Fix crossScalaVersions default, derived from scalaVersion.

Fixes #1828.

main/src/main/scala/sbt/Defaults.scala

@@ -238,7 +238,7 @@ object Defaults extends BuildCommon {
     javacOptions :== Nil,
     scalacOptions :== Nil,
     scalaVersion := appConfiguration.value.provider.scalaProvider.version,
-    crossScalaVersions := Seq(scalaVersion.value),
+    derive(crossScalaVersions := Seq(scalaVersion.value)),
     derive(compilersSetting),
     derive(scalaBinaryVersion := binaryScalaVersion(scalaVersion.value))
   ))

notes/0.13.9/cross-scala-versions-default.markdown

@@ -0,0 +1,12 @@
+
+  [@dwijnand]: http://github.com/dwijnand
+  [1828]: https://github.com/sbt/sbt/issues/1828
+  [1922]: https://github.com/sbt/sbt/pull/1992
+
+### Fixes with compatibility implications
+
+### Improvements
+
+### Bug fixes
+
+- Fix `crossScalaVersions` default, now correctly derives from the configured `scalaVersion`. [#1828][1828]/[#1992][1992] by [@dwijnand][@dwijnand]

sbt/src/sbt-test/project/cross-scala-versions-default/build.sbt

@@ -0,0 +1,6 @@
+scalaVersion := "2.11.6"
+
+TaskKey[Unit]("check") := {
+  assert(crossScalaVersions.value == Seq("2.11.6"),
+    s"""crossScalaVersions should be Seq("2.11.6") but is ${crossScalaVersions.value}""")
+}

sbt/src/sbt-test/project/cross-scala-versions-default/test

@@ -0,0 +1 @@
+> check