When I do the following the compiler fails to give an error

class A
{
  Str? val1
}

class B : A
{
  Str? val2
}

class Test
{
  Void main()
  {
    A[] aa := [A(),A()]
    someFunc(aa)
  }

  Void someFunc(B[] para)
  {
    para.each |elem|
    {
      echo(elem.val2)
    }
  }
}

That is by design because the type system only flags types errors that are known to be wrong at compile time. Same principle without using lists:

class Test
{
  static Void foo(Str x) {}
  static Void main() { Obj x := 3; foo(x) }
}

That is true, but Hmm I had somewhere a case where it somehow failed in JS, but can not find it any more :(