Unexpected compiler error when trying to call sync function alongside async code

[icanzilb (Marin Todorov)]

I'm trying to understand the reason for this unexpected compiler error I'm getting. Here is how to reproduce the issue:

1. Make two functions with the same name - one sync and one async

2. I can call each one separately.

I'm using the 'task' SwiftUI modifier which takes an async closure:

and

3. But if I use any await in the task body, calling the sync version of the function produces compiler error. The compiler complains that I need to use 'await' with async functions. For example:

(The await call doesn't need to be to the same function, also a call to Task.sleep(...) trigger the compiler error)

I don't see the reason why I can't call the sync version - is this a bug or a feature?

[ksluder (Kyle Sluder)]

What is .task? Is this a SwiftUI view?

[icanzilb (Marin Todorov)]

Yes, it's the task SwiftUI modifier - it takes an async closure

[ksluder (Kyle Sluder)]

And what is the compiler error?

[icanzilb (Marin Todorov)]

Need to use 'await' when calling an async function (not on the laptop rn for the exact error)

[ksluder (Kyle Sluder)]

Ah, ok. The issue is that the async version wins out in overload resolution. The workaround is explained in SE-0296—wrap the synchronous call in a closure:

func f() async {
  let f2 = {
    // In a synchronous context, the non-async overload is preferred:
    doSomething()
  }
  f2()
}

[icanzilb (Marin Todorov)]

Oh I see - it's covered by the last paragraph in that subsection; thanks for the link!