Fails to expand indexed access type when used as a generic type parameter

Bug Report

This logic is part of a proprietary DSL where the user wants to work with a number of types, specified as a record. Most of the time the user calls a function similar to

declare function someOtherFunction<T extends DiscriminatorRecord>(record: T): { [K in keyof T]: DiscriminatedUnion<T[K]> }

in which case there are no problems with the inference, but sometimes T needs to be restricted to have certain keys and values, as shown in the code below. One part of the problem that I wasn't able to reproduce in a code example was the fact that it did work with four types in the union, but failed when I added a fifth. Whereas in the example it fails with any number except one. I could spend more time on reproduction, but hopefully this will be helpful to start with.

🔎 Search Terms

generic indexed access type inference

🕗 Version & Regression Information

• This is the behavior in every version (5.0.4, 5.1.3, v5.2.0-dev.20230625) I tried, and I reviewed the FAQ for entries about Type System Behavior, Generics

⏯ Playground Link

Playground link with relevant code

💻 Code

type Discriminator =
  | 'First'
  | 'Second'
  | 'Third'

type DiscriminatedUnion<D extends Discriminator> = { discriminator: D } & (
  | { discriminator: 'First', first: boolean }
  | { discriminator: 'Second', second: boolean }
  | { discriminator: 'Third', third: boolean }
  )

type DiscriminatorRecord = Record<string, Discriminator>

function someFunction<T extends DiscriminatorRecord & { override: 'Third' }>() {
  type DiscriminatorFromT = T['override'] // If this expands to 'Third'

  type DiscriminatedFromLiteral = DiscriminatedUnion<'Third'> // then this

  type DiscriminatedFromT = DiscriminatedUnion<DiscriminatorFromT> // should be equivalent to this

  const f1 = (d: DiscriminatedFromLiteral) => d.third; // yet this works
  const f2 = (d: DiscriminatedFromT) => d.third; // but not this
}

🙁 Actual behavior

const f2 = (d: DiscriminatedFromT) => d.third; fails with

Property 'third' does not exist on type 'DiscriminatedUnion<DiscriminatorFromT>'.
  Property 'third' does not exist on type '{ discriminator: DiscriminatorFromT; } & { discriminator: "First"; first: boolean; }'.

🙂 Expected behavior

The property third is inferred to exist on d in const f2 = (d: DiscriminatedFromT) => d.third;

Inizia con il TypeScript Playground collegato e la riproduzione nell’issue, quindi confronta l’argomento generico letterale con il tipo di accesso indicizzato usato in DiscriminatedUnion. Conferma il comportamento nelle versioni segnalate e determina quale modifica fa sì che d.third venga risolto correttamente senza compromettere il comportamento esistente dell’unione.