sbt - defining custom configuration for sequential tests - scala

Question

Some of my tests require that they are sequentially executed. I read about custom configurations on http://www.scala-sbt.org/0.13.0/docs/Detailed-Topics/Testing.html but I am missing something because my configuration is not working properly.
Here it is:
import sbt._
import Keys._
object SchedulingBackendBuild extends Build {
lazy val SequentialTest = config("sequentialTest") extend(Test)
def sequentialTestFilter(name: String): Boolean = {
println("===seq test filter")
name endsWith "SeqSpec"
}
def unitTestFilter(name: String): Boolean = {
println("===unit test filter")
!sequentialTestFilter(name)
}
lazy val root = Project(id = "scheduling-backend",
base = file("."),
settings = Project.defaultSettings
).configs(SequentialTest)
.settings(inConfig(SequentialTest)(Defaults.testTasks): _*)
.settings(
testOptions in Test ++= Seq(Tests.Filter(unitTestFilter)),
testOptions in SequentialTest ++= Seq(Tests.Filter(sequentialTestFilter))
)
}
I want test to only execute tests that are not ending with SeqSpec and this is working, but when I try to execute sequentialTest:test no tests are executed. I added println to my filters and I can see that even if I execute sequentialTest:test I am getting
===unit test filter
===seq test filter
===seq test filter
so both filters are executed.
When I type inspect sequentialTest:testOptions I am getting
[info] Task: scala.collection.Seq[sbt.TestOption]
[info] Description:
[info] Options for running tests.
[info] Provided by:
[info] {file:/path/to/project/scheduling-backend/}scheduling-backend/sequentialTest:testOptions
[info] Defined at:
[info] /path/to/project/scheduling-backend/project/Build.scala:22
[info] Reverse dependencies:
[info] sequentialTest:testOnly::testOptions
[info] sequentialTest:testQuick::testOptions
[info] sequentialTest:test::testOptions
[info] Delegates:
[info] sequentialTest:testOptions
[info] test:testOptions
[info] runtime:testOptions
[info] compile:testOptions
[info] *:testOptions
[info] {.}/sequentialTest:testOptions
[info] {.}/test:testOptions
[info] {.}/runtime:testOptions
[info] {.}/compile:testOptions
[info] {.}/*:testOptions
[info] */sequentialTest:testOptions
[info] */test:testOptions
[info] */runtime:testOptions
[info] */compile:testOptions
[info] */*:testOptions
[info] Related:
[info] sequentialTest:testOnly::testOptions
[info] sequentialTest:testQuick::testOptions
[info] test:testOptions
[info] test:testQuick::testOptions
[info] */*:testOptions
[info] test:testOnly::testOptions
[info] test:test::testOptions
[info] sequentialTest:test::testOptions
so for me it looks ok, line 22 is testOptions in SequentialTest ++= Seq(Tests.Filter(sequentialTestFilter))

Answer 1

Partial answer: if I change ++= operator (appending to the sequence) to := (replacing the sequence) everything works fine (and := operators are used in the documentation). Even so I would want to know why ++= in this case is bad choice.