Lossy argument names when spreading across parameter types

Bug Report

🔎 Search Terms

argument names spread, parameter names spread

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

type Equality = (a: number, b: number) => boolean;

type GOOD = (...args: [...Parameters<Equality>]) => void;

type BAD = (...args: [...Parameters<Equality>, string]) => void;

🙁 Actual behavior

As shown here, the argument names are lost in if the argument list expression involves any arguments without names:

🙂 Expected behavior

It should infer the type of BAD as (a: number, b: number, args_2: string) => void.

🤔 Workaround

Declaring an extra function type with the additional arguments seems to work:

type Equality = (a: number, b: number) => boolean;

type Additional = (lol: string) => void; // 👈

type GOOD = (...args: [...Parameters<Equality>]) => void;

type BAD = (...args: [...Parameters<Equality>, ...Parameters<Additional>]) => void;

Inlining a function type seems to work too:

type Equality = (a: number, b: number) => boolean;

type GOOD = (...args: [...Parameters<Equality>]) => void;

type BAD = (...args: [...Parameters<Equality>, ...Parameters<(lol: string) => void>]) => void;

So there are known workarounds, although I would say these are non-obvious - it took me a long time to figure this out, and it’s just surprising how the second part of the parameter type expression seems to determine the outcome of the first part.

[RyanCavanaugh]

The short answer is that the parameter -> tuple name feature is “best effort” (since nothing in the type system works/doesn’t work depending on these names), and the effort to handle this particular case simply hasn’t been done yet

[jcalz]

I had no idea you could name tuple members! (is that a documented feature? I’ve never seen this before.)

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#labeled-tuple-elements