...infer X extends TypeParameter[] in conditional type inappropriate binding

[webstrand]

Bug Report

🔎 Search Terms

conditional type, extends, infer, spread, tuple

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type Foo<K, T extends readonly unknown[]> = T extends readonly [any, ...infer X extends readonly unknown[]] ? X : never;
type Bar<K, T extends readonly unknown[]> = T extends readonly [any, ...infer X extends readonly K[]] ? X : never;
type Baz<K, T extends readonly unknown[]> = T extends readonly [any, ...infer X extends readonly (K | "a" | "b")[]] ? X : never;

type x = Foo<"a" | "b", ["a", "b", "b"]>
//   ^? - type x = ["b", "b"]
type y = Bar<"a" | "b", ["a", "b", "b"]> 
//   ^? - type y = readonly ("a" | "b")[]
type z = Baz<"a" | "b", ["a", "b", "b"]>
//   ^? - type z = ["b", "b"]

🙁 Actual behavior

The conditional types do not produce the same output:

type x = Foo<"a" | "b", ["a", "b", "b"]> // ["b", "b"]
type y = Bar<"a" | "b", ["a", "b", "b"]> // readonly ("a" | "b")[]
type z = Baz<"a" | "b", ["a", "b", "b"]> // ["b", "b"]

🙂 Expected behavior

The conditional types should all produce the same output. The fact that K is a type parameter should not affect the output.

type x = Foo<"a" | "b", ["a", "b", "b"]> // ["b", "b"]
type y = Bar<"a" | "b", ["a", "b", "b"]> // ["b", "b"]
type z = Baz<"a" | "b", ["a", "b", "b"]> // ["b", "b"]

Alternatively if that is infeasible for technical reasons, the conditional type should fail. Inferring the constraint makes traversing the tuple impossible.

[warriordog]

Here's another reproduction, showing how it works if the tuple is reversed.
I've tested in 5.0.4 and 5.1.3.

type Heads<T extends any[]> = T extends [... infer Rest, any] ? Rest : never;
type Tails<T extends any[]> = T extends [any, ... infer Rest] ? Rest : never;

// Expected: [1, 2]
// Actual:   never
type heads = Heads<[1, 2, 3]>;

// Expected: [2, 3]
// Actual:   [2, 3]
type tails = Tails<[1, 2, 3]>;