Oh, this is because we haven't implemented code generation for `@asyncHandler`

@DougGregor I'm gonna venture that there can't be a workaround, because that's what `@asyncHandler` was intended for, workarounds...

Let me know when this gets unblocked. The reason I'm writing this weird code is because I was trying to implement a RxSwift bridging layer to Swift-async.

In general, code generation for concurrency isn't implemented yet. It's coming up in stages, but right now the GitHub main is mostly useful for type checking. You can dodge the `@asyncHandler` issue by making it a synchronous function that does `Task.runDetached`.

@DougGregor what do you think makes this crash?

extension Array {
    func mapAsync<T>(_ transform: (Element) async throws -> T) async rethrows -> [T] {
        try await Task.withGroup(resultType: (Int, T).self) { group in
            var results = [T?].init(repeating: nil, count: self.count)
            for i in self.indices {
                await group.add {
                    (i, try await transform(self[i]))
                }
            }
            while let (index, result) = await try group.next() {
                results[index] = result
            }
            return results.map { $0! }
        }
    }
}