Typescript infer keyword inferring wrong type?

Question

With a simple construct like this:

type Test<Source extends string[]> = 
    Source extends [infer Head, ...infer Tail] ? `${Head}${Head}` : never;

Typescript complains about the string interpolation inside ${Head}${Head} saying: Type `'Head'` is not assignable to type 'string | number | bigint | boolean'. Type 'Head' is not assignable to type 'number'.ts(2322)

...which does not make much sense to me but my searches did not turn fruitful. Source must be a string[] so Head, if it exists, must be a string right? So ${Head} must be totally valid. Why is TS complaining about not being able to assign it to a number? String is also assignable to "string | number | bigint | boolean" so this error does not make much sense to me. I can fix it by putting the string interpolation between a "Head extends string ? ... : ... clause but I am wondering why it is necessary in the first place.

Answer 1

Yes and no. Take a look on next example:

type Test0<Source extends string[]> =
    Source extends [infer Head, ...infer Tail] ? Head : never;

type O = Test0<[never, 'a']> // no error, because that is how NEVER works

Because string|never returns string.

I think it is much safer to check if Head is string or not in above case.