Nested array incorrectly infer children type to unknown when given a single element

[ycmjason]

Bug Report

🔎 Search Terms

nested, infer, inference, unknown

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

type Magic<X> = X extends [[infer Y, ...infer _], ...infer _] ? Y : never;

type R = Magic<[[number]]> // ❌ unknown
type R2 = Magic<[[number, 2]]> // ✅ number
type R3 = Magic<[[number], []]> // ✅ number

🙁 Actual behavior

• Magic<[[whatever]]> is unknown

🙂 Expected behavior

• Magic<[[whatever]]> should be whatever

[ycmjason]

Upon further inspection, it looks like removing one of the ... infer would yield correct results.

Playground Link

type Magic2<X> = X extends [[infer Y, ...infer _]] ? Y : never;

type S = Magic2<[[number]]> // ✅ number
type S2 = Magic2<[[number, 2]]> // ✅ number
type S3 = Magic2<[[number], []]> // ✅ never

/////////

type Magic3<X> = X extends [[infer Y], ...infer _] ? Y : never;

type T = Magic3<[[number]]> // ✅ number
type T2 = Magic3<[[number, 2]]> // ✅ never
type T3 = Magic3<[[number], []]> // ✅ number