need help with member const pointer conversion

hi,

I'm working on a binding for Box2D 2.1.2 and stumbled across this:

struct b2FixtureDef
{
...
const b2Shape* shape
...
};

class b2Shape { .... };
class b2PolygonShape : public b2Shape { ... };

I can bind b2Fixture and b2Shape and create both, but when I try to assign a 
shape to the fixture member, luabind complains. Here's the code from the 
Tutorial ported to Lua:

  dynamicBox = box2d.b2PolygonShape()
  dynamicBox:SetAsBox(1,1)
  
  fixtureDef = box2d.b2FixtureDef()
  fixtureDef.shape = dynamicBox

The last line produces the following error:

  No matching overload found, candidates:
  void <unknown>(b2FixtureDef&,b2Shape const* const&)

Wrapping it in a setter works:

  void shape_setter(b2FixtureDef* fd, b2Shape* sh)
  {
    fd->shape = sh;
  }

but I'm puzzled why luabind won't allow me to assign a Shape* to a const Shape* 
?
Any help appreciated,
thanks,
Tony

p.s. Mac, gcc 4.2.1, boost 1.44, luabind 0.9.1


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