Bug 291255 – `export async function` changes prototype to Function instead of AsyncFunction

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Jarred Sumner]

The following code doesn't throw an error in node/v8 and does throw an error in JSC:

```
export async function a() {}

async function b() {}
async function c() {}

if (Object.getPrototypeOf(a) !== Object.getPrototypeOf(c)) {
  throw new Error("Expected export declaration & export function to have same prototype");
}

if (Object.getPrototypeOf(a) !== Object.getPrototypeOf(b)) {
  throw new Error("Expected export & non-exported functions to have same prototype");
}

if (Object.getPrototypeOf(a).constructor !== Object.getPrototypeOf(c).constructor) {
  throw new Error("Expected export declaration & export function to have same constructor");
}

if (Object.getPrototypeOf(a).constructor !== Object.getPrototypeOf(b).constructor) {
  throw new Error("Expected export & non-exported functions to have same constructor");
}

if (Object.getPrototypeOf(a).constructor.name !== "AsyncFunction") {
  throw new Error("Expected AsyncFunction");
}
globalThis.print ??= console.log;

const AsyncFunction = Object.getPrototypeOf(a).constructor;

// This should not throw
new AsyncFunction("await 42")();

export { c };
```

Originally reported in https://github.com/oven-sh/bun/issues/18137

[Radar WebKit Bug Importer]

<rdar://problem/149083588>

[Yusuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/43970

[EWS]

Committed 293602@main (910c1b00a197): <https://commits.webkit.org/293602@main>

Reviewed commits have been landed. Closing PR #43970 and removing active labels.