Scala New Added 26 September 2026

Scala 3: `_` is no longer supported for a type wildcard, use `?`

Underscore is being reclaimed for type lambdas, so existing type wildcards have to be written with a question mark instead. It arrives as a warning during a Scala 3 upgrade and becomes an error under -source:future, and the rewrite is mechanical because only type positions changed.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Before
// def sizes(xs: List[Seq[_]]): List[Int] = xs.map(_.size)

// After: ? in a type position, _ still means a placeholder in a term
def sizes(xs: List[Seq[?]]): List[Int] = xs.map(_.size)

// Java interop and existential style signatures are where it turns up
val cls: Class[?] = classOf[String]
val pending: List[scala.concurrent.Future[?]] = Nil

// Let the compiler rewrite the codebase for you
// build.sbt
scalacOptions ++= Seq("-source:3.4-migration", "-rewrite")
// then remove both flags once it compiles clean

# Find what is left by hand
grep -rn '\[_\]' src/main/scala | grep -v '=>'

// A type lambda is the reason the syntax was needed: [X] =>> Map[String, X]

How to diagnose Scala errors

Scala's most distinctive error class is binary incompatibility: libraries are published per Scala major version, and mixing artifacts built for different versions produces confusing "not a member of" or NoSuchMethodError failures. The %% operator in sbt exists precisely to append the right suffix, and most unresolved-dependency errors are a % where %% was needed.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Check the Scala version suffix on every dependency. Use %% for Scala libraries and plain % only for Java ones.
Run sbt evicted to list dependencies that were evicted by version conflict. A frequent source of runtime NoSuchMethodError.
Use sbt dependencyTree to see the resolved graph and locate the transitive dependency pulling in an old version.
For inference failures, add explicit type annotations at the boundary rather than at the error site; inference failures usually propagate from further up.
Clear stale state with sbt clean and remove ~/.ivy2/cache or ~/.cache/coursier entries only for the specific failing artifact.

Tools worth reaching for

sbt evicted
sbt dependencyTree
coursier resolve
scalac -explain
MiMa (binary compat checks)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

docs.scala-lang.org docs.scala-lang.org

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.