Add link to Swift forums

docs/design/generics/appendix-coherence.md

@@ -235,3 +235,8 @@ either:
     like
     [dynamic implementation binding](#rejected-variation-dynamic-implementation-binding)
     (though perhaps with a monomorphization cost instead of a runtime cost).
+
+In addition, this
+[can create unsoundness when combined with dynamic downcasts and a more complex, less predictable implementation model](https://forums.swift.org/t/retroactive-conformances-dynamic-downcast-type-unsoundness/73890).
+This approach would be particularly complex in Carbon due to supporting
+[impl specialization](terminology.md#specialization).