illegal_async_generator_return_type

Functions marked 'async*' must have a return type that is a supertype of 'Stream' for some type 'T'.

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 either Stream or a supertype of Stream.

The following code produces this diagnostic because the body of the function f has the 'async*' modifier even though the return type int isn't a supertype of Stream:

int f() async* {}

If the function should be asynchronous, then change the return type to be either Stream or a supertype of Stream:

Stream<int> f() async* {}

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

int f() => 0;