class A1 {}

class B1<T> {}

extension A2 on A1 {
  A1 method1() {}
  A1 method2<T>(T o) {}
  A1 method3<T>([T? o]) {}
  A1 method4<T>({T? o}) {}
}

extension B2<T> on B1<T> {
  B1<T> method1() {}
  B1<T> method2<S>(S o) {}
}

main() {}