cross-building with source:3.0-migration compiler option

[RustedBones]

I'm cross-building a library with scala 2.12 and 2.13 and wanted to add support for scala 3.2.1
I added the -source:3.0-migration scalac option for scala 3, hoping to get some warnings from the compiler on the shared code.

However, this setup does not work if my module contains a specific scala-3 source folder.

if for instance I have in src/main/scala-3/example/Test.scala

package example

trait Test:
  def greet: Unit =
    println("Hello")

The compiler will not accept the new syntax and prints

[error] -- Error: /path/src/main/scala-3/example/Test.scala:4:2 
[error] 4 |  def greet: Unit =
[error]   |  ^^^
[error]   |  indented definitions expected, def found

Shouldn't the compiler consider scala-3 source folder as already migrated ?

[jchyb]

I'm afraid that the scalac option is applied to the whole compilation classpath by sbt. In the compiler, we don't differentiate directories based on their name (like the scala-3 directory here, which sbt uses to filter the compilation classpath based on the used scala version). Moreover, it is not possible to apply an option to only a part of compilation classpath. Best idea I have for this case is to temporarily split the codebase into separate modules:

1. one with only the files in scala-3 directory, which does not use the "-source:3.0-migration" option
2. one with all of the other cross-compiled files, which uses the "-source:3.0-migration"

This could be problematic, since the (2.) would have to depend on (1.) (or vice versa), which may not always be possible.