illegal_async_return_type

Functions marked 'async' must have a return type which is a supertype of 'Future'.

The analyzer produces this diagnostic when the body of a function has the async modifier even though the return type of the function isn't assignable to Future.

The following code produces this diagnostic because the body of the function f has the async modifier even though the return type isn't assignable to Future:

int f() async {
  return 0;
}

If the function should be asynchronous, then change the return type to be assignable to Future:

Future<int> f() async {
  return 0;
}

If the function should be synchronous, then remove the async modifier:

int f() => 0;