Description

Tao Zhou (taozhou-glean)

opened on Jul 6, 2023

Issue body actions

Bug Report

🔎 Search Terms

conditionally generic return type, union vs intersection, valueof etc

🕗 Version & Regression Information

• This changed between versions 3.3.3 and 3.5

⏯ Playground Link

Playground link with relevant code

💻 Code

export interface ConfigA {
  user: string
}

export interface ConfigB {
  [key: string]: number | string | boolean
}

export interface Config {
  A?: ConfigA
  B?: ConfigB
}

export class ConfigClass {
  config: Config = {}
  get(key: keyof Config) {
    return this.config[key]
  }
}

type ValueOf<T> = T[keyof T]

const config = new ConfigClass()

function a<K extends keyof Config>(key: K): Config[K] {
  return config.get(key)
}

🙁 Actual behavior

Type 'ConfigA | ConfigB | undefined' is not assignable to type 'Config[K]'.
Type 'ConfigA' is not assignable to type 'Config[K]'.
Type 'ConfigA' is not assignable to type 'ConfigA & ConfigB'.
Type 'ConfigA' is not assignable to type 'ConfigB'.
Index signature for type 'string' is missing in type 'ConfigA'.

🙂 Expected behavior

I would expect no error of returning config.get(key) within function a

Activity

1. olivermrose commented on Jul 6, 2023

   

   Oliver Rose (olivermrose)

   on Jul 6, 2023

   More actions

   get needs to be generic; otherwise, the return type is essentially Config[keyof T].
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Jul 6, 2023
3. taozhou-glean commented on Jul 7, 2023

   

   Tao Zhou (taozhou-glean)

   on Jul 7, 2023

   Author

   More actions

   get needs to be generic; otherwise, the return type is essentially Config[keyof T].

   thanks! But even with Config[keyof T], should not it be ConfigA | ConfigB instead of ConfigA & ConfigB ?

   The example is a simplified case just for the minimum repro, in reality we are having this issue with a IPC proxy, and the final xx.get is also generic, but I am getting exact same error, maybe I should update the playground with a more realistic case.
4. fatcerberus commented on Jul 7, 2023

   

   Bruce Pascoe (fatcerberus)

   on Jul 7, 2023

   More actions

   But even with Config[keyof T], should not it be ConfigA | ConfigB instead of ConfigA & ConfigB ?

   It’s a side effect of #30769.
5. typescript-bot commented on Jul 9, 2023

   

   TypeScript Bot (typescript-bot)

   on Jul 9, 2023

   Contributor

   More actions

   This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
6. TypeScript Bot (typescript-bot)

   closed this as completedon Jul 9, 2023
7. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025

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