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

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[].

Start with the linked TypeScript Playground reproduction and confirm that the rest parameter is inferred as unknown[] rather than string[]. Trace the compiler's conditional-type inference for the shown Join type, then verify that the rest is inferred as string[] without changing the expected behavior for other conditional types.