Inter-project dependencies break on variant Scala binary versions #4847
Description
steps
sbt version: 1.2.8
build.sbt
val foo = project in file(".")
ThisBuild / organization := "com.example"
ThisBuild / scalaVersion := "2.13.0"
aggregateProjects(bar, baz)
val bar = project.settings(scalaVersion := "2.12.8")
val baz = project.dependsOn(bar % Runtime)problem
> baz/run
[... running log and sbt internal stacktrace ...]
[error] (baz / update) sbt.librarymanagement.ResolveException: Error downloading com.example:bar_2.13:0.1.0-SNAPSHOT
[error] Not found
[error] not found: /Users/dnw/.ivy2/local/com.example/bar_2.13/0.1.0-SNAPSHOT/ivys/ivy.xml
[error] not found: https://repo1.maven.org/maven2/com/example/bar_2.13/0.1.0-SNAPSHOT/bar_2.13-0.1.0-SNAPSHOT.pom
[error] not found: https://repo.typesafe.com/typesafe/ivy-releases/com.example/bar_2.13/0.1.0-SNAPSHOT/ivys/ivy.xml
[error] not found: https://repo.scala-sbt.org/scalasbt/sbt-plugin-releases/com.example/bar_2.13/0.1.0-SNAPSHOT/ivys/ivy.xml
[error] Total time: 1 s, completed 08-Jul-2019 16:52:21
expectation
I expect baz / update to resolve the project bar that it depends on, compile it and then put it on project baz's runtime classpath for baz's execution.
notes
The use of a 2.12 library in a 2.13 library is safeguarded by the use of classloaders and java reflection.