Generic function return type inference regression in 3.7-beta

[dloehr]

TypeScript Version: 3.7.0-beta

Search Terms: 3.7-beta, inference, infer, return type

Code

interface Person {
  readonly age: number
  readonly firstName: string
}

declare function getProperty<K extends keyof Person>(name: K): Person[K]

interface Selector<Args extends any[], R> { (...args: Args): R }

type SelectorReturnType<
  Args extends any[],
  Fn extends Selector<Args, any>
> = Fn extends Selector<Args, infer R> ? R : never

declare function select<Args extends any[], Fn extends Selector<Args, any>>(
  selector: Fn,
  ...args: Args
): SelectorReturnType<Args, Fn>

const age = select(getProperty, 'age' as const)
const firstName = select(getProperty, 'firstName' as const)

Expected behavior:
The type of age should be inferred as number and the type of firstName should be string. This is how it works with TypeScript 3.6.3.

Actual behavior:
age and firstName are both string | number

Playground Link: https://www.typescriptlang.org/play/?ts=3.7-Beta#code/JYOwLgpgTgZghgYwgAgArQM4HsTIN4BQyyUEcAJjgDYCeycA5hAFzIgCuAtgEbQDcREmUohayGMCgYwAOTicWyaVFAMBAXwIFyEBFTilx7EAjDAcyJmFRQsAB2hgaAHgDSyCAA9IIchmQA1hA0WDBomDgAfAAUIPKKrgCUrOhSOADargC6AgSgkLCIKADKEFS6YFhQzgCCUAz+Xj5+9CA06VkANMgASpH4gtEAdCMGDax1Dcm9GlpODsil5aZVPRBg7FAgACo0Ds6Ck43eEL7+cG0dnYIAYrhNpy1LFVW19RjdFzSRBP0AvMg7h4TmdFmUXtUjt1QDBoL1+gB+XrIVggCAAN34Wh0egMKBgxlM5lwGHBpjeDWBzXOly6gPuIKeZMqkPenzakRiglJyxZrDu12IIyGYwwE3eBGmzxWUDWGy2u32UPpkVyCBw0noTGQAJ5FWiVhs9kcNG6ACJGBAzfR-OqQNJEgI7ZqJFJZPEdUpmQb1kaHFAnObXdI5AprXBbRqwI6CEA

[ahejlsberg]

This is an intended effect of #33228. The type of the selector parameter is the naked type parameter Fn, and we now infer the exact type of getProperty instead of instantiating getProperty in the context of the constraint of Fn and inferring from that. While this produces a more general inference for Fn, it also erases the connection what we've already inferred for Args.

However, there is a much simpler way to write this that has the desired effect:

declare function select<A extends any[], R>(selector: (...args: A) => R, ...args: A): R;

const age = select(getProperty, 'age' as const)
const firstName = select(getProperty, 'firstName' as const)

This works with both 3.6.3 and 3.7.

[dloehr]

Thanks @ahejlsberg!