diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f0e0c85..aeb17eb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         java: [8, 11, 17]
-        scala: [2.13.8]
+        scala: [2.13.8, 3.1.2-RC1]
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
diff --git a/build.sbt b/build.sbt
index 3aa152c..22a2868 100644
--- a/build.sbt
+++ b/build.sbt
@@ -1,4 +1,5 @@
-ThisBuild / scalaVersion := "2.13.8"
+ThisBuild / scalaVersion := "3.1.2-RC1"
+ThisBuild / crossScalaVersions := Seq((ThisBuild / scalaVersion).value, "2.13.8")
 
 lazy val root = project.in(file("."))
   .aggregate(collectionContrib.jvm, collectionContrib.js)
@@ -15,10 +16,20 @@ lazy val collectionContrib = crossProject(JVMPlatform, JSPlatform)
   .settings(ScalaModulePlugin.scalaModuleSettings)
   .settings(
     name := "scala-collection-contrib",
-    versionPolicyIntention := Compatibility.BinaryCompatible,
+    versionPolicyIntention := Compatibility.None,
     scalaModuleAutomaticModuleName := Some("scala.collection.contrib"),
-    Compile / compile / scalacOptions ++= Seq("-opt-warnings", "-language:higherKinds", "-deprecation", "-feature", "-Xfatal-warnings", "-Wconf:origin=scala.collection.IterableOps.toIterable:s"),
-    Compile / doc / scalacOptions ++= Seq("-implicits", "-groups", "-nowarn"),
+    Compile / compile / scalacOptions ++= {
+      CrossVersion.partialVersion(scalaVersion.value) match {
+        case Some((2, _)) => Seq("-opt-warnings", "-Werror", "-Wconf:origin=scala.collection.IterableOps.toIterable:s")
+        case Some((3, _)) => Seq("-Xfatal-warnings", "-Yscala-release:3.0", "-Wconf:cat=deprecation:s")
+      }
+    },
+    Compile / doc / scalacOptions ++= {
+      CrossVersion.partialVersion(scalaVersion.value) match {
+        case Some((2, _)) => Seq("-implicits", "-groups", "-nowarn")
+        case Some((3, _)) => Seq.empty
+      }
+    },
     testOptions += Tests.Argument(TestFrameworks.JUnit, "-q", "-v", "-s", "-a"),
     Test / parallelExecution := false,  // why?
     libraryDependencies ++= Seq(
diff --git a/src/main/scala/scala/collection/decorators/package.scala b/src/main/scala/scala/collection/decorators/package.scala
index 547891a..e13ccb9 100644
--- a/src/main/scala/scala/collection/decorators/package.scala
+++ b/src/main/scala/scala/collection/decorators/package.scala
@@ -8,13 +8,13 @@ package object decorators {
   implicit def iteratorDecorator[A](it: Iterator[A]): IteratorDecorator[A] =
     new IteratorDecorator[A](it)
 
-  implicit def IterableDecorator[C](coll: C)(implicit it: IsIterable[C]): IterableDecorator[C, it.type] =
+  implicit def iterableDecorator[C](coll: C)(implicit it: IsIterable[C]): IterableDecorator[C, it.type] =
     new IterableDecorator(coll)(it)
 
-  implicit def SeqDecorator[C](coll: C)(implicit seq: IsSeq[C]): SeqDecorator[C, seq.type] =
+  implicit def seqDecorator[C](coll: C)(implicit seq: IsSeq[C]): SeqDecorator[C, seq.type] =
     new SeqDecorator(coll)(seq)
 
-  implicit def MapDecorator[C](coll: C)(implicit map: IsMap[C]): MapDecorator[C, map.type] =
+  implicit def mapDecorator[C](coll: C)(implicit map: IsMap[C]): MapDecorator[C, map.type] =
     new MapDecorator(coll)(map)
 
   implicit def bitSetDecorator[C <: BitSet with BitSetOps[C]](bs: C): BitSetDecorator[C] =
diff --git a/src/main/scala/scala/collection/immutable/MultiDict.scala b/src/main/scala/scala/collection/immutable/MultiDict.scala
index 1a750ae..1114b98 100644
--- a/src/main/scala/scala/collection/immutable/MultiDict.scala
+++ b/src/main/scala/scala/collection/immutable/MultiDict.scala
@@ -69,7 +69,7 @@ object MultiDict extends MapFactory[MultiDict] {
 
   def from[K, V](source: IterableOnce[(K, V)]): MultiDict[K, V] =
     source match {
-      case mm: MultiDict[K, V] => mm
+      case mm: MultiDict[K, V] @unchecked => mm
       case _ => (newBuilder[K, V] ++= source).result()
     }
 
diff --git a/src/main/scala/scala/collection/immutable/MultiSet.scala b/src/main/scala/scala/collection/immutable/MultiSet.scala
index 14d0b80..a674c3b 100644
--- a/src/main/scala/scala/collection/immutable/MultiSet.scala
+++ b/src/main/scala/scala/collection/immutable/MultiSet.scala
@@ -78,7 +78,7 @@ object MultiSet extends IterableFactory[MultiSet] {
 
   def from[A](source: IterableOnce[A]): MultiSet[A] =
     source match {
-      case ms: MultiSet[A] => ms
+      case ms: MultiSet[A] @unchecked => ms
       case _ => (newBuilder[A] ++= source).result()
     }
 
diff --git a/src/main/scala/scala/collection/immutable/SortedMultiDict.scala b/src/main/scala/scala/collection/immutable/SortedMultiDict.scala
index 08af58f..2ea5bed 100644
--- a/src/main/scala/scala/collection/immutable/SortedMultiDict.scala
+++ b/src/main/scala/scala/collection/immutable/SortedMultiDict.scala
@@ -72,7 +72,7 @@ object SortedMultiDict extends SortedMapFactory[SortedMultiDict] {
 
   def from[K: Ordering, V](it: IterableOnce[(K, V)]): SortedMultiDict[K, V] =
     it match {
-      case smm: SortedMultiDict[K, V] => smm
+      case smm: SortedMultiDict[K, V] @unchecked => smm
       case _ => (newBuilder[K, V] ++= it).result()
     }
 
diff --git a/src/main/scala/scala/collection/immutable/SortedMultiSet.scala b/src/main/scala/scala/collection/immutable/SortedMultiSet.scala
index c1d593f..1509d1d 100644
--- a/src/main/scala/scala/collection/immutable/SortedMultiSet.scala
+++ b/src/main/scala/scala/collection/immutable/SortedMultiSet.scala
@@ -55,7 +55,7 @@ object SortedMultiSet extends SortedIterableFactory[SortedMultiSet] {
 
   def from[A: Ordering](source: IterableOnce[A]): SortedMultiSet[A] =
     source match {
-      case sms: SortedMultiSet[A] => sms
+      case sms: SortedMultiSet[A] @unchecked => sms
       case _ => (newBuilder[A] ++= source).result()
     }
 
diff --git a/src/main/scala/scala/collection/mutable/MultiSet.scala b/src/main/scala/scala/collection/mutable/MultiSet.scala
index e2098a8..a387e90 100644
--- a/src/main/scala/scala/collection/mutable/MultiSet.scala
+++ b/src/main/scala/scala/collection/mutable/MultiSet.scala
@@ -16,7 +16,7 @@ trait MultiSet[A]
   override protected def newSpecificBuilder: mutable.Builder[A, MultiSet[A]] = iterableFactory.newBuilder
   override def empty: MultiSet[A] = iterableFactory.empty
 
-  override def knownSize = super[Growable].knownSize
+  override def knownSize: Int = super[Growable].knownSize
 }
 
 class MultiSetImpl[A] private[mutable] (val elems: Map[A, Int]) extends MultiSet[A] {