Generic return type (T[K]) is intersection among all properties of T instead of union

[taozhou-glean]

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

[xiBread]

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

[taozhou-glean]

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.

[fatcerberus]

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

It’s a side effect of #30769.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.