E4150

Async function call must be marked with !!.

MoonBit require all async function calls to be marked with !!, as a way to to inform the programmer that the function call is async and the control flow will be suspended here.

Erroneous example

async fn f() -> Int {
  ...
}

async fn g() -> Int {
  f() // Error: async function call must be marked with `!!`
}

Suggestion

Mark the async function call with !!:

async fn g() -> Int {
  f!!()
}