[3.5.0-dev.20190516] Incorrect type error for mixin

[canonic-epicure]

When using the mixin pattern, there are 2 main notations to define the type of the mixin entity.
The mixin pattern:

export type AnyFunction<A = any>        = (...input : any[]) => A
export type AnyConstructor<A = object>  = new (...input : any[]) => A
export type Mixin<T extends AnyFunction> = InstanceType<ReturnType<T>>

export const Box = <T extends AnyConstructor<object>>(base : T) =>
class Box extends base {
    value       : any
}

1st notation:

export type Box = Mixin<typeof Box> {}

2nd notation:

export interface Box extends Mixin<typeof Box> {}

The 1st notation can not be used for recursive definitions (#29872). Because of that we primarily use 2nd notation. It works fine in most cases, however I found a case, when it produces invalid type error. The full snippet to reproduce the problem below.

Note:

• The typechecker correctly figures out that there's no zxc property on this, inside the observe method of Quark mixin.
• In that method, it does not complain about the this.value usage
• It does complain, when value is used on function argument
• If you'll switch the Quark mixin to the 1st notation, the error will disappear

Expected behavior:

• No type errors for the definition of test function below

export type AnyFunction<A = any>        = (...input : any[]) => A
export type AnyConstructor<A = object>  = new (...input : any[]) => A
export type Mixin<T extends AnyFunction> = InstanceType<ReturnType<T>>

export const Box = <T extends AnyConstructor<object>>(base : T) =>
class Box extends base {
    value       : any
}
export interface Box extends Mixin<typeof Box> {}

export const Observable = <T extends AnyConstructor<object>>(base : T) =>
class Observable extends base {
    observe () : Quark {
        return
    }
}
export interface Observable extends Mixin<typeof Observable> {}

export const Quark = <T extends AnyConstructor<Box & Observable>>(base : T) =>
class Quark extends base {

    observe () : Quark {
        // No error here!
        this.value
        
        // error: Error:(28, 14) TS2339: Property 'zxc' does not exist on type 'Quark'.
        this.zxc
        
        return
    }
}
export interface Quark extends Mixin<typeof Quark> {}

const test = (a : Quark) => a.value // <-- Error:(35, 28) TS2339: Property 'value' does not exist on type 'Quark'.

[orta]

Interesting, so .value occurs in the intellisense for a Quark in these examples, perhaps the error is overly greedy here.

[canonic-epicure]

Glad to see the progress on this issue!

[canonic-epicure]

@orta Perhaps, while you are in the context, you can check why the 2nd notation (#29872) is needed at all? There are some other problems with it, for example, casting a class type to interface loses the static methods information.

[orta]

Putting this issue into backlog instead of milestoned, figuring out this issue was first evaluated in #32770 and then a more permentent fix was explored in #33050 (comment) but was considered too expensive on perf for the feature.