Add mention of opaque types

_sips/sips/2014-06-27-42.type.md

@@ -575,7 +575,7 @@ applications which work with large datasets.
 ## Follow on work from this SIP
 
 Whilst the authors of this SIP believe that it stands on its own merits, we think that there are two
-areas where follow on work is desirable.
+areas where follow on work is desirable, and one area where another SIP might improve the implementation of SIP-23.
 
 ### Infix and prefix types
 
@@ -606,6 +606,16 @@ valuable.
 A prototype of this syntax extension existed at an early stage in the development of Typelevel Scala
 but never matured. The possibility of useful literal types adds impetus.
 
+### Opaque types
+
+In the reference implementation of SIP-23, the `ValueOf[A]` type is implemented as a value class.
+This means that implicit evidence of `ValueOf[A]` will erase to `A` (the value associated with
+the singleton types). This is desirable, but due to value class restrictions, ends up boxing
+primitive types (such as `Int`).
+
+If we implemented `ValueOf[A]` as an opaque type instead of a value class, then this boxing
+would be ellided, and the `valueOf[A]` method would be compiled to an identity function.
+
 ## Related Scala issues resolved by the literal types implementation
 
 + [SI-1273](https://github.com/scala/bug/issues/1273) Singleton type has wrong bounds