microsoft / microsoft/TypeScript

Need a dummy overload to improve inference

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Bug Domain: check: Type Inference
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Description

TypeScript Version: 3.x, 4.0.0-beta, nightly

Search Terms: dummy fake overload infer

Code

class FieldEvent<T extends Field> {
    constructor(readonly field: T, readonly message: string) { }
}

class Field {
    listen<T extends FieldEvent<Field>>(type: new (...args: any[]) => T, listener: (event: T) => void): void { }
}

class BeforeSetEvent<T extends number | string> extends FieldEvent<ValueField<T>> {
    constructor(field: ValueField<T>, public newValue: T) { super(field, ""); }
}

class AfterSetEvent<T extends number | string> extends FieldEvent<ValueField<T>> {
    constructor(field: ValueField<T>, readonly oldValue: T) { super(field, ""); }
}

class ValueField<T extends number | string> extends Field {
    constructor(public value: T) { super(); }

    // listen<U extends FieldEvent<Field>>(type: new (arg: this) => U, listener: (event: U) => void): void;
    // listen<U extends FieldEvent<Field>>(type: new (...args: any[]) => U, listener: (event: U) => void): void;
    listen<U extends FieldEvent<Field>>(type: new (...args: any[]) => U, listener: (event: U) => void): void {
        super.listen(type, listener);
    }
}

let field = new ValueField(0);
field.listen(FieldEvent, (event) => event.field);                           // event: FieldEvent<any>
field.listen(BeforeSetEvent, (event) => { event.field; event.newValue; });  // event: BeforeSetEvent<any>
field.listen(AfterSetEvent, (event) => { event.field; event.oldValue; });   // event: AfterSetEvent<any>

Expected behavior:
event to be better inferred than any.

Actual behavior:
event inferred as any.

But if we uncomment the 2 overloads, then event is correctly inferred:

field.listen(FieldEvent, (event) => event.field);                           // event: FieldEvent<ValueField<0>>
field.listen(BeforeSetEvent, (event) => { event.field; event.newValue; });  // event: BeforeSetEvent<0>
field.listen(AfterSetEvent, (event) => { event.field; event.oldValue; });   // event: AfterSetEvent<0>

What's surprising (so the reason I consider it as a bug):

  • The 1st overload doesn't match any signature.
  • But it influences the way the other overload is inferred.

Note that if we replace (arg: this) by () or another type, then:

field.listen(FieldEvent, (event) => event.field);                           // event: FieldEvent<Field>
field.listen(BeforeSetEvent, (event) => { event.field; event.newValue; });  // event: BeforeSetEvent<string | number>
field.listen(AfterSetEvent, (event) => { event.field; event.oldValue; });   // event: AfterSetEvent<string | number>

Playground Link: Playground Link

Related Issues: I first posted #36226 but was not able to solve my issue, until I found the (arg: this) trick.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the example in the linked TypeScript Playground, comparing inference with and without the two overloads. Trace the compiler's generic inference and overload-resolution behavior for the listen calls; done means the callbacks infer FieldEvent, BeforeSetEvent, and AfterSetEvent with the relevant concrete types without the dummy overloads.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
25/100

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