Note that neither this change, nor the PR by Nicolas Stucki (Add prototype conversion with dependent functions support by nicolasstucki · Pull Request #8523 · lampepfl/dotty · GitHub) would support implicit conversions with dependencies in the implicit parameter list:
implicit def x(a: A)(implicit tc: TC[a.T]): tc.Out
The signature of Conversion cannot support such conversions at all. There are quite a few such conversions in akka-http and other libraries I listed in the issue in dotty