Unfortunately, this requires an existing Scala compiler to build, so it won't be useful as a bootstrap compiler for Scala on the JVM. Does anyone here know of an alternative implementation of Scala that could be used to build the libraries and tools of the reference implementation from source?
It is a problem that many compilers cannot be bootstrapped from source without a trusted binary of a previous release.
It's a universal truism that all compilers cannot be bootstrapped from source without a trusted binary. It's true that in the world of standardized languages (which mostly means "C" in practice) there are compilers (mostly just gcc and clang) that can bootstrap themselves with a trusted binary of a previous release of some other compiler.
I wrote "a trusted binary of a previous release" not just "a trusted binary". There is obviously a difference between having a small set of trusted binaries to bootstrap and having a bootstrap binary for every language or build system.
There are several compiler implementations that enable bootstrapping from alternative implementations, which shifts the problem to a simpler language, which may already have a bootstrap path.
It is a practical problem for people who want to have a correspondence between source and binary. Some users would like to have to rely on as few opaque binaries as possible. There are efforts underway to build a minimal C compiler in a subset of Scheme that can be implemented on bare metal. The goal is to reduce the set of binaries that needs to be trusted or audited manually.
It is a problem that many compilers cannot be bootstrapped from source without a trusted binary of a previous release.