Submit a SIP on existential containers

Author: kyouko-taiga

content/existential-containers.md

@@ -69,9 +69,8 @@ In broad strokes, our solution generalizes the following possible implementation
 
 ```scala
 trait AnyPolygon:
-  type Value
+  type Value: Polygon as witness
   val value: Value
-  val witness: Polygon { type Self = Value }
 
 def largest(xs: Seq[AnyPolygon]): Option[AnyPolygon] =
   xs.maxByOption((a) => a.witness.area(a.value))
@@ -153,6 +152,11 @@ More specifically, given an existential container `c`, accessing `c.value` _open
 
 Since no change in implicit resolution is needed, this proposal cannot create unforeseen negative interactions with existing features.
 
+### Other concerns
+
+This document has been written under the experimental modularity improvements for Scala 3.
+Although the proposed feature is fully expressible without those changes, the encoding of existential containers can only work with the "old" (i.e., the one currently used in production) or "new" type class style.
+
 ### Open questions
 
 One problem not addressed by the proposed encoding is the support of multiple type classes to form the interface of a specific container.