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)

まず、issue の例を動作の基準として使用し、TypeScript のコンテキスト型付けとオーバーロードシグネチャの動作を調べます。オーバーロード間でパラメーター型をどのように組み合わせるべきかを判断し、示されている Reserve の代入に対するカバレッジを追加します。互換性のあるオーバーロード実装が推論されたパラメーター型を受け取り、実行時の出力を変更しないことが完了の条件です。