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When using keyof typeof, downstream code inference breaks in unexpected ways #23649

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AlexGalays opened this issue Apr 24, 2018 · 0 comments
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Bug A bug in TypeScript Fixed A PR has been merged for this issue

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@AlexGalays
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TypeScript Version: 2.8.3 and next

Search Terms:
keyof inference

Code

export function Set<K extends string>(...keys: K[]): Record<K, true | undefined> {
  const result = {} as Record<K, true | undefined>
  keys.forEach(key => result[key] = true)
  return result
}

export function keys<K extends string, V>(obj: Record<K, V>): K[] {
  return Object.keys(obj) as K[]
}

type Obj = { code: LangCode }


/* No duplication - Derived */
const langCodeSet = Set('fr', 'en', 'es', 'it', 'nl')
export type LangCode = keyof typeof langCodeSet
export const langCodes = keys(langCodeSet)

// Does not compile (???)
const arr: Obj[] = langCodes.map(code => ({ code }))



/* Duplication */
export type LangCode2 = 'fr' | 'en' | 'es' | 'it' | 'nl'
export const langCodes2: LangCode2[] = ['fr', 'en', 'es', 'it', 'nl']

// Compiles
const arr2: Obj[] = langCodes2.map(code => ({ code }))

Expected behavior:
Everything should compile

Actual behavior:
Only the straightforward code compiles. The two are functionally equivalent.

Playground Link:
https://www.typescriptlang.org/play/#src=%0Aexport%20function%20Set%3CK%20extends%20string%3E(...keys%3A%20K%5B%5D)%3A%20Record%3CK%2C%20true%20%7C%20undefined%3E%20%7B%0A%20%20const%20result%20%3D%20%7B%7D%20as%20Record%3CK%2C%20true%20%7C%20undefined%3E%0A%20%20keys.forEach(key%20%3D%3E%20result%5Bkey%5D%20%3D%20true)%0A%20%20return%20result%0A%7D%0A%0Aexport%20function%20keys%3CK%20extends%20string%2C%20V%3E(obj%3A%20Record%3CK%2C%20V%3E)%3A%20K%5B%5D%20%7B%0A%20%20return%20Object.keys(obj)%20as%20K%5B%5D%0A%7D%0A%0Atype%20Obj%20%3D%20%7B%20code%3A%20LangCode%20%7D%0A%0A%0A%2F*%20No%20duplication%20-%20Derived%20*%2F%0Aconst%20langCodeSet%20%3D%20Set('fr'%2C%20'en'%2C%20'es'%2C%20'it'%2C%20'nl')%0Aexport%20type%20LangCode%20%3D%20keyof%20typeof%20langCodeSet%0Aexport%20const%20langCodes%20%3D%20keys(langCodeSet)%0A%0A%2F%2F%20Does%20not%20compile%20(%3F%3F%3F)%0Aconst%20arr%3A%20Obj%5B%5D%20%3D%20langCodes.map(code%20%3D%3E%20(%7B%20code%20%7D))%0A%0A%0A%0A%2F*%20Duplication%20*%2F%0Aexport%20type%20LangCode2%20%3D%20'fr'%20%7C%20'en'%20%7C%20'es'%20%7C%20'it'%20%7C%20'nl'%0Aexport%20const%20langCodes2%3A%20LangCode2%5B%5D%20%3D%20%5B'fr'%2C%20'en'%2C%20'es'%2C%20'it'%2C%20'nl'%5D%0A%0A%2F%2F%20Compiles%0Aconst%20arr2%3A%20Obj%5B%5D%20%3D%20langCodes2.map(code%20%3D%3E%20(%7B%20code%20%7D))

@mhegazy mhegazy added the Bug A bug in TypeScript label Apr 24, 2018
@mhegazy mhegazy added this to the TypeScript 2.9 milestone Apr 24, 2018
@ahejlsberg ahejlsberg added the Fixed A PR has been merged for this issue label May 22, 2018
@microsoft microsoft locked and limited conversation to collaborators Jul 31, 2018
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