Typescript: Typescript 3.4 breaks some type inference with method overloads. (unknown)

[garronej]

TypeScript Version: 3.4 and up

Search Terms:
unknown, 3.4, type inference fails, method overload

Code

type Circle = {
    type: "CIRCLE";
    radius: number;
};

type Square = {
    type: "SQUARE";
    sideLength: number;
};

type Shape = Circle | Square;


class Evt<T> {

    public attach<U>(
        matcher: (data: T)=> [U] | undefined,
        callback: (transformedData: U)=> void
    ): Promise<U>;
    public attach(
        matcher: (data: T) => boolean,
        callback: (data: T) => void
    ): Promise<T>;
    public attach(...args: any[]){
        return null as any;
    }

}

const evtShape = new Evt<Shape>();

//OK
const prRadius = evtShape.attach(
    shape => shape.type === "CIRCLE" ? [shape.radius] : undefined,
    radius => { }
);


const prShape = evtShape.attach(
    shape => true,
    shape => { } //<====== In typescript 3.4 and up shape is unknown 
);

Expected behavior:

In the last line, shape should be of type Shape. In typescript 3.4 and up it is of type unknown.

Actual behavior:

Playground Link

[DanielRosenwasser]

Slightly shorter

declare class Stream<T> {
    compactAndThen<U>(matcher: (data: T) => [U] | undefined, callback: (transformedData: U) => void): Promise<U>;
    compactAndThen(matcher: (data: T) => boolean, callback: (data: T) => void): Promise<T>;
}

const thing = new Stream<string>();

//OK
const prRadius = thing.compactAndThen(
    str => str.length > 0 ? [str.toLowerCase()] : undefined,
    str => { }
);


const prShape = thing.compactAndThen(
    foo => true,
    bar => { } //<====== In typescript 3.4+, 'bar' is unknown 
);

Looks like bar gets unknown due to no inferences from the first overload. Not sure if there's a way to fix this. @ahejlsberg?

[garronej]

After further investigation, I noticed that event in typescript prior to 3.4 altho the type Shape is inferred it can only be used as a {}.

Thanks to the typescript's team, you guys do amazing work <3

[garronej]

Hello,
Just a quick update to say that this limitation is a big deal for the library I am working on: EVT.
The overload of the "attach" method had to be split into two groups "attach" and "$attach" and peoples get confused about why and when they need to add the "$".
I know this is really pushing the already amazing type inference system but it would be great for EVT to be able to merge all the overload together.

Regards

[ahejlsberg]

The two overloads are just too close for us to distinguish between them. Best alternative I can come up with is a single signature that uses a conditional type:

declare class Stream<T> {
    compactAndThen<U extends [any] | boolean>(matcher: (data: T) => U | undefined, callback: (data: U extends [infer V] ? V : T) => void): Promise<U extends [infer V] ? V : T>;
}

This seems to deliver the desired results.

[garronej]

@ahejlsberg Thank you very much for taking the time to come out with a workaround, it is very clever.