Scala default visibility: missing private‑by‑default enforcement

Scala default visibility: mechanisms to enforce private-by-default discipline

Scala does not offer a compiler flag or language keyword to make members private-by-default. Any class member without an explicit private, protected, or package-private modifier is public by default, visible from any other code in the same or a different package. The language has discussed this trade-off for years, but no Scala Improvement Proposal has been adopted to change the default, not even in Scala 3.

The following compile-time tools can warn or block implicitly public members:

1. Wartremover - the Wart.PublicInfer wart emits a warning when a member lacks an explicit visibility modifier. In sbt, enable it with wartremoverWarnings ++= Seq(Wart.PublicInfer).
2. Scalafix - the NoPublicMembers rule can be configured to error on missing modifiers, blocking compilation until each member is annotated. Enable via scalafixOnCompile := true and the required dependency.
3. Scalastyle - rule PublicFinalizer flags members that are publicly accessible without explicit marking, useful as a supplementary check.

Typical migration path:

1. Run the linter in warning mode across the existing codebase.
2. Add private or protected to every flagged member.
3. Switch the configuration to error mode so new code cannot omit visibility modifiers.
4. Optionally complement with a project style guide that also disallows package-private (no-modifier) access when stricter encapsulation is desired.

Verification step: create a Scala class with a field that has no visibility modifier, run sbt wartremover with the PublicInfer wart enabled; a warning about PublicInfer should appear. For Scalafix, configure the rule as described and run sbt scalafix; the rule will suggest or enforce adding private/protected.

To refine the exact sbt, Maven, or Gradle setup and Scala version-specific flags, please specify your build tool and Scala version. This detail helps confirm the correct plugin keys and linter integration steps.