diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 1e0beb5f30bd..e5cc94883dd1 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -7,7 +7,7 @@ import Types._, Flags._, Decorators._, DenotTransformers._, StdNames._, Scopes._
 import NameOps._, NameKinds._
 import Scopes.Scope
 import collection.mutable
-import collection.immutable.BitSet
+import collection.BitSet
 import scala.reflect.io.AbstractFile
 import Decorators.SymbolIteratorDecorator
 import ast._
@@ -1412,7 +1412,7 @@ object SymDenotations {
           to
       }
       val bcs = classSymbol :: addParentBaseClasses(classParents, Nil)
-      val scbits = seen.toImmutable
+      val scbits = seen
       if (isFullyCompleted) {
         myBaseClasses = bcs
         mySuperClassBits = scbits
diff --git a/project/Build.scala b/project/Build.scala
index cc6b5a6c3735..db403959c3d1 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -14,7 +14,7 @@ object Build {
 
   projectChecks()
 
-  val scalacVersion = "2.11.5" // Do not rename, this is grepped in bin/common.
+  val scalacVersion = "2.11.11" // Do not rename, this is grepped in bin/common.
 
   val dottyOrganization = "ch.epfl.lamp"
   val dottyVersion = {
@@ -82,10 +82,6 @@ object Build {
     javacOptions in Global ++= Seq("-Xlint:unchecked", "-Xlint:deprecation")
   )
 
-  /** Enforce 2.11.5. Do not let it be upgraded by dependencies. */
-  private val overrideScalaVersionSetting =
-    ivyScala := ivyScala.value.map(_.copy(overrideScalaVersion = true))
-
   // set sources to src/, tests to test/ and resources to resources/
   lazy val sourceStructure = Seq(
     scalaSource       in Compile    := baseDirectory.value / "src",
@@ -448,8 +444,6 @@ object Build {
     settings(sourceStructure).
     settings(dottyCompilerSettings).
     settings(
-      overrideScalaVersionSetting,
-
       // Disable scaladoc generation, it's way too slow and we'll replace it
       // by dottydoc anyway. We still publish an empty -javadoc.jar to make
       // sonatype happy.
@@ -520,8 +514,6 @@ object Build {
     dependsOn(`dotty-compiler`).
     settings(sourceStructure).
     settings(
-      overrideScalaVersionSetting,
-
       cleanSbtBridge := {
         val dottySbtBridgeVersion = version.value
         val dottyVersion = (version in `dotty-compiler`).value
@@ -609,8 +601,6 @@ object DottyInjectedPlugin extends AutoPlugin {
     enablePlugins(ScalaJSPlugin).
     settings(sourceStructure).
     settings(
-      overrideScalaVersionSetting,
-
       /* Remove the Scala.js compiler plugin for scalac, and enable the
        * Scala.js back-end of dotty instead.
        */
@@ -644,8 +634,6 @@ object DottyInjectedPlugin extends AutoPlugin {
     dependsOn(`dotty-compiler` % "compile->test").
     settings(sourceStructure).
     settings(
-      overrideScalaVersionSetting,
-
       baseDirectory in (Test,run) := (baseDirectory in `dotty-compiler`).value,
 
       libraryDependencies += "com.storm-enroute" %% "scalameter" % "0.6" % Test,