> + compile
$ exists lib/target/scala-2.12
$ exists lib/target/scala-2.13.0-M5
$ exists sbt-foo/target/scala-2.12
-$ exists sbt-foo/target/scala-2.13.0-M5

> clean
> + libProj/compile
$ exists lib/target/scala-2.12
$ exists lib/target/scala-2.13.0-M5
-$ exists sbt-foo/target/scala-2.12
-$ exists sbt-foo/target/scala-2.13.0-M5

# test safe switching
> clean
> ++ 2.12.10 -v compile
$ exists lib/target/scala-2.12
-$ exists lib/target/scala-2.13.0-M5
$ exists sbt-foo/target/scala-2.12
-$ exists sbt-foo/target/scala-2.13.0-M5

# Test legacy cross build with command support
# > clean
# > + build
# $ exists lib/target/scala-2.12
# $ exists lib/target/scala-2.13.0-M5
# $ exists sbt-foo/target/scala-2.12
# -$ exists sbt-foo/target/scala-2.13.0-M5

# Test ++ leaves crossScalaVersions unchanged
> clean
> ++2.12.10
> +extrasProj/compile
$ exists extras/target/scala-2.13.0-M5
$ exists extras/target/scala-2.12

# test safe switching
> clean
> ++ 2.13.0-M5 -v compile
$ exists lib/target/scala-2.13.0-M5
-$ exists lib/target/scala-2.12
# -$ exists sbt-foo/target/scala-2.12
-$ exists sbt-foo/target/scala-2.13.0-M5
