This is a spin-off issue from:
http://d.puremagic.com/issues/show_bug.cgi?id=10727#c3

----

DMD does not crib about
missing override for function Foo.frop. But starts doing that if "alias toArray
this" in CirBuff(T) is commented out.

struct CirBuff(T) {
  import std.traits: isArray;
  CirBuff!T opAssign(R)(R ) if(isArray!R) { }

  T[] toArray() {
    T[] ret; //  = new T[this.length];
    return ret;
  }
  alias toArray this;
}

class Bar (T=int){
  CirBuff!T _bar;
}

class Once {
  Bar!Foo _foobar;
}

class Foo: Frop {
  // override
  public int frop() {return 1;}
}

class Frop {
  public int frop() {return 0;}
}