`fn` does not infer parameter labels correctly if `internalSignature` has optional parameters

[aswinsvijay]

Report a bug

🔎 Search Terms

• Optional parameters
• internalSignature
• parameter label

🧩 Context

• ArkType version: ^2.2.0
• TypeScript version (5.1+): All versions from 5.1 to 6.0
• Other context you think may be relevant (JS flavor, OS, etc.): In TS 5.1 and 5.2, fooIncorrect is inferred as TypedFn<(...args: never) => void>, but expected TypedFn<(a: string, b: number) => void

🧑‍💻 Repro

Playground Link: https://www.typescriptlang.org/play/?ts=5.9.3#code/JYWwDg9gTgLgBAbzjAnmApgGjgMwHZwC+uUEIcA5AIZQDWqGFA3ALABQ7AxhHgM7w4IEOAF5ceABQV+UYHgDmFbBTwBXEACN0UCgEoJEqtg27RAPkSFdrNtz4ChASTzcoUdJ3hj8UmXMXKapraegZGcBoA-KYiFghWNuyCEDYA9KkAepFJTi7Q7p5pmdkctjz8ETQACjRUILyiyGjoEgDa0jCyCkqUQVo6ALq6AHTQbRQaQgA26FR4FEM2dhUaNM6uBV7iUsO7PatQNVB1vPqGxtHmltbs7Afr+R4wRVnsQA

import { type, fn } from 'arktype';

const foo = fn('string', 'number')((a, b) => {});
const fooIncorrect = fn('string', 'number')((a, b?) => {});

foo;
//^?
fooIncorrect;
//^?

const barParams = type(['string', 'number']).or(['boolean']);
const barIncorrect = fn('...', barParams)((a, b?) => {});

barIncorrect;
//^?

[pullfrog]

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