Regression(?) in kubukoz/slick-effect - type projections in separate compilation runs

[jchyb]

OpenCB: https://github.com/VirtusLab/community-build3/actions/runs/13209505213/job/36881103468

Compiler version

Working(?): 3.6.3
Broken(?): 3.6.4-RC1

Minimized code

Has to be split into 2 modules/separate compilation runs, for simplicity let's use scala-cli:

main.scala

package slickeffect

object Transactor {
  def fromDatabase(dbF: JdbcBackend#Database): Unit = ???
}

trait JdbcProfile {
  val backend: JdbcBackend = ??? 
}
object H2Profile extends JdbcProfile

trait JdbcBackend {
  type Database = DatabaseFactory
  val Database: DatabaseFactory = ???

  class DatabaseFactory {
    def forURL(url: String): DatabaseFactory = ???
  }
}

main.test.scala

package slickeffect

class TransactorTests {
  Transactor.fromDatabase(H2Profile.backend.Database.forURL("jdbc:h2:mem:"))
}

scala-cli test main.scala main.test.scala

Output

In 3.6.4-RC1 and before - no error.
In 3.7.0-RC1-bin-20250207-d60a914-NIGHTLY:

[error] -- [E007] Type Mismatch Error: /Users/jchyb/workspace/slick-effect/transactor/src/main/scala/slickeffect/Transactor.scala:28:61 
[error] 28 |    Transactor.fromDatabase(H2Profile.backend.Database.forURL("jdbc:h2:mem:"))
[error]    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |                 Found:    slickeffect.H2Profile.backend.DatabaseFactory
[error]    |                 Required: slickeffect.JdbcBackend#Database
[error]    |
[error]    | longer explanation available when compiling with `-explain`

Expectation

Not sure. Type projections like this should be unsupported in scala 3, and this error can also appear in previous compiler versions if both files are a part of the same compilation run. This leads me to believe this is more of a bugfix then regression, but I still wanted to log somewhere the OpenCB error and minimization.

[jchyb]

Errors in melivinlou/hammer-scala look similar https://github.com/VirtusLab/community-build3/actions/runs/13209436977/job/36880173932

[prolativ]

@jchyb Are you sure JdbcBackend#Database should be an illegal type projection? I'm looking at the specs https://docs.scala-lang.org/scala3/reference/dropped-features/type-projection.html and to me it's not quite clear if a trait should be considered an abstract type. Even if so, according to this specification and assuming I didn't miss something, changing JdbcBackend from a trait to a class should make this compile, but with the newest compiler this doesn't work either so IMO this is a regression anyway

[jchyb]

You are right, it also errors out with a class. What bothered me mainly was the fact that in 3.6.3 and earlier this error did appear, but only if both the type definition and method call were in the same compilation run, which led to to think the error might be correct (although, If I recall correctly, the compiler should just inform us directly if an illegal type projection is used).