Allow static members to reference class type parameters

[jaredru]

Search Terms

static, generic, type, getDerivedPropsFromState

Suggestion

Static class members should be allowed to reference the class's type parameters.

Use Cases

React 16.3 introduced a static lifecycle method to Component. Its definition should look something like:

class Component<P, S> {
  static getDerivedStateFromProps?(p: P, s: S): Partial<S> | null;
}

However, that definition results in compiler error TS2302 "Static members cannot reference class type parameters", so it doesn't exist on the DefinitelyTyped definition for Component. In practice, this means we get less help from the editor in the form of code completion, and we get less type safety, as it's possible to misdefine the types when implementing getDerivedStateFromProps.

FWIW, Flow supports this.

Examples

Checklist

My suggestion meets these guidelines:
[x] This wouldn't be a breaking change in existing TypeScript / JavaScript code
[x] This wouldn't change the runtime behavior of existing JavaScript code
[x] This could be implemented without emitting different JS based on the types of the expressions
[x] This isn't a runtime feature (e.g. new expression-level syntax)

[ghost]

Without inheritance that wouldn't make much sense:

class Super<T> {
    static m(x: T): void;
}
Super.m(); // What's `T`?

I see what you mean though, when a subclass is introduced:

class Sub extends Super<number> {}
Sub.m(); // T is number

Although we can't be sure that a subclass will actually provide a concrete type argument and not just another type parameter.

Could you maybe flesh out your example into something independent from react?

(By the way, seems like this issue might be related to #14600.)

[jaredru]

In your example, I would expect it to be inferred:

class Super<T> {
    static m(x: T): void;
}
Super.m();  // Error: Expected 1 argument, but got 0.
Super.m(3); // `T` is number

As a return type, where you can't infer it, I would expect the same functionality as a normal "static"/free function:

function fn<T>(): T;
const a = fn(); // a is typed to {}
const b = fn<number>(); // b is typed to `number`

I understand the "danger" of that - it's effectively a lie, given that there's no runtime information available to fn to actually return the appropriate type - but that syntax is allowed today.

Thank you for the link to #14600. This does seem related, and it might even be safer to only allow abstract static methods to utilize class type params.

Could you maybe flesh out your example into something independent from react?

May I ask why? My motivation is React. I could spend time coming up with a separate contrived example, but really I'd like working with React to require as few concessions as possible.

[RyanCavanaugh]

@jaredru if you want that behavior, you can write

class Super<T> {
    static m<U>(x: U): void;
}

The only thing trying to use Super's T does is change a call that should look like this:

foo.bar<T>(expr);

into this

foo<T>.bar(expr);

[jaredru]

Right, except in the subclass case, which is the React case.

I think you all are right that this ticket is not really what I want. Ultimately, what I'd like is a way to accurately type React's new lifecycle method. So perhaps I should ask, simply, is this something you're thinking about? It seems to me abstract static getDerivedStateFromProps?(p: P, s: S): Partial<S> | null; would work well, but perhaps there's an issue with that, too, even in the face of the use case.

[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.