Template literal value not assignable to template literal type #56659

🔎 Search Terms

template literal not assignable

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs
• There is an error since version 4.1.5 (I think this is when template literal types have been introduced)

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAShDmBLAzsATiKBeKBvAUFEVAIYBcepAjBbgL5R2HEBGtULNeDTT+okKAGkIIAPYAzADwAVAHzYoAa1GSoMqADIoqNIgB28fPgkBXfQGNgiMfqgSqU5kQDKFsYIgAPYBH0ATZGFVaTgkXRA5ABpnKABRADc-YChvXwCgkXFQhBR0EABtNw8IAF05fDkACggk-WAABRJgAAsKAAMAElxiyDoybsTkunaASjxY931UKALkd0go1LrgUsVa5KbWgDpkMAAbRGAqgCIyE9HJ22QxfYht-bF4KvmSpY36y74zS2tbewATE5iFBetA0n5AsFslIwnkMNFYkN6qkfJDMiFYbkIkUFmUKtVXpAKGD3isKMjgOMCCCHFUurgiRB+t0PsARl98EA

💻 Code

type Registry = {
    a: { a1: {} }
    b: { b1: {} }
}

type Keyof<T> = keyof T & string

function f1<
    Scope extends Keyof<Registry>,
    Event extends Keyof<Registry[Scope]>
>(eventPath: `${Scope}:${Event}`) {
    const [scope, event] = eventPath.split(":")
    console.log(scope, event)
}

function f2<
    Scope extends Keyof<Registry>,
    Event extends Keyof<Registry[Scope]>
>(scope: Scope, event: Event) {
    // It errors here with:
    // Argument of type '`${Scope}:${Event}`' is not assignable to parameter of type '`${Scope}:${Keyof<Registry[`${Scope}`]>}`'.(2345)
    f1(`${scope}:${event}`)
}

🙁 Actual behavior

When calling the function f1 with a template literal value built with the same types than the template literal type parameter, it errors with:

Argument of type '${Scope}:${Event}' is not assignable to parameter of type '${Scope}:${Keyof<Registry[${Scope}]>}'.(2345)

🙂 Expected behavior

When calling the function f1 with a template literal value built with the same types than the template literal type parameter, I would expect no error.

Additional information about the issue

The message error message has changed over time, in versions 4.1.5 and 4.2.3 it was:

Argument of type 'string' is not assignable to parameter of type 'never'. (2345)

Since version 4.3.5 it is:

Argument of type '${Scope}:${Event}' is not assignable to parameter of type '${Scope}:${Keyof<Registry[${Scope}]>}'.(2345)