Foo3.swift

class FooSubClass: SomeFoo {}

extension Foo where Self: FooSubClass {
  func bar() {
    print("Foo.FooSubclass.bar()")
  }
}

let fooSubClassOne: Foo = FooSubClass()
fooSubClassOne.bar() // Foo.SomeFoo.bar() 🤨

let fooSubClassTwo: SomeFoo = FooSubClass()
fooSubClassTwo.bar() // Foo.SomeFoo.bar() 🧐

let fooSubClassThree: FooSubClass = FooSubclass()
fooSubClassThree.bar() // Foo.FooSubClass.bar() 🤯