Abstract property specified on multiple classes loses modifier after intersection

[ssalbdivad]

🔎 Search Terms

abstract property class intersection InstanceType typeof mixin trait

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://tsplay.dev/mAL7ZN

💻 Code

abstract class A {
	abstract a(): number
}

abstract class A2 {
	abstract a(): number
}

abstract class B  {
	abstract b(): number
}

declare const Base: abstract new () => A & B

// correctly has type error requiring the implementation of 'a' and 'b'
class Foo extends Base {}

declare const FalsePositiveBase: abstract new () => A & A2

// this sould be an error
// if an abstract property appears on multiple classes, it is not treated as abstract
class Bar extends FalsePositiveBase {}

Seems like there must be some flag that associates the abstractness of the original class instance when it is used as a class instance like that, but there is a logic error that checks if the prop exists on other intersection members and erases the modifier, even if all of them are also abstract.

I previously thought this was a workaround:

// can workaround this as follows:
declare const WorkaroundBase: abstract new () => InstanceType<typeof A & typeof A2>

// now gives the correct error that 'a' is missing
class Baz extends WorkaroundBase {}

But realized that it is just treating the constructor as an overload and only using one of the instance types.

🙁 Actual behavior

Behavior for multiple props of the same name was inconsistent with behavior for props with differing names

🙂 Expected behavior

Intersecting multiple props of the same name that all of the abstract modifier should preserve it

Additional information about the issue

No response