Inferring rest of array in a conditional results in unknown[]

[avaly]

Bug Report

🔎 Search Terms

infer conditional array rest unknown

🕗 Version & Regression Information

• I was unable to test this on prior versions than 4.1 because it uses template literal types added in 4.1

⏯ Playground Link

Playground link with relevant code

💻 Code

Using code from the #40336 description restricted to strings only:

type Join<S extends string[], D extends string> =
	S extends [] ? '' :
	S extends [string] ? `${S[0]}` :
	S extends [string, ...infer R] ? `${S[0]}${D}${Join<R, D>}` :
	string;

🙁 Actual behavior

R is inferred as being unknown[].

🙂 Expected behavior

R should be inferred as being string[].

[cypherfunc]

Has there been any movement on this?
The example can be simplified further, if it helps:

type Join<S extends string[]> =
    S extends [unknown, ...infer R] ? Join<R> // R is inferred as unknown[], but should be string[]
    : DoesntMatter;

I've been able to get around this by adding an extra conditional check, but it's awkward/confusing in the middle of more complex types:

type Join<S extends string[]> =
    S extends [unknown, ...infer R] ? Join<R extends string[] ? R : never>
    : DoesntMatter;

[nag]

Not only the rest of the array is misinferred, but any element as well:

type Foobar<T extends string[]> = T extends [infer F, ...infer R]
  ? [`${F}`, `${R[0]}`]
  : never

// Type 'F' is not assignable to type 'string | number | bigint | boolean | null | undefined'.

// Type 'R[0]' is not assignable to type 'string | number | bigint | boolean | null | undefined'.
// Type 'unknown' is not assignable to type 'string | number | bigint | boolean | null | undefined'.

[cypherfunc]

For any future folks, this is now easily solved with constraints in 4.7:
https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#extends-constraints-on-infer-type-variables