const D = ... 

This made a major contribution to my problem.
So this PR improvement does not make any sense to my original goal: verify that custom IConstructables corresponds TS expectations. My hypothesis was that I could check new ()... method signature vs T extends new(args: any[]) condition. I was wrong. For example, new(...args: null) in some interface passes the check.

type InstanceType<T> = T extends new (...args: any[]) => infer R ? R : any;

interface Foo { foo: string }

interface WeirdConstructable {
  new (...args: never): Foo
}

const instance: InstanceType<WeirdConstructable> = { foo: 1 } // $ExpectError