«Scala 3 is already backwards compatible today: it can consume libraries built with Scala 2.13 (except for libraries that define macros, see the Q&A on macros below). To achieve forwards compatibility, the Scala 3 compiler will provide a mechanism to ensure that the public interface is in the common language subset, so that it can be consumed from Scala 2.13. This means that, as a library author, you can adopt (some of) Scala 3 without requiring your users to upgrade from Scala 2.13.»

will provide

И этот механизм означает ограничение по языковым фичам

«Scala 3 is already backwards compatible today: it can consume libraries built with Scala 2.13 (except for libraries that define macros, see the Q&A on macros below). To achieve forwards compatibility, the Scala 3 compiler will provide a mechanism to ensure that the public interface is in the common language subset, so that it can be consumed from Scala 2.13. This means that, as a library author, you can adopt (some of) Scala 3 without requiring your users to upgrade from Scala 2.13.»

ну и более того
We might eventually still opt to release a Scala 2.14, if during the Scala 2 to 3 migration period, there’s a compelling need for evolving the standard library outside of these constraints.

очень бомбит

лучше б они 2.14 пилили, а не индентацию и не 10 версий имплиситов

надо написать шим для макросов 2.13->3.0 и продавать либописцам за мульоны