steps

When defining an external module with ProjectRef

lazy val externalProj = ProjectRef(path, "external")

cross building fails with

[error] Expected ';'
[error] ++  2.12.18 all libProj/Compile/compile fooPlugin/Compile/compile external/Compile/compile
[error]                                                                                           ^ 

problem

The expanded command from cross build do not take in account the build location of the project ref. external/Compile/compile is not known.

expectation

sbt should have used {build}external/Compile/compile instead

notes