DMD [-O flag] vs. [memory allocation in a synchronized class]

[realhet via Digitalmars-d-learn]

Hi,

This code works well with the unoptimized compilation with DMD.
import std.array;

synchronized class Obj{
  private int[] arr;
  void trigger(){ arr.length += 1; }
}

void main(){
  auto k = new shared Obj;
  k.trigger;
}

And when I use the -O option, it shows the following error in the 
trigger() method:

core.exception.InvalidMemoryOperationError@src\core\exception.d(696): Invalid 
memory operation

Why? o.O