Scala 3 migration "Not found: X" in CI

By Kaveh Alemi·Latchkey

The Scala 3 compiler could not resolve a name that compiled under Scala 2. Common causes are a library that has no Scala 3 build, a moved package, or Scala 2 only syntax the new compiler rejects.

What this error means

A build cross-compiled or migrated to Scala 3 fails with "Not found: X" or "Not found: type X" on names that were fine on 2.13.

[error] -- [E006] Not Found Error: /src/main/scala/App.scala:5:20
[error] 5 |    val p = new ProcessBuilder
[error]   |                ^^^^^^^^^^^^^^
[error]   |                Not found: type ProcessBuilder

Common causes

A dependency has no Scala 3 artifact

A %% library still only publishes _2.13, so on Scala 3 the module (and its names) do not resolve.

A moved or renamed symbol between versions

A type moved package or a Scala 2 only construct is not available, so the Scala 3 compiler cannot find the name.

How to fix it

Use withDottyCompat or a Scala 3 build of the library

1. Check whether the dependency publishes a Scala 3 artifact.
2. If yes, upgrade to it; if not, use .cross(CrossVersion.for3Use2_13) to consume the 2.13 build on Scala 3.
3. Recompile and fix any remaining moved-symbol imports.

libraryDependencies += ("com.example" %% "lib" % "1.0")
  .cross(CrossVersion.for3Use2_13)

Run the Scala 3 migration rewrites

The -rewrite -source 3.0-migration flags let scalac auto-fix many Scala 2 to 3 syntax gaps.

scalacOptions ++= Seq("-source", "3.0-migration", "-rewrite")

How to prevent it

• Confirm each dependency ships a Scala 3 build before migrating.
• Migrate with -source 3.0-migration to surface issues incrementally.
• Cross-build 2.13 and 3 together so regressions are caught in CI.

Frequently asked questions

Related guides

References

• Apache Maven guides
• Gradle user manual
• GitHub Actions documentation