Smarter case selection for overloaded functions

[bcherny]

Please close if this has been asked before - I didn't find anything via search.

Search Terms

Function, overload, multiple signature, discriminate, union

Suggestion

TypeScript should take advantage of refinement to infer which case of an overload we're in.

Use Cases

This would improve the ergonomics of using overloaded function call signatures, and make implementing overloaded functions less tedious.

Examples

type Reservation = void

type Reserve = {
  (from: Date, to: Date, destination: string): Reservation // 1
  (from: Date, destination: string): Reservation // 2
}

let reserve: Reserve = (from: Date, toOrDestination: Date | string, destination?: string) => {
  if (toOrDestination instanceof Date) {
     destination.toUpperCase() // TSC should infer we're in case 1, so destination must be a string
  }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
  • It might catch real errors
• 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)

Related

• Why do we need to manually type unified overload signatures? #25352
• TS fails to refine mapped type case when switching on key #23616

[RyanCavanaugh]

Looks like #14515 ?

[bcherny]

Dup of #14515.