Type parameter in template literal type is inferred as never

[zyf0330]

Bug Report

🔎 Search Terms

Template Literal Type
Default Value of Generic
Property Chain

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Template Literay Type_

⏯ Playground Link

Playground link with relevant code

💻 Code

🙁 Actual behavior

1. F, F1, F2 in function chain is infered wrongly as never, never, never when only first level property field is passed
2. F2 in function chain("a.b") is infered as never

🙂 Expected behavior

1. F in function chain is infered correctly as "a", undefined, undefined when only one argument field is passed
2. F2 in function chain("a.b") is infered as undefined

[zyf0330]

I need Type is infered as undefined not never, it is because that I write code like this

type A<T = undefined, T1 = undefined, T2 = undefined> = T1 extends undefined ? T : T2 extends undefined ? T1 : T2  

I cannot make it work if use extends never instead of extends undefined to judge.

[DanielRosenwasser]

Shorter example

type Schema = {
    a: {
        b: {
            c: number
        }
    }
}

declare function chain<F extends keyof Schema>(field: F | `${F}.${F}`): void;
chain("a")

I'm guessing what happens is that when we try to infer from a string literal type to a template string type, we assume that we should infer never for each type parameter rather than not inferring at all.

[zyf0330]

@ahejlsberg Hello, may I know when this fix will be released?

[ahejlsberg]

Hello, may I know when this fix will be released?

It'll be in the TS 4.5 beta, due to release any day now.

[DanielRosenwasser]

You can also use it in our nightly releases.