`this` type in constructor interface will not be extended in sub class

[ktsn]

TypeScript Version: 3.3.0-dev.20181208

Search Terms: constructor interface this extends sub class

Code

interface AConstructor {
    new (): AInstance
    a(): this
}

interface AInstance {
    a: string
}

declare const A: AConstructor 

declare function test<T extends typeof A>(a: T): T

class B extends A {
    b: string
}

const B1: typeof B = B.a() // Unexpected: Return type is AConstructor
const B2: typeof B = test(B) // Unexpected: Type parameter is inferred to AConstructor

Expected behavior:
The return type of B.a() and test(B) is typeof B.

Actual behavior:
The return type of B.a() and test(B) is AConstructor.

Playground Link:
https://www.typescriptlang.org/play/#src=interface%20AConstructor%20%7B%0D%0A%20%20%20%20new%20()%3A%20AInstance%0D%0A%20%20%20%20a()%3A%20this%0D%0A%7D%0D%0A%0D%0Ainterface%20AInstance%20%7B%0D%0A%20%20%20%20a%3A%20string%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20A%3A%20AConstructor%20%0D%0A%0D%0Adeclare%20function%20test%3CT%20extends%20typeof%20A%3E(a%3A%20T)%3A%20T%0D%0A%0D%0Aclass%20B%20extends%20A%20%7B%0D%0A%20%20%20%20b%3A%20string%0D%0A%7D%0D%0A%0D%0Aconst%20B1%3A%20typeof%20B%20%3D%20B.a()%20%2F%2F%20Unexpected%3A%20Return%20type%20is%20AConstructor%0D%0Aconst%20B2%3A%20typeof%20B%20%3D%20test(B)%20%2F%2F%20Unexpected%3A%20Type%20parameter%20is%20inferred%20to%20AConstructor

Related Issues:
vuejs/vue-class-component#294

Background: I've found this behaviour while investigating the above issue. This is caused by recent changes for Vue's type declaration. It looks like the extending class does not match with the decorator type because this type is still base constructor type.

I'm still not sure if my guess is correct. If it is not true, please just let me know about that.

[HerringtonDarkholme]

It works when only interface involves. I don't know if mixing up this, new, interface and constructor is good practice.

interface AConstructor {
    new (): AInstance
    a(): this
}

interface AInstance {
    c: string
}

declare const A: AConstructor 

declare function test<T extends AConstructor>(a: T): T

interface BConstructor extends AConstructor {
    b: string
}

declare const B: BConstructor

const B1: typeof B = B.a() // Good
const B2: typeof B = test(B) // Good

https://www.typescriptlang.org/play/#src=interface%20AConstructor%20%7B%0D%0A%20%20%20%20new%20()%3A%20AInstance%0D%0A%20%20%20%20a()%3A%20this%0D%0A%7D%0D%0A%0D%0Ainterface%20AInstance%20%7B%0D%0A%20%20%20%20c%3A%20string%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20A%3A%20AConstructor%20%0D%0A%0D%0Adeclare%20function%20test%3CT%20extends%20AConstructor%3E(a%3A%20T)%3A%20T%0D%0A%0D%0Ainterface%20BConstructor%20extends%20AConstructor%20%7B%0D%0A%20%20%20%20b%3A%20string%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20B%3A%20BConstructor%0D%0A%0D%0Aconst%20B1%3A%20typeof%20B%20%3D%20B.a()%20%2F%2F%20Unexpected%3A%20Return%20type%20is%20AConstructor%0D%0Aconst%20B2%3A%20typeof%20B%20%3D%20test(B)%20%2F%2F%20Unexpected%3A%20Type%20parameter%20is%20inferred%20to%20AConstructor

[fenying]

Maybe it same as this issue #26206 (comment)

[DanielRosenwasser]

I believe what you really want is #5863.

[DanielRosenwasser]

I would suggest patching over vuejs/vue#8595 to return VueConstructor<V> instead of this.

[DanielRosenwasser]

...like in vuejs/vue#9173 😄

[ktsn]

Oh, I didn't realize #5863 is related. Thank you for pointing it out and suggestion!
Yes, it looks better to patch Vue side 🙂

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.