await_of_incompatible_type

The 'await' expression can't be used for an expression with an extension type that is not a subtype of 'Future'.

The analyzer produces this diagnostic when the type of the expression in an await expression is an extension type, and the extension type isn't a subclass of Future.

The following code produces this diagnostic because the extension type E isn't a subclass of Future:

extension type E(int i) {}

void f(E e) async {
  await e;
}

If the extension type is correctly defined, then remove the await:

extension type E(int i) {}

void f(E e) {
  e;
}

If the extension type is intended to be awaitable, then add Future (or a subtype of Future) to the implements clause (adding an implements clause if there isn't one already), and make the representation type match:

extension type E(Future<int> i) implements Future<int> {}

void f(E e) async {
  await e;
}