library test;

class A {
  A f(A x, {A? y}) => throw '';
  A g(A x, {A? y}) => throw '';
  A h(A x, {A? y}) => throw '';
}

class B extends A implements I {
  f(x, {y}) => throw '';
  g(x, {y}) => throw '';
  h(x, {y}) => throw '';
}

class I {
  I f(I x, {I? y}) => throw '';
  A g(I x, {I? y}) => throw '';
  A h(A x, {I? y}) => throw '';
}

main() {}