maybe use ~= here?  it makes it explicit to sbt that this depends upon the value of libraryDependencies.

```
libraryDependencies ~= { dependencies =>
  libraryDependencies ++ CrossVersion.partialVersion(scalaVersion.value) match {
    case ... => Seq(scala-parser-combinators)
    case _ => Seq.empty
  }
}
```

or we can just shove that Seq into the original libraryDependencies Seq.