weird timing, given that initial parts of project valhalla were recently integrated (as a preview feature) into mainline java: JEP 401: Value Objects (Preview) and that suggests the overall design is stable enough for java authors to progress with implementation of more parts of project valhalla.
iirc, years ago there was an effort to make scala linker, which could theoretically do the specialization only for generic parameter combinations that are used in a given project. that would make it similar to c++, iiuc. it would make a lot of sense especially now, as we have scala-native, scala.js and scala-wasm, which all have linking as final step.
scala-on-jvm would get solid generic specialization boost from future project valhalla parts, like probably JEP 218: Generics over Primitive Types (or some iteration of that jep). efforts in jvm area combine java’s laziness (individual *.class objects can be compiled and loaded whenever needed, there’s no single big bang compilation or linking phase needed) with strong jit optimizations.
rebooting compile-time specialization, that is just some enhanced form of what scala 2 had, feels like some unsatisfactory stopgap solution, unless there’s a plan to extend it to a full linker with full monomorphisation (so it would reduce the performance and efficiency gap between future scala-on-jvm and scala-on-non-jvm).