Diamond shape inheritance: why downcasting is not working?

class DiamondBase
{
      public:
             int DiamondBaseData1;
};

// virtual base class will share base-class-subobject with DiamondDerived2 class
class DiamondDerived1: virtual public DiamondBase
{
      public:
             int DiamondBaseData2;

             void DiamondDerived1Fun(void){};

};

// virtual base class will share base-class-subobject with DiamondDerived1 class
class DiamondDerived2: virtual public DiamondBase
{
      public:
             int DiamondBaseData3;

             void DiamondDerived2Fun(void){};
};

class VirtualBaseDerived: public DiamondDerived1, public DiamondDerived2
{
      public:
             int DiamondBaseData4;

};

void Main()
{
   DiamondDerived2 obj1;
   DiamondBase* pBase = &obj1;  // upcasting of DiamondDerived2

  ((DiamondDerived1*)pBase)->DiamondDerived1Fun();  // error: cannot convert from base to derived via "virtual-base"

  ((DiamondDerived2*)pBase)->DiamondDerived2Fun(); // error: cannot convert from base to derived via "virtual-base"

}