class A {
  foo() => 23;
}

extension B on A? {
  foo() => 42;
  bar() => 87;
}

extension C on A {
  bar() => 123;
}

extension D on int Function()? {
  int call() => 76;
}

main() {}
testA(A? a) {}
testFunction(int Function()? f) {}
expect(expected, actual) {}