class C [A] {
  f : A;
  
  foo () : void {
    when (f != null) // E: comparing a value type A.* to .*null
    
    {}
  }

  foo[B] (f : void -> B) : void
  {
    when (f () != null) // E: comparing a value type B.* to .*null
    {}
  }
}