steps

sbt 2.x allows dependsOn(...) mismatched Scala subproject dependencies by design, but the error state might be confusing.

expectations

1. Provide a setting allowMismatchScala, which by default should be set to false at the global level.
2. When allowMismatchScala is set to false, error out when mismatch Scala subprojects are detected, with the exception of Scala 2.13 and (3.0 through 3.7).
3. When allowMismatchScala is set to true for the subproject, allow dependsOn(...) to an arbitrary Scala version as it is coded right now in develop branch.