Leverage generic type identification

[nugaon]

Bug Report

🔎 Search Terms

inferring, infer, generic type, generic, user-defined type guard, conditional type

🕗 Version & Regression Information

• This is a bug in all versions of TypeScript

⏯ Playground Link

Playground link with relevant code

💻 Code

type MainT = 'john' | 'doe'
type ValueT<T extends MainT> = T extends 'john' ? string : T extends 'doe' ? number : never
type ReturnT<T extends MainT> = T extends 'john' ? Uint32Array : T extends 'doe' ? Uint8Array : never

function isMainT<T extends MainT>(value: unknown, mainType: T): value is T {
    return value === mainType
}

function test<T extends MainT>(key: T, value: ValueT<T>): ReturnT<T> {
    if(isMainT(key, 'john')) {
        const n = value.split('') // ValueT<T> is not inferred when T is known by using typeguard on `key`
        return new Uint32Array() // ReturnT<T> is not inferred when T is known
    }
}

🙁 Actual behavior

When the type of the main generic type (MaintT) is identified by a User-Defined Type Guard function, the other conditional types based on this main type are not identified and TS keeps it generic.

🙂 Expected behavior

ReturnT<T> and ValueT<T> became inferred after T has been identified

[andrewbranch]

Duplicate of #33912 and #35873

[typescript-bot]

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