TASTy Compatibility After a Scala 2.13→3 Upgrade: Clean Rollback

Direct answer

sbt clean does not remove Scala 3 TASTy files that may be cached in your local Ivy2 or Coursier repositories; those artifacts must be purged manually before a clean rollback to Scala 2.13 will succeed.

Explanation vs. confirmed facts

Explanation: Scala 3 compiles to TASTy with a format version that the Scala 2.13 compiler cannot read. If any TASTy‑containing JAR remains on the classpath (e.g., in the local cache), the 2.13 compiler will fail with a "TASTy version" error, even after you have reverted scalaVersion and removed the _3 suffix from dependencies.

Confirmed facts from the research:

• sbt clean deletes only the target/ directories of your project; it does not touch ~/.ivy2/local or ~/.coursier where locally published JARs (and their embedded TASTy) reside.
• Leftover TASTy files can be found by searching for *.tasty under those cache directories, or by inspecting JARs with unzip -l and grepping for .tasty. The file magic number starts with 0x00 0x01 0x0A (UTF‑8 "TASTy").
• To revert Scala‑specific suffixes (_3 → _2.13), set scalaVersion := "2.13.x" and rely on SBT’s cross‑version operators (%% or %%%). A full reload (sbt reload) or an update (sbt update) will then resolve all dependencies with the correct suffix.

Steps for a clean rollback

1. Revert the Scala version in build.sbt (or wherever it is defined):

   scalaVersion := "2.13.12"   // or your preferred 2.13.x

2. Remove any explicit _3 suffixes from libraryDependencies (if you added them manually); otherwise keep the cross‑version operators (%%) so SBT will automatically apply the _2.13 suffix.

3. Run a clean to delete compiled class files:

   sbt clean

4. Purge locally cached Scala 3 artifacts:

   • For Ivy2: rm -rf ~/.ivy2/local/<your‑organization> (or search and delete only the relevant modules).
   • For Coursier: rm -rf ~/.coursier/cache/v1/https/repo1.maven.org/maven2/<your‑organization> (or use coursier purge if available).

   Alternatively, run a search to verify no TASTy remains before proceeding:

   find ~/.ivy2/local ~/.coursier -type f -name "*.tasty" -print
   # or, to inspect JARs:
   find ~/.ivy2/local ~/.coursier -type f -name "*.jar" -exec sh -c 'unzip -l {} | grep -q \.tasty' \; -print
   

5. Reload the build and update dependencies so that SBT resolves them with the _2.13 suffix:

   sbt reload
   sbt update
   

6. Compile to confirm the rollback succeeded:

   sbt compile
   
   If the console shows no "TASTy version" warnings, the classpath is free of Scala 3 TASTy files.
   (Optional) Verify dependency resolution:
   sbt "show libraryDependencies"
   
   All entries should display the _2.13 suffix (or no suffix for purely cross‑built artifacts).
   

When to ask for a missing diagnostic detail

If you are unable to locate the exact cache directories used by your organization (e.g., you publish to a custom local Ivy2 repository), please provide the base path of that repository so the purge command can be adjusted accordingly.