Operator error ?? for instances of two classes

Both instances are instances of a class. What is the problem?
Operator ‘??’ cannot be applied to operands of type ‘A’ and ‘B’.

A myA = new A(); 
B myB = new B(); 

object? Ok1 = myA ?? myA; //Ok
object? Ok2 = myB ?? myB; //Ok

object? Error = myA ?? myB; //Error. Operator '??' cannot be applied to operands of type 'A' and 'B'

class A
{

}

class B
{

}

>Solution :

The problem is that the compiler can’t work out the "best type" to use for the expression. (It tries for either A or B, but doesn’t look for common types in the hierarchy.)

From the ECMA standard, section 12.15:

The type of the expression a ?? b depends on which implicit conversions are available on the operands. In order of preference, the type of a ?? b is A₀, A, or B, where A is the type of a (provided that a has a type), B is the type of b(provided that b has a type), and A₀ is the underlying type of A if A is a nullable value type, or A otherwise.

If you cast either of the operands to object?, it should be fine:

object? success = myA ?? (object?) myB;