Property observers ABI stability

I’ve been taking a stab at SR-5982, and so far I’ve been able to check for whether or not oldValue was used.
Whenever it’s not used, I’m changing the didSet function signature to remove the oldValue parameter, and removing the corresponding parameter list and call to the getter/property access.
However I’m starting to wonder now if this would affect ABI stability? I’m thinking of a case like so:

//Module A
class Foo {
     var a: Int {
         didSet {
               print("I am using oldValue: \(oldValue)")
         }
     }
}

//Module B
class Bar: Foo {
     var a: Int {
           didSet {
               print("Didn't use oldValue")
           }
     }
}

If module A is compiled before the optimisation and module B after, the SIL generated by B wouldn’t pass in oldValue to didSet in module A.

Would it be possible instead to maybe just pass in a null value for oldValue? Is it possible to do this with the type checking system at the stage where property observers are synthesised?

Those types should be classes, no? struct Bar : Foo is not supported.

Yup, my bad

Property observers are not ABI. The ABI for a property is always its generic get/set/materializeForSet entry points.