Scala New Added 19 September 2026

Scala: warning: match may not be exhaustive

The compiler proved a case is missing from a match on a sealed type, and with fatal warnings enabled the build stops. It is worth reading rather than silencing: the same warning is how a new subtype added elsewhere in the codebase tells you every match that has not been updated.

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

// The compiler prints the input that would not match, including nested shapes
sealed trait Event
case class Created(id: Long) extends Event
case class Deleted(id: Long) extends Event
case object Ping extends Event

def handle(e: Event): String = e match {
  case Created(id) => s"created $id"
  case Deleted(id) => s"deleted $id"
  case Ping        => "ping"          // the case the warning was about
}

// Option and Either are sealed too, so a missing None is the same warning
// Adding a catch all hides future additions: prefer listing the cases

// If a case really is unreachable, say so explicitly rather than widening
(e: @unchecked) match { case Created(id) => id }

// Turn it into an error everywhere so it cannot be ignored in review
// build.sbt
scalacOptions ++= Seq("-Wunused:all", "-Werror")

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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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.