To use a locally built Scala version, define the scala-home setting, which is of type Option[File]. This Scala version will only be used for the build and not for sbt, which will still use the version it was compiled against.
Example: scala scalaHome := Some(file("/path/to/scala"))
Using a local Scala version will override the scala-version setting and will not work with cross building.
sbt reuses the class loader for the local Scala version. If you recompile your local Scala version and you are using sbt interactively, run text > reload
to use the new compilation results.