F bar() => foo;
F? baz() => foo;
Function()? foobar(Function()? x) => null;

class A<T> {}

class B extends A<Function()?> {
  Function()? method(Function()? x) => null;
}

main() {}
typedef F = void Function();
void Function() hest() => foo;
void Function()? fisk() => foo;
void foo() {}