keyof R is not assignable to extends keyof R

[artaommahe]

TypeScript Version: 2.7.1

Code

class Store<R> {
  protected events: { type: keyof R, data: R[keyof R] }[] = [];

  public on<A extends keyof R>(type: A): R[A] {
    return this.events
      .find(event => event.type === type)
      .data;
  }
}

interface IEvents {
  smth: string;
  smth2: number;
  smth3: boolean;
}

const store = new Store<IEvents>();

const smthEvent = store.on('smth');

Expected behavior:
No error, smthEvent has proper string type

Actual behavior:

Type 'R[keyof R]' is not assignable to type 'R[A]'.
  Type 'keyof R' is not assignable to type 'A'.

or with

public on(type: keyof R): R[keyof R]

smthEvent has parameter) data: string | number | boolean type instead if string

No error with TS 2.6.2

Playground Link:
https://www.typescriptlang.org/play/#src=class%20Store%3CR%3E%20%7B%0D%0A%20%20protected%20events%3A%20%7B%20type%3A%20keyof%20R%2C%20data%3A%20R%5Bkeyof%20R%5D%20%7D%5B%5D%20%3D%20%5B%5D%3B%0D%0A%0D%0A%20%20public%20on%3CA%20extends%20keyof%20R%3E(type%3A%20A)%3A%20R%5BA%5D%20%7B%0D%0A%20%20%20%20return%20this.events%0D%0A%20%20%20%20%20%20.find(event%20%3D%3E%20event.type%20%3D%3D%3D%20type)%0D%0A%20%20%20%20%20%20.data%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Ainterface%20IEvents%20%7B%0D%0A%20%20smth%3A%20string%3B%0D%0A%20%20smth2%3A%20number%3B%0D%0A%20%20smth3%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Aconst%20store%20%3D%20new%20Store%3CIEvents%3E()%3B%0D%0A%0D%0Aconst%20smthEvent%20%3D%20store.on('smth')%3B

[jcalz]

Looks like things are working as intended here. R[keyof R] can be any of the property value types of R; there is no guarantee in { type: keyof R, data: R[keyof R] } that the type of data corresponds to the same key in type. Probably you should just use type assertions, if you're sure you set it up properly.

The type-safe way to get this done would probably require existential types (#14466) which TypeScript doesn't have built-in support for. For fun, here's how you'd need to do it today:

type Event<R, K extends keyof R> = { type: K, data: R[K] };
type SomeEvent<R> = <O>(f: <K extends keyof R>(e: Event<R, K>) => O) => O
type SomeParticularEvent<R, K extends keyof R> = <O>(f: (e: Event<R, K>) => O) => O

const narrowSomeEvent = <R, K extends keyof R>(k: K) =>
  (someEvent: SomeEvent<R>): someEvent is SomeParticularEvent<R, K> =>
    someEvent(e => e.type === k as string);

class Store<R> {
  protected events: SomeEvent<R>[] = [];

  public on<A extends keyof R>(type: A): R[A] | undefined {
    const theEvent = this.events.find(narrowSomeEvent(type))
    return theEvent ? theEvent(e => e.data) : undefined;
  }
}

Might not be worth it to you. Anyway, this doesn't look like a bug to me and perhaps this kind of question belongs on StackOverflow instead.

[DanielRosenwasser]

The title has it backwards - it's that keyof R is not assignable to A which extends keyof R.

[DanielRosenwasser]

You could also use a type predicate function to convey the narrowing to find:

class Store<R> {
  protected events: { type: keyof R, data: R[keyof R] }[] = [];

  public on<A extends keyof R>(type: A): R[A] {
    return this.events
      .find((event): event is { type: A, data: R[A] } => event.type === type)!
      .data;
  }
}

interface IEvents {
  smth: string;
  smth2: number;
  smth3: boolean;
}

const store = new Store<IEvents>();

const smthEvent = store.on('smth');

[artaommahe]

thx for solutions

• why has this been broken from 2.6.2 to 2.7?
• will be there more obvious solution for such cases?

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.