Conditional type not narrowed inside of conditional type

Bug Report

🔎 Search Terms

nested conditional type narrow

🕗 Version & Regression Information

• This is the behavior in every version I tried (4.4 up to 4.7.4)

⏯ Playground Link

Playground Link

💻 Code

type EventType = {
    [key: string]: unknown[]
} | string;

type EventName<TEvents extends EventType> = TEvents extends string ? TEvents : keyof TEvents;

type EventArgs<TEvents extends EventType, TName extends EventName<TEvents>> = TEvents extends string ? undefined[] : TEvents[TName];

🙁 Actual behavior

🙂 Expected behavior

TName can be used to index TEvents as the same narrowing occurs.

[RyanCavanaugh]

TEvents[TName & keyof TEvents] is a safe workaround.

It’s not 100% identical in functionality, but another way would be

type EventArgs<TEvents extends EventType, TName extends EventName<TEvents>> = TName extends keyof TEvents ? TEvents[TName] : undefined[];

[douglasg14b]

Gotcha, thanks for that! Very helpful.