TS 2.4.1: ReadonlyArray<Identifiable> not assignable to type ReadonlyArray<T> even though <T extends Identifiable>

[mmc41]

TypeScript Version: 2.4.1

Code

export interface Identifiable {
 id: string;
}

export function test<T extends Identifiable>(): ReadonlyArray<T> {
    const result: ReadonlyArray<Identifiable> = [];
    return result;
}

Expected behavior:
Should compile ok.

Actual behavior:

Error: 'Type 'ReadonlyArray<Identifiable>' is not assignable to type 'ReadonlyArray<T>'.
  Type 'Identifiable' is not assignable to type 'T'.
```'

[kitsonk]

export interface Identifiable {
 id: string;
}

export function test<T extends Identifiable>(): ReadonlyArray<T> {
    const result: ReadonlyArray<T> = [];
    return result;
}

2.4 is correct, Identifiable is not assignable to T. Previously the compiler ignored these types of errors. Now it correctly flags them. See #16368 and #15104.

[mmc41]

@kitsonk Thanks for info. Then the Q is, how to change the signature of test method above to work returning an readonly array of either Identifiables or subclasses of Identifiables ?

[mmc41]

Ok. Got it myself:

export function test<T extends Identifiable>(): ReadonlyArray<T> {
    const result: ReadonlyArray<T> = [];
    return result;
}

[kitsonk]

For the record, I did exactly that in the example I provided back... 😁