The last SIP meeting ended up being private-only, but we did discuss this SIP. We will discuss again in public on November 27th, but here is nevertheless the gist of the feedback from the SIP committee.
There seem to be two main aspects for the motivation of the proposal: type safety and performance.
For type safety, we’re basically not very convinced that the use cases cannot be implemented using appropriate usage of implicit evidences and/or union types and/or HLists.
For performance, we believe that it should be possible to implement a forced expansion using macros, perhaps even just with inline in Scala 3.
Therefore, the feedback of the committee is that we think you should try and implement this using existing tools in the language.