SBT cross-building multi-build project with dependsOn example

This examples shows how to cross-build a project consisting of multiple sub-projects where the sub-projects of different Scala versions depend on a parent project. Also, it attempts to answer StackOverflow question How to make different subclass for each scala version?

Given scala-parent project, and scala-2.11 and scala-2.12 sub-projects that dependsOn scala-parent, then set crossScalaVersions on scala-parent, whilst setting scalaVersion on sub-projects like so

lazy val scalaParent = (project in file("scala-parent")).settings(crossScalaVersions := Seq("2.11.0", "2.12.0"))
lazy val scala211  = (project in file("scala-2.11")).dependsOn(scalaParent).settings(scalaVersion := "2.11.0")
lazy val scala212  = (project in file("scala-2.12")).dependsOn(scalaParent).settings(scalaVersion := "2.12.0")

Also set crossScalaVersions := Seq() on the root project that aggregates other projects:

lazy val root = (project in file("."))
  .aggregate(scalaParent, scala211, scala212)
  .settings(
    name := "sbt-cross-build-dependson-example",
    libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.5" % Test,
    crossScalaVersions := Seq() // https://github.com/sbt/sbt/issues/4262#issuecomment-405607763
  )

Now executing sbt +compile should correctly cross-build the project:

sbt:sbt-cross-build-dependson-example> +compile
[info] Setting Scala version to 2.12.0 on 2 projects.
...
[info] Updating scala212...
[info] Compiling 1 Scala source to /Users/mario/IdeaProjects/sbt-cross-build-dependson-example/scala-parent/target/scala-2.12/classes ...
[info] Compiling 1 Scala source to /Users/mario/IdeaProjects/sbt-cross-build-dependson-example/scala-2.12/target/scala-2.12/classes ...
...
[info] Setting Scala version to 2.11.0 on 2 projects.
...
[info] Updating scala211...
[info] Compiling 1 Scala source to /Users/mario/IdeaProjects/sbt-cross-build-dependson-example/scala-parent/target/scala-2.11/classes ...
[info] Compiling 1 Scala source to /Users/mario/IdeaProjects/sbt-cross-build-dependson-example/scala-2.11/target/scala-2.11/classes ...

Note how scala-parent has been cross-compiled for 2.11 and 2.12

> tree -d -L 1 scala-parent/target/
scala-parent/target/
├── scala-2.11
├── scala-2.12

whilst sub-projects were build just for their respective versions

> tree -d -L 1 scala-2.11/target/
scala-2.11/target/
├── scala-2.11

> tree -d -L 1 scala-2.12/target/
scala-2.12/target/
├── scala-2.12