Generics Not Working With This Property

[MartinLoeper]

TypeScript Version: 2.9.0-dev.20180515

Search Terms: generics, property, incompatible, polymorphic this, extends constraint

Code

/* taken from mongoose.Document */
interface Document {
    increment(): this;
}

/* our custom model extends the mongoose document */
interface CustomDocument extends Document { }

export class Example<Z extends CustomDocument> {
    constructor() {
        // types of increment not compatible??
        this.test<Z>();
    }

    public test<Z extends Document>() { }
}

Expected behavior:
The type parameter Z should be assignable to the method test as Z is a subtype of CustomDocument which is a subtype of Document.

Actual behavior:
Polymorphic this somehow breaks the functionality.

TS-Error:

error TS2344: Type 'Z' does not satisfy the constraint 'Document'.
  Type 'CustomDocument' is not assignable to type 'Document'.
    Types of property 'increment' are incompatible.
      Type '() => CustomDocument' is not assignable to type '() => Z'.
        Type 'CustomDocument' is not assignable to type 'Z'.

Playground Link: typescriptlang playground

Related Issues: maybe #17316? or #7818?

[weswigham]

Fixed by #21210

[weswigham]

@mhegazy technically this is a dupe of #21117, but this example is much simpler 😉