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Source Compatibility

Scala 3 does NOT guarantee source compatibility between different minor language versions (e.g. some syntax valid in 3.x might get deprecated and then phased out in 3.y for y > x). There are also some syntax structures that were valid in Scala 2 but are not anymore in Scala 3. However the compiler provides a possibility to specify the desired version of syntax used in a particular file or globally for a run of the compiler to make migration between versions easier.

The default Scala language syntax version currently supported by the Dotty compiler is 3.2. There are also other language versions that can be specified instead:

  • 3.0-migration: Same as 3.0 and 3.1, but with a Scala 2 compatibility mode that helps moving Scala 2.13 sources over to Scala 3. In particular, it

    • flags some Scala 2 constructs that are disallowed in Scala 3 as migration warnings instead of hard errors,
    • changes some rules to be more lenient and backwards compatible with Scala 2.13
    • gives some additional warnings where the semantics has changed between Scala 2.13 and 3.0
    • in conjunction with -rewrite, offer code rewrites from Scala 2.13 to 3.0.
  • 3.0, 3.1: the default set of features included in scala versions 3.0.0 to 3.1.3.

  • 3.2: the same as 3.0 and 3.1, but in addition:

    • stricter pattern bindings are now enabled (part of future in earlier 3.x releases), producing warnings for refutable patterns. These warnings can be silenced to achieve the same runtime behavior, but in future they become errors and refutable patterns will not compile.
    • Nonlocal returns now produce a warning upon usage (they are still an error under future).
  • 3.2-migration: the same as 3.2, but in conjunction with -rewrite, offer code rewrites from Scala 3.0/3.1 to 3.2.

  • future: A preview of changes that will be introduced in 3.x versions after 3.2. Some Scala 2 specific idioms are dropped in this version. The feature set supported by this version may grow over time as features become stabilised for preview.

  • future-migration: Same as future but with additional helpers to migrate from 3.2. Similarly to the helpers available under 3.0-migration, these include migration warnings and optional rewrites.

There are two ways to specify a language version :

  • with a -source command line setting, e.g. -source 3.0-migration.
  • with a scala.language import at the top of a source file, e.g:
package p
import scala.language.`future-migration`

class C { ... }

Language imports supersede command-line settings in the source files where they are specified. Only one language import specifying a source version is allowed in a source file, and it must come before any definitions in that file.

Note: The Scala 3 Migration Guide gives further information to help the Scala programmer moving from Scala 2.13 to Scala 3.