Behavior difference: tsgo no longer infer variadic `args` type when `arguments[...]` is referenced inside plain js function body

[hkleungai]

There are (somewhat) related sections on CHANGES.md, but none of them are describing what I observe here.

• https://github.com/microsoft/typescript-go/blob/main/CHANGES.md#jsdoc-variadic-types-are-now-only-synonyms-for-array-types
• https://github.com/microsoft/typescript-go/blob/main/CHANGES.md#with-strict-false-corsa-no-longer-allows-omitting-arguments-for-parameters-with-type-undefined-unknown-or-any

---

Steps to reproduce

1. Start a plain new project
   • "@typescript/native-preview": "7.0.0-dev.20260606.1",
   • "typescript": "6.0.3"
2. Run npx tsgo --init to get the default tsconfig. Turn on these flags as well.
   • "allowJs": true,
   • "checkJs": true,
   • "stableTypeOrdering": true,
3. Add src/main.js. Run npx tsc & npx tsgo to see the difference.

// src/main.js

/**
 * @param {number} length
 */
export function rectArea(length) {
    const width = arguments[1];

    if (typeof width === 'number') {
        return length * width;
    }

    return length * length;
}

rectArea(3);
rectArea(4, 5);

Behavior with typescript@6.0

Emit

/**
 * @param {number} length
 */
export function rectArea(length: number, ...args: any[]): number;
//# sourceMappingURL=main.d.ts.map

With no error.

Behavior with tsgo

Emit

/**
 * @param {number} length
 */
export declare function rectArea(length: number): number;
//# sourceMappingURL=main.d.ts.map

With error.

src/main.js:17:13 - error TS2554: Expected 1 arguments, but got 2.

17 rectArea(4, 5);
               ~


Found 1 error in src/main.js:17