As usual for minor releases, Scala 2.12.9 is binary compatible with the whole Scala 2.12 series.
Scala.js users must update to 0.6.28+ or 1.0.0-M8+ for compatibility with Scala 2.12.9. (#7375)
- Improved compiler performance. Benchmarks of compiler performance have improved by between 5% and 10% since 2.12.8, thanks to many performance optimizations (mostly by Jason Zaugg and Diego E. Alonso-Blas: kudos!)