Why do we need to manually type unified overload signatures?

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Search Terms

overload, function, multiple, default, combine, unify, top

Suggestion

Today, when defining an overloaded call signature you have to manually type the implementation. Would it be possible to extend contextual types so TS can infer the implementation's types from context, just like TS already does for non-overloaded signatures?

Use Cases

Today, contextual typing isn't able to infer parameter types for overloaded call signatures. Instead, users have to manually type the implementation.

Examples

Before

type Reserve = {
  (from: Date, to: Date, destination: string): Reservation
  (from: Date, destination: string): Reservation
}

let reserve: Reserve = (
  from: Date,
  toOrDestination: Date | string,
  destination?: string
) => { /* ... */ }

After

type Reserve = {
  (from: Date, to: Date, destination: string): Reservation
  (from: Date, destination: string): Reservation
}

let reserve: Reserve = (
  from,             // inferred as Date | Date = Date
  toOrDestination,  // inferred as Date | string
  destination       // inferred as string | undefined
) => { /* ... */ }

Checklist

My suggestion meets these guidelines:

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

Commencez par étudier le typage contextuel de TypeScript et le comportement des signatures surchargées, en utilisant les exemples de l’issue comme référence comportementale. Déterminez comment les types des paramètres doivent être combinés entre les surcharges et ajoutez une couverture pour l’affectation de Reserve présentée. Le travail est terminé lorsque les implémentations surchargées compatibles reçoivent des types de paramètres inférés sans modifier la sortie à l’exécution.