The following error is reported in the code editor:
Companions 'class MyTest' and 'object MyTest' must be defined in same file: Found in /Users/d061578/dev/MAPlatform/base_project/src/test/scala/scalaTest.scala and /core/src/test/scala/scalaTest.scala

However, the file exists only once. It looks like ScalaIDE sees the file once in the actual file system and once in Eclise's project tree.

I'm in a mixed JavaEE / Scala project which started as JavaEE project. I added the "Scala nature" to the existing project after installing the Eclipse plugins. Maven build and tests run fine.

Stats:

Scala IDE version:
4.0.0.v-2_11-201501121809-027320c
Scala compiler version:
2.11.5
Scala library version:
2.11.5
Eclipse version:
4.4.0.v20140925-0400
Class loader store: 1 entries
Compiler v. 2.11.5(/Users/d061578/dev/eclipse 3/plugins/org.scala-lang.scala-compiler_2.11.5.v20150101-184742-3fafbc204f.jar)