An object is polymorphic when its specific type may vary.
The types a specific value may take, is called class.

It is trivial to copy an object if its type is known:

int x;
int y = x;
Here x is not polymorphic, so y is declared of same type (int) as x.
But if the specific type of x were unknown, then y could not be declared of any specific type.

The task: let a polymorphic object contain an instance of some specific type S derived from a type T.
The type T is known.
The type S is possibly unknown until run time.
The objective is to create an exact copy of such polymorphic object (not to create a reference, nor a pointer to).
Let further the type T have a method overridden by S.
This method is to be called on the copy to demonstrate that the specific type of the copy is indeed S.

type T() =
  // expose protected MemberwiseClone method (and downcast the result)
  member x.Clone() = x.MemberwiseClone() :?> T
  // virtual method Print with default implementation
  abstract Print : unit -> unit
  default x.Print() = printfn "I'm a T!"
 
type S() =
  inherit T()
  override x.Print() = printfn "I'm an S!"
 
let s = new S()
let s2 = s.Clone()  // the static type of s2 is T, but it "points" to an S
s2.Print() // prints "I'm an S!"