c ++多态（（X *）y） - > foo（）vs（（X）* y）.foo（）

Suppose Y is a derived class from class X and X declares foo to be virtual. Suppose y is of type (Y*). Then ((X*)y)->foo() will execute the Y version of foo(), but ((X)*y).foo() will execute the X version. Can you tell me why polymorphism does not apply in the dereferenced case? I would expect either syntax would yield the Y version of foo().Suppose Y is a derived class from class X and X