Never never() => throw "Never";

class A<X extends Object, Y extends Object?> {
  X? bar() => null;
  X foo() => never();
  Y baz() => never();
}

class B<X extends List<Y>, Y extends Object?> {
  foo(X x, Y y) {}
}

class C<X extends List<Y>?, Y extends List<X>?> {
  foo(X x, Y y) {}
}

class D<X extends Y, Y extends Z, Z> {
  foo(X x, Y y, Z z) {}
}

main() {}