Inconsistent error between different versions of the same function with required parameter following an optional one

🔎 Search Terms

error function optional required rest

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240227#code/GYVwdgxgLglg9mABAIwIYCcAUqBciwgC2yApugDQoD8eBxZlEtRpFiAdJxgOYDOevKOhhhuAbQC6ASkQBvAL4AoRWnSKA9OsQA9KolCRYCFBmzN6bZDXwsGiJjYuVO7Hv0SDhoyVLwA3OBgAE2VVTABGKQBuRE1EAFEADwAHEmgSIMRUKEQAGxJUQUQAZiz0biISMCheSmQQHO44HPD2TAAmAFZuqWUDaHgkNAAvM0dWOus6CY4ucvcxB2m7FzcBIRFxCWk5JRVUYY0tXX1wAeMRseXLKds2JbvnOb51ry3fRADg0IOI6NitElUulMtk8gUiqUeJVqrUUA1EE0Wm0uj1FEA

💻 Code

function bar(a: number, b?: number, c: number, ...args: string[]) {}

bar
// ^? function bar(a: number, b?: number, c: number, ...args: string[]): void

bar(1); // Expected at least 3 arguments, but got 1.(2555)

function baz(a: number, b?: number, ...args: [c: number, ...args: string[]]) {}

baz
// ^? function baz(a: number, b?: number, c: number, ...args: string[]): void

baz(1); // Expected at least 3 arguments, but got 1.(2555)

🙁 Actual behavior

We get an error within the bar declaration ("'c' is declared but its value is never read.(6133)") but we get none in baz. They are pretty much equivalent.

🙂 Expected behavior

I'd expect this error to be raised consistently here (or not raised consistently).

In addition to that, b comes with the question mark in both quick infos here. TypeScript understands that at least 3 arguments are required here. It means that effectively b is required in both cases and ? is somewhat misleading here. I think it would be good to normalize this. It already is normalized with tuples:

type A = [a: number, b?: number, ...[c: number, ...args: string[]]]
//   ^? type A = [a: number, b: number | undefined, c: number, ...args: string[]]

Additional information about the issue

No response

Start with the supplied TypeScript Playground example and compare the diagnostics for bar and baz, including their quick-info signatures. Done means deciding whether the unused-parameter diagnostic and optional-parameter normalization should be consistent, then adding or updating compiler coverage for the chosen behavior.