Can't infer the type of this self-calculated

Bug Report

🔎 Search Terms

• infer
• self-calculated
• generic

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type Inference

⏯ Playground Link

type Exactify<T, X extends T> = T & {
  [K in keyof X]: K extends keyof T ? X[K] : never
}

declare function pick<T, X extends Exactify<{
  [K in keyof T]?: true
}, X>>(t: T, x: X): [Pick<T, keyof X>,...

Playground Link

💻 Code

type Exactify<T, X extends T> = T & {
  [K in keyof X]: K extends keyof T ? X[K] : never
}

declare function pick<T, X extends Exactify<{
  [K in keyof T]?: true
}, X>>(t: T, x: X): [Pick<T, keyof X>, X, X extends Exactify<infer A, infer B> ? B : never]

const t0 = pick({ a: '1', b: 1 }, { a: true })
const [
  a0,
  a1,
// ^? Exactify<{ a?: string | undefined; b?: number | undefined }, { a: boolean }>
  a2
// ^? { a?: true | undefined; b?: true | undefined }
] = pick({ a: '1', b: 1 }, { a: true })

🙁 Actual behavior

The a2 variable is { a?: true | undefined; b?: true | undefined } type.

🙂 Expected behavior

Infer the a2 variable is { a: boolean } or { a: true } type.

Inizia dalla riproduzione collegata in TypeScript Playground e dall’esempio generico pick/Exactify nell’issue. Analizza perché il tipo a2 inferito è Exactify<...> invece di { a: boolean } o { a: true }, quindi verifica l’inferenza prevista con un test mirato del compilatore.