diff --git a/core-macros/src/main/scala/sbt/internal/util/appmacro/ContextUtil.scala b/core-macros/src/main/scala/sbt/internal/util/appmacro/ContextUtil.scala
index e40cdeac9..4bec6e63d 100644
--- a/core-macros/src/main/scala/sbt/internal/util/appmacro/ContextUtil.scala
+++ b/core-macros/src/main/scala/sbt/internal/util/appmacro/ContextUtil.scala
@@ -20,7 +20,7 @@ object ContextUtil {
    * Constructs an object with utility methods for operating in the provided macro context `c`.
    * Callers should explicitly specify the type parameter as `c.type` in order to preserve the path dependent types.
    */
-  def apply[C <: blackbox.Context with Singleton](c: C): ContextUtil[C] = new ContextUtil(c)
+  def apply[C <: blackbox.Context with Singleton](c: C): ContextUtil[C] = new ContextUtil(c: C)
 
   /**
    * Helper for implementing a no-argument macro that is introduced via an implicit.
@@ -33,6 +33,7 @@ object ContextUtil {
       c: blackbox.Context
   )(f: (c.Expr[Any], c.Position) => c.Expr[T]): c.Expr[T] = {
     import c.universe._
+
     c.macroApplication match {
       case s @ Select(Apply(_, t :: Nil), _) => f(c.Expr[Any](t), s.pos)
       case a @ Apply(_, t :: Nil)            => f(c.Expr[Any](t), a.pos)
@@ -106,7 +107,10 @@ final class ContextUtil[C <: blackbox.Context](val ctx: C) {
             if isWrapper(nme.decodedName.toString, tpe.tpe, qual) =>
           ()
         case tree =>
-          if (tree.symbol ne null) defs += tree.symbol;
+          if (tree.symbol ne null) {
+            defs += tree.symbol
+            ()
+          }
           super.traverse(tree)
       }
     }
diff --git a/core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala b/core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala
index 3f5066d66..489ec3d4f 100644
--- a/core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala
+++ b/core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala
@@ -158,7 +158,7 @@ object Instance {
       val param =
         treeCopy.ValDef(variable, util.parameterModifiers, variable.name, variable.tpt, EmptyTree)
       val typeApplied =
-        TypeApply(util.select(instance, MapName), variable.tpt :: TypeTree(treeType) :: Nil)
+        TypeApply(util.select(instance, MapName), variable.tpt :: (TypeTree(treeType): Tree) :: Nil)
       val f = util.createFunction(param :: Nil, body, functionSym)
       val mapped = ApplyTree(typeApplied, input.expr :: f :: Nil)
       if (t.isLeft) mapped else flatten(mapped)
diff --git a/core-macros/src/main/scala/sbt/internal/util/appmacro/MixedBuilder.scala b/core-macros/src/main/scala/sbt/internal/util/appmacro/MixedBuilder.scala
index 2d4f68423..abcde7c80 100644
--- a/core-macros/src/main/scala/sbt/internal/util/appmacro/MixedBuilder.scala
+++ b/core-macros/src/main/scala/sbt/internal/util/appmacro/MixedBuilder.scala
@@ -19,7 +19,9 @@ object MixedBuilder extends TupleBuilder {
   def make(
       c: blackbox.Context
   )(mt: c.Type, inputs: Inputs[c.universe.type]): BuilderResult[c.type] = {
-    val delegate = if (inputs.size > TupleNBuilder.MaxInputs) KListBuilder else TupleNBuilder
+    val delegate =
+      if (inputs.size > TupleNBuilder.MaxInputs) (KListBuilder: TupleBuilder)
+      else (TupleNBuilder: TupleBuilder)
     delegate.make(c)(mt, inputs)
   }
 }
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/Attributes.scala b/internal/util-collection/src/main/scala/sbt/internal/util/Attributes.scala
index 931c8592e..0c1d2cf2c 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/Attributes.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/Attributes.scala
@@ -216,16 +216,17 @@ private class BasicAttributeMap(private val backing: Map[AttributeKey[_], Any])
   def contains[T](k: AttributeKey[T]) = backing.contains(k)
 
   def put[T](k: AttributeKey[T], value: T): AttributeMap =
-    new BasicAttributeMap(backing.updated(k, value))
+    new BasicAttributeMap(backing.updated(k, value: Any))
 
   def keys: Iterable[AttributeKey[_]] = backing.keys
 
   def ++(o: Iterable[AttributeEntry[_]]): AttributeMap =
-    new BasicAttributeMap(o.foldLeft(backing)((b, e) => b.updated(e.key, e.value)))
+    new BasicAttributeMap(o.foldLeft(backing)((b, e) => b.updated(e.key, e.value: Any)))
 
   def ++(o: AttributeMap): AttributeMap = o match {
-    case bam: BasicAttributeMap => new BasicAttributeMap(backing ++ bam.backing)
-    case _                      => o ++ this
+    case bam: BasicAttributeMap =>
+      new BasicAttributeMap(Map(backing.toSeq ++ bam.backing.toSeq: _*))
+    case _ => o ++ this
   }
 
   def entries: Iterable[AttributeEntry[_]] =
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/HListFormats.scala b/internal/util-collection/src/main/scala/sbt/internal/util/HListFormats.scala
index a2aede11d..4143861b2 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/HListFormats.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/HListFormats.scala
@@ -13,7 +13,7 @@ import sjsonnew._
 import Types.:+:
 
 trait HListFormats {
-  implicit val lnilFormat1: JsonFormat[HNil] = forHNil(HNil)
+  implicit val lnilFormat1: JsonFormat[HNil] = forHNil(HNil: HNil)
   implicit val lnilFormat2: JsonFormat[HNil.type] = forHNil(HNil)
 
   private def forHNil[A <: HNil](hnil: A): JsonFormat[A] = new JsonFormat[A] {
@@ -73,7 +73,7 @@ trait HListFormats {
       }
     }
 
-  implicit val lnilHListJF1: HListJF[HNil] = hnilHListJF(HNil)
+  implicit val lnilHListJF1: HListJF[HNil] = hnilHListJF(HNil: HNil)
   implicit val lnilHListJF2: HListJF[HNil.type] = hnilHListJF(HNil)
 
   implicit def hnilHListJF[A <: HNil](hnil: A): HListJF[A] = new HListJF[A] {
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/IDSet.scala b/internal/util-collection/src/main/scala/sbt/internal/util/IDSet.scala
index bf8d42ed3..b3cacdfe1 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/IDSet.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/IDSet.scala
@@ -25,9 +25,11 @@ trait IDSet[T] {
 
 object IDSet {
   implicit def toTraversable[T]: IDSet[T] => Traversable[T] = _.all
-  def apply[T](values: T*): IDSet[T] = apply(values)
+  def apply[T](values: T*): IDSet[T] = fromIterable(values)
 
-  def apply[T](values: Iterable[T]): IDSet[T] = {
+  def apply[T](values: Iterable[T]): IDSet[T] = fromIterable(values)
+
+  private def fromIterable[T](values: Iterable[T]): IDSet[T] = {
     val s = create[T]
     s ++= values
     s
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/INode.scala b/internal/util-collection/src/main/scala/sbt/internal/util/INode.scala
index 8cac982b4..3d7c86646 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/INode.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/INode.scala
@@ -42,8 +42,8 @@ abstract class EvaluateSettings[ScopeType] {
       )
     case b: Bind[s, A1$] @unchecked           => new BindNode[s, A1$](transform(b.in), x => transform(b.f(x)))
     case v: Value[A1$] @unchecked             => constant(v.value)
-    case v: ValidationCapture[A1$] @unchecked => strictConstant(v.key)
-    case t: TransformCapture                  => strictConstant(t.f)
+    case v: ValidationCapture[A1$] @unchecked => strictConstant(v.key: A1$)
+    case t: TransformCapture                  => strictConstant(t.f: A1$)
     case o: Optional[s, A1$] @unchecked =>
       o.a match {
         case None    => constant(() => o.f(None))
@@ -120,7 +120,8 @@ abstract class EvaluateSettings[ScopeType] {
 
     private[this] def keyString =
       (static.toSeq.flatMap {
-        case (key, value) => if (value eq this) init.showFullKey.show(key) :: Nil else Nil
+        case (key, value) =>
+          if (value eq this) init.showFullKey.show(key) :: Nil else List.empty[String]
       }).headOption getOrElse "non-static"
 
     final def get: T = synchronized {
@@ -130,7 +131,10 @@ abstract class EvaluateSettings[ScopeType] {
 
     final def doneOrBlock(from: INode[_]): Boolean = synchronized {
       val ready = state == Evaluated
-      if (!ready) blocking += from
+      if (!ready) {
+        blocking += from
+        ()
+      }
       registerIfNew()
       ready
     }
@@ -191,9 +195,10 @@ abstract class EvaluateSettings[ScopeType] {
       registerIfNew()
       state match {
         case Evaluated => submitCallComplete(by, value)
-        case _         => calledBy += by
+        case _ =>
+          calledBy += by
+          ()
       }
-      ()
     }
 
     protected def dependsOn: Seq[INode[_]]
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/KList.scala b/internal/util-collection/src/main/scala/sbt/internal/util/KList.scala
index 3274a2872..2761d8172 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/KList.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/KList.scala
@@ -62,7 +62,7 @@ sealed abstract class KNil extends KList[NothingK] {
   final def apply[N[x], Z](f: KNil => Z)(implicit ap: Applicative[N]): N[Z] = ap.pure(f(KNil))
 
   final def traverse[N[_], P[_]](f: NothingK ~> (N ∙ P)#l)(implicit np: Applicative[N]): N[KNil] =
-    np.pure(KNil)
+    np.pure(KNil: KNil)
 }
 
 case object KNil extends KNil {
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/PMap.scala b/internal/util-collection/src/main/scala/sbt/internal/util/PMap.scala
index 3fc5b231f..356f6973b 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/PMap.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/PMap.scala
@@ -72,8 +72,8 @@ object IMap {
       val mapped = backing.iterator.map {
         case (k, v) =>
           f(v) match {
-            case Left(l)  => Left((k, l))
-            case Right(r) => Right((k, r))
+            case Left(l)  => Left((k, l)): Either[(K[_], VL[_]), (K[_], VR[_])]
+            case Right(r) => Right((k, r)): Either[(K[_], VL[_]), (K[_], VR[_])]
           }
       }
       val (l, r) = Util.separateE[(K[_], VL[_]), (K[_], VR[_])](mapped.toList)
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala b/internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala
index 73de8f284..7d15ee2f7 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala
@@ -11,6 +11,7 @@ import scala.language.existentials
 
 import Types._
 import sbt.util.Show
+import Util.{ nil, nilSeq }
 
 sealed trait Settings[ScopeType] {
   def data: Map[ScopeType, AttributeMap]
@@ -177,7 +178,7 @@ trait Init[ScopeType] {
     val (defaults, others) = Util.separate[Setting[_], DefaultSetting[_], Setting[_]](ss) {
       case u: DefaultSetting[_] => Left(u); case s => Right(s)
     }
-    defaults.distinct ++ others
+    (defaults.distinct: Seq[Setting[_]]) ++ others
   }
 
   def compiled(init: Seq[Setting[_]], actual: Boolean = true)(
@@ -382,10 +383,16 @@ trait Init[ScopeType] {
 
   def flattenLocals(compiled: CompiledMap): Map[ScopedKey[_], Flattened] = {
     val locals = compiled flatMap {
-      case (key, comp) => if (key.key.isLocal) Seq[Compiled[_]](comp) else Nil
+      case (key, comp) =>
+        if (key.key.isLocal) Seq(comp)
+        else nilSeq[Compiled[_]]
     }
     val ordered = Dag.topologicalSort(locals)(
-      _.dependencies.flatMap(dep => if (dep.key.isLocal) Seq[Compiled[_]](compiled(dep)) else Nil)
+      _.dependencies.flatMap(
+        dep =>
+          if (dep.key.isLocal) Seq[Compiled[_]](compiled(dep))
+          else nilSeq[Compiled[_]]
+      )
     )
     def flatten(
         cmap: Map[ScopedKey[_], Flattened],
@@ -394,7 +401,9 @@ trait Init[ScopeType] {
     ): Flattened =
       new Flattened(
         key,
-        deps.flatMap(dep => if (dep.key.isLocal) cmap(dep).dependencies else dep :: Nil)
+        deps.flatMap(
+          dep => if (dep.key.isLocal) cmap(dep).dependencies else Seq[ScopedKey[_]](dep).toIterable
+        )
       )
 
     val empty = Map.empty[ScopedKey[_], Flattened]
@@ -405,8 +414,7 @@ trait Init[ScopeType] {
 
     compiled flatMap {
       case (key, comp) =>
-        if (key.key.isLocal)
-          Nil
+        if (key.key.isLocal) nilSeq[(ScopedKey[_], Flattened)]
         else
           Seq[(ScopedKey[_], Flattened)]((key, flatten(flattenedLocals, key, comp.dependencies)))
     }
@@ -515,8 +523,8 @@ trait Init[ScopeType] {
               d.outputs ++= out
               out
             } else
-              Nil
-        } getOrElse Nil
+              nilSeq
+        } getOrElse nilSeq
       }
       derivedForKey.flatMap(localAndDerived)
     }
@@ -527,7 +535,7 @@ trait Init[ScopeType] {
     def process(rem: List[Setting[_]]): Unit = rem match {
       case s :: ss =>
         val sk = s.key
-        val ds = if (processed.add(sk)) deriveFor(sk) else Nil
+        val ds = if (processed.add(sk)) deriveFor(sk) else nil
         addDefs(ds)
         process(ds ::: ss)
       case Nil =>
@@ -539,7 +547,7 @@ trait Init[ScopeType] {
     val allDefs = addLocal(init)(scopeLocal)
     allDefs.flatMap {
       case d: DerivedSetting[_] => (derivedToStruct get d map (_.outputs)).toSeq.flatten
-      case s                    => s :: Nil
+      case s                    => s :: nil
     }
   }
 
@@ -608,7 +616,8 @@ trait Init[ScopeType] {
   ) extends SettingsDefinition {
     def settings = this :: Nil
     def definitive: Boolean = !init.dependencies.contains(key)
-    def dependencies: Seq[ScopedKey[_]] = remove(init.dependencies, key)
+    def dependencies: Seq[ScopedKey[_]] =
+      remove(init.dependencies.asInstanceOf[Seq[ScopedKey[T]]], key)
     def mapReferenced(g: MapScoped): Setting[T] = make(key, init mapReferenced g, pos)
 
     def validateReferenced(g: ValidateRef): Either[Seq[Undefined], Setting[T]] =
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/Signal.scala b/internal/util-collection/src/main/scala/sbt/internal/util/Signal.scala
index 857c38807..c8188e9ac 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/Signal.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/Signal.scala
@@ -16,7 +16,7 @@ object Signals {
       try {
         val signals = new Signals0
         signals.withHandler(signal, handler, action)
-      } catch { case _: LinkageError => Right(action()) }
+      } catch { case _: LinkageError => Right(action()): Either[Throwable, T] }
 
     result match {
       case Left(e)  => throw e
diff --git a/internal/util-collection/src/main/scala/sbt/internal/util/Util.scala b/internal/util-collection/src/main/scala/sbt/internal/util/Util.scala
index 0e71371cd..a42f3c380 100644
--- a/internal/util-collection/src/main/scala/sbt/internal/util/Util.scala
+++ b/internal/util-collection/src/main/scala/sbt/internal/util/Util.scala
@@ -55,4 +55,12 @@ object Util {
 
   lazy val isNonCygwinWindows: Boolean = isWindows && !isCygwin
   lazy val isCygwinWindows: Boolean = isWindows && isCygwin
+
+  def nil[A]: List[A] = List.empty[A]
+  def nilSeq[A]: Seq[A] = Seq.empty[A]
+  def none[A]: Option[A] = (None: Option[A])
+
+  implicit class AnyOps[A](private val value: A) extends AnyVal {
+    def some: Option[A] = (Some(value): Option[A])
+  }
 }
diff --git a/internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala b/internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala
index 85e696313..d23bc91ff 100644
--- a/internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala
+++ b/internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala
@@ -108,8 +108,10 @@ private[sbt] object JLine {
       case "jline.UnsupportedTerminal"                      => "none"
       case x                                                => x
     }
-    if (newValue != null) System.setProperty(TerminalProperty, newValue)
-    ()
+    if (newValue != null) {
+      System.setProperty(TerminalProperty, newValue)
+      ()
+    }
   }
 
   protected[this] val originalIn = new FileInputStream(FileDescriptor.in)
@@ -149,7 +151,7 @@ private[sbt] object JLine {
       cr.setBellEnabled(false)
       val h = historyPath match {
         case None       => new MemoryHistory
-        case Some(file) => new FileHistory(file)
+        case Some(file) => (new FileHistory(file): MemoryHistory)
       }
       h.setMaxSize(MaxHistorySize)
       cr.setHistory(h)
diff --git a/internal/util-complete/src/main/scala/sbt/internal/util/complete/HistoryCommands.scala b/internal/util-complete/src/main/scala/sbt/internal/util/complete/HistoryCommands.scala
index f7625be56..6c45f54e9 100644
--- a/internal/util-complete/src/main/scala/sbt/internal/util/complete/HistoryCommands.scala
+++ b/internal/util-complete/src/main/scala/sbt/internal/util/complete/HistoryCommands.scala
@@ -9,6 +9,7 @@ package sbt.internal.util
 package complete
 
 import sbt.io.IO
+import Util.{ AnyOps, nil }
 
 object HistoryCommands {
   val Start = "!"
@@ -57,7 +58,7 @@ object HistoryCommands {
   lazy val last = Last ^^^ { execute(_.!!) }
 
   lazy val list = ListCommands ~> (num ?? Int.MaxValue) map { show => (h: History) =>
-    { printHistory(h, MaxLines, show); Some(Nil) }
+    { printHistory(h, MaxLines, show); nil[String].some }
   }
 
   lazy val execStr = flag('?') ~ token(any.+.string, "<string>") map {
@@ -70,7 +71,7 @@ object HistoryCommands {
       execute(h => if (neg) h !- value else h ! value)
   }
 
-  lazy val help = success((h: History) => { printHelp(); Some(Nil) })
+  lazy val help = success((h: History) => { printHelp(); nil[String].some })
 
   def execute(f: History => Option[String]): History => Option[List[String]] = (h: History) => {
     val command = f(h).filterNot(_.startsWith(Start))
@@ -79,7 +80,7 @@ object HistoryCommands {
     h.path foreach { h =>
       IO.writeLines(h, lines)
     }
-    Some(command.toList)
+    command.toList.some
   }
 
   val actionParser: Parser[complete.History => Option[List[String]]] =
diff --git a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parser.scala b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parser.scala
index db29830d8..464ab1916 100644
--- a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parser.scala
+++ b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parser.scala
@@ -319,7 +319,7 @@ trait ParserMain {
   implicit def richParser[A](a: Parser[A]): RichParser[A] = new RichParser[A] {
     def ~[B](b: Parser[B]) = seqParser(a, b)
     def ||[B](b: Parser[B]) = choiceParser(a, b)
-    def |[B >: A](b: Parser[B]) = homParser(a, b)
+    def |[B >: A](b: Parser[B]) = homParser[B](a, b)
     def ? = opt(a)
     def * = zeroOrMore(a)
     def + = oneOrMore(a)
@@ -327,7 +327,9 @@ trait ParserMain {
     def id = a
 
     def ^^^[B](value: B): Parser[B] = a map (_ => value)
-    def ??[B >: A](alt: B): Parser[B] = a.? map { _ getOrElse alt }
+    def ??[B >: A](alt: B): Parser[B] = a.? map { x =>
+      x.getOrElse[B](alt)
+    }
     def <~[B](b: Parser[B]): Parser[A] = (a ~ b) map { case av ~ _ => av }
     def ~>[B](b: Parser[B]): Parser[B] = (a ~ b) map { case _ ~ bv => bv }
     def !!!(msg: String): Parser[A] = onFailure(a, msg)
@@ -477,7 +479,7 @@ trait ParserMain {
           if (ci >= s.length)
             a.resultEmpty.toEither.left.map { msgs0 => () =>
               val msgs = msgs0()
-              val nonEmpty = if (msgs.isEmpty) "Unexpected end of input" :: Nil else msgs
+              val nonEmpty = if (msgs.isEmpty) Seq("Unexpected end of input") else msgs
               (nonEmpty, ci)
             } else
             loop(ci, a derive s(ci))
@@ -601,7 +603,8 @@ trait ParserMain {
 
   def seq0[T](p: Seq[Parser[T]], errors: => Seq[String]): Parser[Seq[T]] = {
     val (newErrors, valid) = separate(p) {
-      case Invalid(f) => Left(f.errors _); case ok => Right(ok)
+      case Invalid(f) => Left(f.errors _): Either[() => Seq[String], Parser[T]]
+      case ok         => Right(ok): Either[() => Seq[String], Parser[T]]
     }
     def combinedErrors = errors ++ newErrors.flatMap(_())
     if (valid.isEmpty) invalid(combinedErrors) else new ParserSeq(valid, combinedErrors)
@@ -966,10 +969,11 @@ private final class Repeat[T](
   lazy val resultEmpty: Result[Seq[T]] = {
     val partialAccumulatedOption =
       partial match {
-        case None                 => Value(accumulatedReverse)
-        case Some(partialPattern) => partialPattern.resultEmpty.map(_ :: accumulatedReverse)
+        case None => (Value(accumulatedReverse): Result[List[T]])
+        case Some(partialPattern) =>
+          partialPattern.resultEmpty.map(_ :: accumulatedReverse)
       }
-    (partialAccumulatedOption app repeatedParseEmpty)(_ reverse_::: _)
+    (partialAccumulatedOption app repeatedParseEmpty)((x, y) => (x reverse_::: y): Seq[T])
   }
 
   private def repeatedParseEmpty: Result[List[T]] = {
diff --git a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
index 3560ef2f6..651ddd8d0 100644
--- a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
+++ b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
@@ -22,6 +22,7 @@ import java.lang.Character.{
 }
 
 import scala.annotation.tailrec
+import sbt.internal.util.Util.nilSeq
 
 /** Provides standard implementations of commonly useful [[Parser]]s. */
 trait Parsers {
@@ -130,7 +131,7 @@ trait Parsers {
    * Matches a non-empty String consisting of whitespace characters.
    * The suggested tab completion is a single, constant space character.
    */
-  lazy val Space = SpaceClass.+.examples(" ")
+  lazy val Space: Parser[Seq[Char]] = SpaceClass.+.examples(" ")
 
   /**
    * Matches a possibly empty String consisting of whitespace characters.
@@ -274,7 +275,7 @@ trait Parsers {
    * The result is the (possibly empty) sequence of results from the multiple `rep` applications.  The `sep` results are discarded.
    */
   def repsep[T](rep: Parser[T], sep: Parser[_]): Parser[Seq[T]] =
-    rep1sep(rep, sep) ?? Nil
+    rep1sep(rep, sep) ?? nilSeq[T]
 
   /**
    * Applies `rep` one or more times, separated by `sep`.
diff --git a/internal/util-logic/src/main/scala/sbt/internal/util/logic/Logic.scala b/internal/util-logic/src/main/scala/sbt/internal/util/logic/Logic.scala
index 5066bd879..31597a8c8 100644
--- a/internal/util-logic/src/main/scala/sbt/internal/util/logic/Logic.scala
+++ b/internal/util-logic/src/main/scala/sbt/internal/util/logic/Logic.scala
@@ -118,9 +118,9 @@ object Logic {
     val (pos, neg) = (posSeq.toSet, negSeq.toSet)
 
     val problem =
-      checkContradictions(pos, neg) orElse
-        checkOverlap(clauses, pos) orElse
-        checkAcyclic(clauses)
+      (checkContradictions(pos, neg): Option[LogicException]) orElse
+        (checkOverlap(clauses, pos): Option[LogicException]) orElse
+        (checkAcyclic(clauses): Option[LogicException])
 
     problem.toLeft(
       reduce0(clauses, initialFacts, Matched.empty)
@@ -249,7 +249,8 @@ object Logic {
         else {
           val unproven = Clauses(unprovenClauses)
           val nextFacts: Set[Literal] =
-            if (newlyProven.nonEmpty) newlyProven.toSet else inferFailure(unproven)
+            if (newlyProven.nonEmpty) newlyProven.toSet[Literal]
+            else inferFailure(unproven)
           reduce0(unproven, nextFacts, newState)
         }
     }
@@ -283,7 +284,7 @@ object Logic {
     }
   }
 
-  private[this] def negated(atoms: Set[Atom]): Set[Literal] = atoms.map(a => Negated(a))
+  private[this] def negated(atoms: Set[Atom]): Set[Literal] = atoms.map(a => (Negated(a): Literal))
 
   /**
    * Computes the set of atoms in `clauses` that directly or transitively take a negated atom as input.
@@ -386,7 +387,9 @@ object Logic {
         None
       else {
         val newLits = lits -- facts
-        val newF = if (newLits.isEmpty) True else And(newLits)
+        val newF =
+          if (newLits.isEmpty) (True: Formula)
+          else (And(newLits): Formula)
         Some(newF) // 1.
       }
     case True => Some(True)
diff --git a/main-actions/src/main/scala/sbt/ForkTests.scala b/main-actions/src/main/scala/sbt/ForkTests.scala
index c2bf44ddd..f83bd71bb 100755
--- a/main-actions/src/main/scala/sbt/ForkTests.scala
+++ b/main-actions/src/main/scala/sbt/ForkTests.scala
@@ -18,6 +18,7 @@ import sbt.util.Logger
 import sbt.ConcurrentRestrictions.Tag
 import sbt.protocol.testing._
 import sbt.internal.util.ConsoleAppender
+import sbt.internal.util.Util.{ AnyOps, none }
 
 private[sbt] object ForkTests {
   def apply(
@@ -56,8 +57,8 @@ private[sbt] object ForkTests {
     std.TaskExtra.task {
       val server = new ServerSocket(0)
       val testListeners = opts.testListeners flatMap {
-        case tl: TestsListener => Some(tl)
-        case _                 => None
+        case tl: TestsListener => tl.some
+        case _                 => none[TestsListener]
       }
 
       object Acceptor extends Runnable {
@@ -124,7 +125,7 @@ private[sbt] object ForkTests {
         val acceptorThread = new Thread(Acceptor)
         acceptorThread.start()
 
-        val fullCp = classpath ++: Seq(
+        val fullCp = classpath ++ Seq(
           IO.classLocationPath[ForkMain].toFile,
           IO.classLocationPath[Framework].toFile
         )
diff --git a/main-actions/src/main/scala/sbt/Package.scala b/main-actions/src/main/scala/sbt/Package.scala
index 82a9717c8..e5ba47012 100644
--- a/main-actions/src/main/scala/sbt/Package.scala
+++ b/main-actions/src/main/scala/sbt/Package.scala
@@ -51,8 +51,8 @@ object Package {
     val entryMap = manifest.getEntries.asScala
     for ((key, value) <- mergeManifest.getEntries.asScala) {
       entryMap.get(key) match {
-        case Some(attributes) => mergeAttributes(attributes, value)
-        case None             => entryMap put (key, value)
+        case Some(attributes) => mergeAttributes(attributes, value); ()
+        case None             => entryMap put (key, value); ()
       }
     }
   }
@@ -81,9 +81,9 @@ object Package {
     val main = manifest.getMainAttributes
     for (option <- conf.options) {
       option match {
-        case JarManifest(mergeManifest)          => mergeManifests(manifest, mergeManifest)
-        case MainClass(mainClassName)            => main.put(Attributes.Name.MAIN_CLASS, mainClassName)
-        case ManifestAttributes(attributes @ _*) => main.asScala ++= attributes
+        case JarManifest(mergeManifest)          => mergeManifests(manifest, mergeManifest); ()
+        case MainClass(mainClassName)            => main.put(Attributes.Name.MAIN_CLASS, mainClassName); ()
+        case ManifestAttributes(attributes @ _*) => main.asScala ++= attributes; ()
         case _                                   => log.warn("Ignored unknown package option " + option)
       }
     }
@@ -98,6 +98,7 @@ object Package {
           if (inChanged || outChanged) {
             makeJar(sources, jar.file, manifest, log)
             jar.file
+            ()
           } else
             log.debug("Jar uptodate: " + jar.file)
         }
diff --git a/main-actions/src/main/scala/sbt/Tests.scala b/main-actions/src/main/scala/sbt/Tests.scala
index 535e09887..aac68dc55 100644
--- a/main-actions/src/main/scala/sbt/Tests.scala
+++ b/main-actions/src/main/scala/sbt/Tests.scala
@@ -24,6 +24,7 @@ import testing.{
   SubclassFingerprint,
   Runner,
   TaskDef,
+  Selector,
   SuiteSelector,
   Task => TestTask
 }
@@ -155,14 +156,15 @@ object Tests {
 
     for (option <- config.options) {
       option match {
-        case Filter(include) => testFilters += include
+        case Filter(include) => testFilters += include; ()
         case Filters(includes) =>
           if (orderedFilters.nonEmpty) sys.error("Cannot define multiple ordered test filters.")
           else orderedFilters = includes
-        case Exclude(exclude)         => excludeTestsSet ++= exclude
-        case Listeners(listeners)     => testListeners ++= listeners
-        case Setup(setupFunction)     => setup += setupFunction
-        case Cleanup(cleanupFunction) => cleanup += cleanupFunction
+          ()
+        case Exclude(exclude)         => excludeTestsSet ++= exclude; ()
+        case Listeners(listeners)     => testListeners ++= listeners; ()
+        case Setup(setupFunction)     => setup += setupFunction; ()
+        case Cleanup(cleanupFunction) => cleanup += cleanupFunction; ()
         case _: Argument              => // now handled by whatever constructs `runners`
       }
     }
@@ -239,16 +241,18 @@ object Tests {
     val setupTasks = fj(partApp(userSetup) :+ frameworkSetup)
     val mainTasks =
       if (config.parallel)
-        makeParallel(loader, runnables, setupTasks, config.tags) //.toSeq.join
+        makeParallel(loader, runnables, setupTasks, config.tags).map(_.toList)
       else
         makeSerial(loader, runnables, setupTasks)
     val taggedMainTasks = mainTasks.tagw(config.tags: _*)
-    taggedMainTasks map processResults flatMap { results =>
-      val cleanupTasks = fj(partApp(userCleanup) :+ frameworkCleanup(results.overall))
-      cleanupTasks map { _ =>
-        results
+    taggedMainTasks
+      .map(processResults)
+      .flatMap { results =>
+        val cleanupTasks = fj(partApp(userCleanup) :+ frameworkCleanup(results.overall))
+        cleanupTasks map { _ =>
+          results
+        }
       }
-    }
   }
   type TestRunnable = (String, TestFunction)
 
@@ -367,13 +371,20 @@ object Tests {
     }
 
   def foldTasks(results: Seq[Task[Output]], parallel: Boolean): Task[Output] =
-    if (results.isEmpty)
-      task { Output(TestResult.Passed, Map.empty, Nil) } else if (parallel)
-      reduced(results.toIndexedSeq, {
-        case (Output(v1, m1, _), Output(v2, m2, _)) =>
-          Output(if (severity(v1) < severity(v2)) v2 else v1, m1 ++ m2, Iterable.empty)
-      })
-    else {
+    if (results.isEmpty) {
+      task { Output(TestResult.Passed, Map.empty, Nil) }
+    } else if (parallel) {
+      reduced[Output](
+        results.toIndexedSeq, {
+          case (Output(v1, m1, _), Output(v2, m2, _)) =>
+            Output(
+              (if (severity(v1) < severity(v2)) v2 else v1): TestResult,
+              Map((m1.toSeq ++ m2.toSeq): _*),
+              Iterable.empty[Summary]
+            )
+        }
+      )
+    } else {
       def sequence(tasks: List[Task[Output]], acc: List[Output]): Task[List[Output]] =
         tasks match {
           case Nil => task(acc.reverse)
@@ -386,7 +397,10 @@ object Tests {
         val (rs, ms) = ress.unzip { e =>
           (e.overall, e.events)
         }
-        Output(overall(rs), ms reduce (_ ++ _), Iterable.empty)
+        val m = ms reduce { (m1: Map[String, SuiteResult], m2: Map[String, SuiteResult]) =>
+          Map((m1.toSeq ++ m2.toSeq): _*)
+        }
+        Output(overall(rs), m, Iterable.empty)
       }
     }
   def overall(results: Iterable[TestResult]): TestResult =
@@ -406,9 +420,11 @@ object Tests {
       acs.flatMap { ac =>
         val companions = ac.api
         val all =
-          Seq(companions.classApi, companions.objectApi) ++
-            companions.classApi.structure.declared ++ companions.classApi.structure.inherited ++
-            companions.objectApi.structure.declared ++ companions.objectApi.structure.inherited
+          Seq(companions.classApi: Definition, companions.objectApi: Definition) ++
+            (companions.classApi.structure.declared.toSeq: Seq[Definition]) ++
+            (companions.classApi.structure.inherited.toSeq: Seq[Definition]) ++
+            (companions.objectApi.structure.declared.toSeq: Seq[Definition]) ++
+            (companions.objectApi.structure.inherited.toSeq: Seq[Definition])
 
         all
       }.toSeq
@@ -446,8 +462,10 @@ object Tests {
     })
     // TODO: To pass in correct explicitlySpecified and selectors
     val tests =
-      for ((df, di) <- discovered; fingerprint <- toFingerprints(di))
-        yield new TestDefinition(df.name, fingerprint, false, Array(new SuiteSelector))
+      for {
+        (df, di) <- discovered
+        fingerprint <- toFingerprints(di)
+      } yield new TestDefinition(df.name, fingerprint, false, Array(new SuiteSelector: Selector))
     val mains = discovered collect { case (df, di) if di.hasMain => df.name }
     (tests, mains.toSet)
   }
diff --git a/main-actions/src/main/scala/sbt/compiler/Eval.scala b/main-actions/src/main/scala/sbt/compiler/Eval.scala
index c024cf577..f1d408af0 100644
--- a/main-actions/src/main/scala/sbt/compiler/Eval.scala
+++ b/main-actions/src/main/scala/sbt/compiler/Eval.scala
@@ -214,7 +214,8 @@ final class Eval(
 
     val (extra, loader) = backing match {
       case Some(back) if classExists(back, moduleName) =>
-        val loader = (parent: ClassLoader) => new URLClassLoader(Array(back.toURI.toURL), parent)
+        val loader = (parent: ClassLoader) =>
+          (new URLClassLoader(Array(back.toURI.toURL), parent): ClassLoader)
         val extra = ev.read(cacheFile(back, moduleName))
         (extra, loader)
       case _ =>
@@ -313,9 +314,10 @@ final class Eval(
       )
     )
 
-    def moduleBody = Template(List(gen.scalaAnyRefConstr), noSelfType, emptyInit :: definitions)
+    def moduleBody =
+      Template(List(gen.scalaAnyRefConstr), noSelfType, (emptyInit: Tree) :: definitions)
     def moduleDef = ModuleDef(NoMods, newTermName(objectName), moduleBody)
-    parser.makePackaging(0, emptyPkg, (imports :+ moduleDef).toList)
+    parser.makePackaging(0, emptyPkg, (imports :+ (moduleDef: Tree)).toList)
   }
 
   private[this] final class TypeExtractor extends Traverser {
diff --git a/main-command/src/main/scala/sbt/BasicCommands.scala b/main-command/src/main/scala/sbt/BasicCommands.scala
index 3f65eb913..90e5abe85 100644
--- a/main-command/src/main/scala/sbt/BasicCommands.scala
+++ b/main-command/src/main/scala/sbt/BasicCommands.scala
@@ -19,6 +19,7 @@ import sbt.internal.util.complete.{
   TokenCompletions
 }
 import sbt.internal.util.Types.{ const, idFun }
+import sbt.internal.util.Util.{ AnyOps, nil, nilSeq, none }
 import sbt.internal.inc.classpath.ClasspathUtilities.toLoader
 import sbt.internal.inc.ModuleUtilities
 import sbt.internal.client.NetworkClient
@@ -67,7 +68,7 @@ object BasicCommands {
     Iterator(Level.Debug, Level.Info, Level.Warn, Level.Error) map (l => token(l.toString)) reduce (_ | _)
 
   private[this] def addPluginSbtFileParser: Parser[File] = {
-    token(AddPluginSbtFileCommand) ~> (":" | "=" | Space) ~> (StringBasic).examples(
+    token(AddPluginSbtFileCommand) ~> (":" | "=" | Space.map(_.toString)) ~> (StringBasic).examples(
       "/some/extra.sbt"
     ) map {
       new File(_)
@@ -76,7 +77,7 @@ object BasicCommands {
 
   private[this] def addPluginSbtFileStringParser: Parser[String] = {
     token(
-      token(AddPluginSbtFileCommand) ~ (":" | "=" | Space) ~ (StringBasic)
+      token(AddPluginSbtFileCommand) ~ (":" | "=" | Space.map(_.toString)) ~ (StringBasic)
         .examples("/some/extra.sbt") map {
         case s1 ~ s2 ~ s3 => s1 + s2 + s3
       }
@@ -99,7 +100,7 @@ object BasicCommands {
   def addPluginSbtFile: Command = Command.arb(_ => addPluginSbtFileParser, addPluginSbtFileHelp) {
     (s, extraSbtFile) =>
       val extraFiles = s.get(BasicKeys.extraMetaSbtFiles).toList.flatten
-      s.put(BasicKeys.extraMetaSbtFiles, extraFiles :+ extraSbtFile)
+      s.put(BasicKeys.extraMetaSbtFiles, (extraFiles: Seq[File]) :+ extraSbtFile)
   }
 
   def help: Command = Command.make(HelpCommand, helpBrief, helpDetailed)(helpParser)
@@ -119,12 +120,12 @@ object BasicCommands {
 
     val (extraArgs, remainingCommands) = s.remainingCommands match {
       case xs :+ exec if exec.commandLine == "shell" => (xs, exec :: Nil)
-      case xs                                        => (xs, Nil)
+      case xs                                        => (xs, nil[Exec])
     }
 
     val topic = (arg.toList ++ extraArgs.map(_.commandLine)) match {
-      case Nil => None
-      case xs  => Some(xs.mkString(" "))
+      case Nil => none[String]
+      case xs  => xs.mkString(" ").some
     }
     val message = try Help.message(h, topic)
     catch { case NonFatal(ex) => ex.toString }
@@ -165,7 +166,8 @@ object BasicCommands {
     val nonDelim = charClass(c => c != '"' && c != '{' && c != '}', label = "not '\"', '{', '}'")
     // Accept empty commands to simplify the parser.
     val cmdPart =
-      matched(((nonSemi & nonDelim) | StringEscapable | braces('{', '}')).*).examples()
+      matched(((nonSemi & nonDelim).map(_.toString) | StringEscapable | braces('{', '}')).*)
+        .examples()
 
     val completionParser: Option[Parser[String]] =
       state.map(s => (matched(s.nonMultiParser) & cmdPart) | cmdPart)
@@ -189,8 +191,8 @@ object BasicCommands {
       var fail = false
       while (it.hasNext && !fail) {
         it.next match {
-          case ""   => fail = it.hasNext
-          case next => result += next
+          case ""   => fail = it.hasNext; ()
+          case next => result += next; ()
         }
       }
       if (fail) Parser.failure(s"Couldn't parse empty commands in ${s.mkString(";")}")
@@ -244,7 +246,7 @@ object BasicCommands {
               val commandArgs =
                 (first.drop(commandName.length).trim :: Nil ::: tail).mkString(";")
               parse(commandArgs, command.parser(state)).toOption
-            case _ => None
+            case _ => none[() => State]
           }
         }.headOption match {
           case Some(s) => s()
@@ -261,7 +263,7 @@ object BasicCommands {
     (s: State) => token(OptSpace ~> combinedLax(s, NotSpaceClass ~ any.*))
 
   def combinedLax(s: State, any: Parser[_]): Parser[String] =
-    matched(s.combinedParser | token(any, hide = const(true)))
+    matched((s.combinedParser: Parser[_]) | token(any, hide = const(true)))
 
   def ifLast: Command =
     Command(IfLast, Help.more(IfLast, IfLastDetailed))(otherCommandParser)(
@@ -286,7 +288,7 @@ object BasicCommands {
     )
 
   def popOnFailure: Command = Command.command(PopOnFailure) { s =>
-    val stack = s.get(OnFailureStack).getOrElse(Nil)
+    val stack = s.get(OnFailureStack).getOrElse(nil)
     val updated =
       if (stack.isEmpty) s.remove(OnFailureStack) else s.put(OnFailureStack, stack.tail)
     updated.copy(onFailure = stack.headOption.flatten)
@@ -323,7 +325,7 @@ object BasicCommands {
     }
 
   def callParser: Parser[(Seq[String], Seq[String])] =
-    token(Space) ~> ((classpathOptionParser ?? Nil) ~ rep1sep(className, token(Space)))
+    token(Space) ~> ((classpathOptionParser ?? nilSeq) ~ rep1sep(className, token(Space)))
 
   private[this] def className: Parser[String] = {
     val base = StringBasic & not('-' ~> any.*, "Class name cannot start with '-'.")
@@ -367,7 +369,7 @@ object BasicCommands {
     }
 
   def oldshell: Command = Command.command(OldShell, Help.more(Shell, OldShellDetailed)) { s =>
-    val history = (s get historyPath) getOrElse Some(new File(s.baseDir, ".history"))
+    val history = (s get historyPath) getOrElse (new File(s.baseDir, ".history")).some
     val prompt = (s get shellPrompt) match { case Some(pf) => pf(s); case None => "> " }
     val reader = new FullReader(history, s.combinedParser)
     val line = reader.readLine(prompt)
@@ -388,12 +390,12 @@ object BasicCommands {
     Command(Client, Help.more(Client, ClientDetailed))(_ => clientParser)(runClient)
 
   def clientParser: Parser[Seq[String]] =
-    (token(Space) ~> repsep(StringBasic, token(Space))) | (token(EOF) map (_ => Nil))
+    (token(Space) ~> repsep(StringBasic, token(Space))) | (token(EOF) map (_ => nilSeq))
 
   def runClient(s0: State, inputArg: Seq[String]): State = {
     val arguments = inputArg.toList ++
       (s0.remainingCommands match {
-        case e :: Nil if e.commandLine == "shell" => Nil
+        case e :: Nil if e.commandLine == "shell" => nil
         case xs                                   => xs map (_.commandLine)
       })
     NetworkClient.run(s0.configuration, arguments)
@@ -511,7 +513,9 @@ object BasicCommands {
     val aliasRemoved = removeAlias(state, name)
     // apply the alias value to the commands of `state` except for the alias to avoid recursion (#933)
     val partiallyApplied = Parser(aliasRemoved.combinedParser)(value)
-    val arg = matched(partiallyApplied & (success(()) | (SpaceClass ~ any.*)))
+    val arg: Parser[String] = matched(
+      partiallyApplied & (success((): Any) | ((SpaceClass ~ any.*): Parser[Any]))
+    )
     // by scheduling the expanded alias instead of directly executing,
     // we get errors on the expanded string (#598)
     arg.map(str => () => (value + str) :: state)
diff --git a/main-command/src/main/scala/sbt/Command.scala b/main-command/src/main/scala/sbt/Command.scala
index 4348160b6..2244660a4 100644
--- a/main-command/src/main/scala/sbt/Command.scala
+++ b/main-command/src/main/scala/sbt/Command.scala
@@ -11,6 +11,7 @@ import sbt.internal.inc.ReflectUtilities
 import sbt.internal.util.complete.{ DefaultParsers, EditDistance, Parser }
 import sbt.internal.util.Types.const
 import sbt.internal.util.{ AttributeKey, AttributeMap, Util }
+import sbt.internal.util.Util.{ nilSeq }
 
 /**
  * An operation that can be executed from the sbt console.
@@ -195,7 +196,7 @@ object Command {
     s"Not a valid $label: $value" + similar(value, allowed)
 
   def similar(value: String, allowed: Iterable[String]): String = {
-    val suggested = if (value.length > 2) suggestions(value, allowed.toSeq) else Nil
+    val suggested = if (value.length > 2) suggestions(value, allowed.toSeq) else nilSeq
     if (suggested.isEmpty) "" else suggested.mkString(" (similar: ", ", ", ")")
   }
 
@@ -240,7 +241,11 @@ private final class Help0(
     val more: Set[String]
 ) extends Help {
   def ++(h: Help): Help =
-    new Help0(Help0.this.brief ++ h.brief, Help0.this.detail ++ h.detail, more ++ h.more)
+    new Help0(
+      Help0.this.brief ++ h.brief,
+      Map(Help0.this.detail.toSeq ++ h.detail.toSeq: _*),
+      more ++ h.more
+    )
 }
 
 object Help {
diff --git a/main-command/src/main/scala/sbt/CommandUtil.scala b/main-command/src/main/scala/sbt/CommandUtil.scala
index 78329fa28..f5f15733c 100644
--- a/main-command/src/main/scala/sbt/CommandUtil.scala
+++ b/main-command/src/main/scala/sbt/CommandUtil.scala
@@ -13,6 +13,7 @@ import java.util.regex.{ Pattern, PatternSyntaxException }
 import sbt.internal.util.AttributeKey
 import sbt.internal.util.complete.Parser
 import sbt.internal.util.complete.DefaultParsers._
+import sbt.internal.util.Util.nilSeq
 
 import sbt.io.IO
 import sbt.io.syntax._
@@ -35,7 +36,7 @@ object CommandUtil {
     catch { case _: NoSuchMethodError => new File(".").getAbsoluteFile }
 
   def aligned(pre: String, sep: String, in: Seq[(String, String)]): Seq[String] =
-    if (in.isEmpty) Nil
+    if (in.isEmpty) nilSeq
     else {
       val width = in.iterator.map(_._1.length).max
       for ((a, b) <- in) yield pre + fill(a, width) + sep + b
@@ -80,9 +81,9 @@ object CommandUtil {
         val keyString = Highlight.bold(keyMatches getOrElse k)
         val contentString = contentMatches getOrElse v
         if (keyMatches.isDefined || contentMatches.isDefined)
-          (keyString, contentString) :: Nil
+          Seq((keyString, contentString))
         else
-          Nil
+          nilSeq
     }
   }
 
diff --git a/main-command/src/main/scala/sbt/State.scala b/main-command/src/main/scala/sbt/State.scala
index 28be57a98..784c0f162 100644
--- a/main-command/src/main/scala/sbt/State.scala
+++ b/main-command/src/main/scala/sbt/State.scala
@@ -365,6 +365,7 @@ object State {
                         jars.toList,
                         () => new UncloseableURLLoader(jars, fullScalaLoader)
                       )
+                      ()
                     case _ =>
                   }
               }
diff --git a/main-command/src/main/scala/sbt/internal/CommandChannel.scala b/main-command/src/main/scala/sbt/internal/CommandChannel.scala
index 2d61c995c..9a0c932f0 100644
--- a/main-command/src/main/scala/sbt/internal/CommandChannel.scala
+++ b/main-command/src/main/scala/sbt/internal/CommandChannel.scala
@@ -30,7 +30,14 @@ abstract class CommandChannel {
     ()
   }
   def append(exec: Exec): Boolean = {
-    registered.forEach(q => q.synchronized { if (!q.contains(this)) q.add(this); () })
+    registered.forEach(
+      q =>
+        q.synchronized {
+          if (!q.contains(this)) {
+            q.add(this); ()
+          }
+        }
+    )
     commandQueue.add(exec)
   }
   def poll: Option[Exec] = Option(commandQueue.poll)
diff --git a/main-command/src/main/scala/sbt/internal/ConsoleChannel.scala b/main-command/src/main/scala/sbt/internal/ConsoleChannel.scala
index 53a2c71d9..3c23e97b2 100644
--- a/main-command/src/main/scala/sbt/internal/ConsoleChannel.scala
+++ b/main-command/src/main/scala/sbt/internal/ConsoleChannel.scala
@@ -13,11 +13,12 @@ import BasicKeys._
 import java.io.File
 import sbt.protocol.EventMessage
 import sjsonnew.JsonFormat
+import Util.AnyOps
 
 private[sbt] final class ConsoleChannel(val name: String) extends CommandChannel {
   private var askUserThread: Option[Thread] = None
   def makeAskUserThread(s: State): Thread = new Thread("ask-user-thread") {
-    val history = (s get historyPath) getOrElse Some(new File(s.baseDir, ".history"))
+    val history = (s get historyPath) getOrElse (new File(s.baseDir, ".history")).some
     val prompt = (s get shellPrompt) match {
       case Some(pf) => pf(s)
       case None     => "> "
diff --git a/main-command/src/main/scala/sbt/internal/LegacyWatched.scala b/main-command/src/main/scala/sbt/internal/LegacyWatched.scala
index 3a818aa9c..dc9860efd 100644
--- a/main-command/src/main/scala/sbt/internal/LegacyWatched.scala
+++ b/main-command/src/main/scala/sbt/internal/LegacyWatched.scala
@@ -42,7 +42,7 @@ private[sbt] object LegacyWatched {
           override def close(): Unit = watchState.close()
         }
         (ClearOnFailure :: next :: FailureWall :: repeat :: s)
-          .put(ContinuousEventMonitor, monitor)
+          .put(ContinuousEventMonitor, monitor: EventMonitor)
       case Some(eventMonitor) =>
         Watched.printIfDefined(watched watchingMessage eventMonitor.state)
         @tailrec def impl(): State = {
diff --git a/main-command/src/main/scala/sbt/internal/classpath/ClassLoaderCache.scala b/main-command/src/main/scala/sbt/internal/classpath/ClassLoaderCache.scala
index eaf3ff1de..dd730db87 100644
--- a/main-command/src/main/scala/sbt/internal/classpath/ClassLoaderCache.scala
+++ b/main-command/src/main/scala/sbt/internal/classpath/ClassLoaderCache.scala
@@ -103,6 +103,7 @@ private[sbt] class ClassLoaderCache(
           case ClassLoaderReference(key, classLoader) =>
             close(classLoader)
             delegate.remove(key)
+            ()
           case _ =>
         }
         clearExpiredLoaders()
@@ -145,8 +146,9 @@ private[sbt] class ClassLoaderCache(
     ManagementFactory.getMemoryPoolMXBeans.asScala
       .exists(b => (b.getName == "Metaspace") && (b.getUsage.getMax > 0))
   private[this] val mkReference: (Key, ClassLoader) => Reference[ClassLoader] =
-    if (metaspaceIsLimited)(_, cl) => new SoftReference(cl, referenceQueue)
-    else ClassLoaderReference.apply
+    if (metaspaceIsLimited) { (_, cl) =>
+      (new SoftReference[ClassLoader](cl, referenceQueue): Reference[ClassLoader])
+    } else ClassLoaderReference.apply
   private[this] val cleanupThread = new CleanupThread(ClassLoaderCache.threadID.getAndIncrement())
   private[this] val lock = new Object
 
diff --git a/main-command/src/main/scala/sbt/internal/client/NetworkClient.scala b/main-command/src/main/scala/sbt/internal/client/NetworkClient.scala
index cedc1aaed..2419f4747 100644
--- a/main-command/src/main/scala/sbt/internal/client/NetworkClient.scala
+++ b/main-command/src/main/scala/sbt/internal/client/NetworkClient.scala
@@ -123,6 +123,7 @@ class NetworkClient(configuration: xsbti.AppConfiguration, arguments: List[Strin
         lock.synchronized {
           pendingExecIds -= execId
         }
+        ()
       case _ =>
     }
   }
diff --git a/main-command/src/main/scala/sbt/internal/client/ServerConnection.scala b/main-command/src/main/scala/sbt/internal/client/ServerConnection.scala
index e8acb0b69..ebde66bd1 100644
--- a/main-command/src/main/scala/sbt/internal/client/ServerConnection.scala
+++ b/main-command/src/main/scala/sbt/internal/client/ServerConnection.scala
@@ -29,7 +29,7 @@ abstract class ServerConnection(connection: Socket) {
         val in = connection.getInputStream
         connection.setSoTimeout(5000)
         var buffer: Vector[Byte] = Vector.empty
-        def readFrame: Array[Byte] = {
+        def readFrame: Vector[Byte] = {
           def getContentLength: Int = {
             readLine.drop(16).toInt
           }
@@ -56,17 +56,17 @@ abstract class ServerConnection(connection: Socket) {
           } else readLine
         }
 
-        def readContentLength(length: Int): Array[Byte] = {
+        def readContentLength(length: Int): Vector[Byte] = {
           if (buffer.size < length) {
             val bytesRead = in.read(readBuffer)
             if (bytesRead > 0) {
               buffer = buffer ++ readBuffer.toVector.take(bytesRead)
-            }
-          }
+            } else ()
+          } else ()
           if (length <= buffer.size) {
             val chunk = buffer.take(length)
             buffer = buffer.drop(length)
-            chunk.toArray
+            chunk
           } else readContentLength(length)
         }
 
@@ -77,7 +77,7 @@ abstract class ServerConnection(connection: Socket) {
               .deserializeJsonMessage(frame)
               .fold(
                 { errorDesc =>
-                  val s = new String(frame.toArray, "UTF-8")
+                  val s = frame.mkString("") // new String(: Array[Byte], "UTF-8")
                   println(s"Got invalid chunk from server: $s \n" + errorDesc)
                 },
                 _ match {
diff --git a/main-settings/src/main/scala/sbt/Append.scala b/main-settings/src/main/scala/sbt/Append.scala
index 6176fa9b3..68aa27d9b 100644
--- a/main-settings/src/main/scala/sbt/Append.scala
+++ b/main-settings/src/main/scala/sbt/Append.scala
@@ -31,8 +31,8 @@ object Append {
 
   implicit def appendSeq[T, V <: T]: Sequence[Seq[T], Seq[V], V] =
     new Sequence[Seq[T], Seq[V], V] {
-      def appendValues(a: Seq[T], b: Seq[V]): Seq[T] = a ++ b
-      def appendValue(a: Seq[T], b: V): Seq[T] = a :+ b
+      def appendValues(a: Seq[T], b: Seq[V]): Seq[T] = a ++ (b: Seq[T])
+      def appendValue(a: Seq[T], b: V): Seq[T] = a :+ (b: T)
     }
 
   implicit def appendSeqImplicit[T, V](implicit ev: V => T): Sequence[Seq[T], Seq[V], V] =
@@ -50,8 +50,8 @@ object Append {
 
   implicit def appendList[T, V <: T]: Sequence[List[T], List[V], V] =
     new Sequence[List[T], List[V], V] {
-      def appendValues(a: List[T], b: List[V]): List[T] = a ::: b
-      def appendValue(a: List[T], b: V): List[T] = a :+ b
+      def appendValues(a: List[T], b: List[V]): List[T] = a ::: (b: List[T])
+      def appendValue(a: List[T], b: V): List[T] = a :+ (b: T)
     }
 
   implicit def appendListImplicit[T, V](implicit ev: V => T): Sequence[List[T], List[V], V] =
@@ -80,14 +80,15 @@ object Append {
 
   implicit def appendSet[T, V <: T]: Sequence[Set[T], Set[V], V] =
     new Sequence[Set[T], Set[V], V] {
-      def appendValues(a: Set[T], b: Set[V]): Set[T] = a ++ b
-      def appendValue(a: Set[T], b: V): Set[T] = a + b
+      def appendValues(a: Set[T], b: Set[V]): Set[T] = a ++ (b.toSeq: Seq[T]).toSet
+      def appendValue(a: Set[T], b: V): Set[T] = a + (b: T)
     }
 
   implicit def appendMap[A, B, X <: A, Y <: B]: Sequence[Map[A, B], Map[X, Y], (X, Y)] =
     new Sequence[Map[A, B], Map[X, Y], (X, Y)] {
-      def appendValues(a: Map[A, B], b: Map[X, Y]): Map[A, B] = a ++ b
-      def appendValue(a: Map[A, B], b: (X, Y)): Map[A, B] = a + b
+      def appendValues(a: Map[A, B], b: Map[X, Y]): Map[A, B] =
+        (a.toSeq ++ (b.toSeq: Seq[(A, B)])).toMap
+      def appendValue(a: Map[A, B], b: (X, Y)): Map[A, B] = a + (b: (A, B))
     }
 
   implicit def appendOption[T]: Sequence[Seq[T], Option[T], Option[T]] =
diff --git a/main-settings/src/main/scala/sbt/Def.scala b/main-settings/src/main/scala/sbt/Def.scala
index 7db51382a..95988ebcc 100644
--- a/main-settings/src/main/scala/sbt/Def.scala
+++ b/main-settings/src/main/scala/sbt/Def.scala
@@ -15,6 +15,7 @@ import sbt.Scope.{ GlobalScope, ThisScope }
 import sbt.internal.util.Types.const
 import sbt.internal.util.complete.Parser
 import sbt.internal.util._
+import Util._
 import sbt.util.Show
 
 /** A concrete settings system that uses `sbt.Scope` for the scope type. */
@@ -173,8 +174,8 @@ object Def extends Init[Scope] with TaskMacroExtra {
   override def deriveAllowed[T](s: Setting[T], allowDynamic: Boolean): Option[String] =
     super.deriveAllowed(s, allowDynamic) orElse
       (if (s.key.scope != ThisScope)
-         Some(s"Scope cannot be defined for ${definedSettingString(s)}")
-       else None) orElse
+         s"Scope cannot be defined for ${definedSettingString(s)}".some
+       else none) orElse
       s.dependencies
         .find(k => k.scope != ThisScope)
         .map(
diff --git a/main-settings/src/main/scala/sbt/DelegateIndex.scala b/main-settings/src/main/scala/sbt/DelegateIndex.scala
index 7c5967790..e187ce931 100644
--- a/main-settings/src/main/scala/sbt/DelegateIndex.scala
+++ b/main-settings/src/main/scala/sbt/DelegateIndex.scala
@@ -21,9 +21,10 @@ private final class DelegateIndex0(refs: Map[ProjectRef, ProjectDelegates]) exte
     refs.get(ref) match {
       case Some(pd) =>
         pd.confs.get(conf) match {
-          case Some(cs) => cs; case None => Select(conf) :: Zero :: Nil
+          case Some(cs) => cs
+          case None     => (Select(conf): ScopeAxis[ConfigKey]) :: (Zero: ScopeAxis[ConfigKey]) :: Nil
         }
-      case None => Select(conf) :: Zero :: Nil
+      case None => (Select(conf): ScopeAxis[ConfigKey]) :: (Zero: ScopeAxis[ConfigKey]) :: Nil
     }
 }
 private final class ProjectDelegates(
diff --git a/main-settings/src/main/scala/sbt/InputTask.scala b/main-settings/src/main/scala/sbt/InputTask.scala
index 3a43877bf..faea5bd00 100644
--- a/main-settings/src/main/scala/sbt/InputTask.scala
+++ b/main-settings/src/main/scala/sbt/InputTask.scala
@@ -12,6 +12,7 @@ import Def.{ Initialize, ScopedKey }
 import std.TaskExtra._
 import sbt.internal.util.{ ~>, AttributeKey, Types }
 import sbt.internal.util.Types._
+import sbt.internal.util.Util._
 
 /** Parses input and produces a task to run.  Constructed using the companion object. */
 final class InputTask[T] private (val parser: State => Parser[Task[T]]) {
@@ -109,7 +110,7 @@ object InputTask {
 
   /** Implementation detail that is public because it is used by a macro.*/
   def initParserAsInput[T](i: Initialize[Parser[T]]): Initialize[State => Parser[T]] =
-    i(Types.const)
+    i(Types.const[State, Parser[T]])
 
   @deprecated("Use another InputTask constructor or the `Def.inputTask` macro.", "0.13.0")
   def apply[I, T](
@@ -147,7 +148,7 @@ object InputTask {
     val key = localKey[Option[I]]
     val f: () => I = () =>
       sys.error(s"Internal sbt error: InputTask stub was not substituted properly.")
-    val t: Task[I] = Task(Info[I]().set(key, None), Pure(f, false))
+    val t: Task[I] = Task(Info[I]().set(key, none), Pure(f, false))
     (key, t)
   }
 
@@ -163,7 +164,7 @@ object InputTask {
         if (t0 == null) {
           val newAction =
             if (t.info.get(marker).isDefined)
-              Pure(() => value.asInstanceOf[A], inline = true)
+              (Pure[A](() => value.asInstanceOf[A], inline = true): Action[A])
             else
               t.work.mapTask(f)
           val newTask = Task(t.info, newAction)
diff --git a/main-settings/src/main/scala/sbt/Previous.scala b/main-settings/src/main/scala/sbt/Previous.scala
index 91340613f..aaa61c320 100644
--- a/main-settings/src/main/scala/sbt/Previous.scala
+++ b/main-settings/src/main/scala/sbt/Previous.scala
@@ -77,7 +77,7 @@ object Previous {
     }
     override def hashCode(): Int = (task.## * 31) ^ enclosing.##
     def cacheKey: AnyTaskKey = {
-      if (task == enclosing) task
+      if (task == enclosing) task.asInstanceOf[ScopedKey[Task[Any]]]
       else {
         val am = enclosing.scope.extra match {
           case Select(a) => a.put(scopedKeyAttribute, task.asInstanceOf[AnyTaskKey])
diff --git a/main-settings/src/main/scala/sbt/Remove.scala b/main-settings/src/main/scala/sbt/Remove.scala
index 8a4fbdaed..e09555f5b 100644
--- a/main-settings/src/main/scala/sbt/Remove.scala
+++ b/main-settings/src/main/scala/sbt/Remove.scala
@@ -27,7 +27,7 @@ object Remove {
   implicit def removeSeq[T, V <: T]: Sequence[Seq[T], Seq[V], V] =
     new Sequence[Seq[T], Seq[V], V] {
       def removeValue(a: Seq[T], b: V): Seq[T] = a filterNot b.==
-      def removeValues(a: Seq[T], b: Seq[V]): Seq[T] = a diff b
+      def removeValues(a: Seq[T], b: Seq[V]): Seq[T] = a diff (b: Seq[T])
     }
   implicit def removeOption[T]: Sequence[Seq[T], Option[T], Option[T]] =
     new Sequence[Seq[T], Option[T], Option[T]] {
diff --git a/main-settings/src/main/scala/sbt/Scope.scala b/main-settings/src/main/scala/sbt/Scope.scala
index fe8f261f3..28c3daa3d 100644
--- a/main-settings/src/main/scala/sbt/Scope.scala
+++ b/main-settings/src/main/scala/sbt/Scope.scala
@@ -10,6 +10,7 @@ package sbt
 import java.net.URI
 
 import sbt.internal.util.{ AttributeKey, AttributeMap, Dag }
+import sbt.internal.util.Util._
 
 import sbt.io.IO
 
@@ -213,7 +214,7 @@ object Scope {
       val zeroConfig = if (showZeroConfig) "Zero /" else ""
       val configPrefix = config.foldStrict(display, zeroConfig, "./")
       val taskPrefix = task.foldStrict(_.label + " /", "", "./")
-      val extras = extra.foldStrict(_.entries.map(_.toString).toList, Nil, Nil)
+      val extras = extra.foldStrict(_.entries.map(_.toString).toList, nil, nil)
       val postfix = if (extras.isEmpty) "" else extras.mkString("(", ", ", ")")
       if (scope == GlobalScope) "Global / " + sep + postfix
       else
@@ -346,6 +347,7 @@ object Scope {
             val t = tLinIt.next()
             if (scope.extra.isSelect) {
               res += Scope(px, c, t, scope.extra)
+              ()
             }
             res += Scope(px, c, t, Zero)
           }
@@ -361,7 +363,8 @@ object Scope {
         val projAxes: Seq[ScopeAxis[ResolvedReference]] =
           resolvedProj match {
             case pr: ProjectRef => index.project(pr)
-            case br: BuildRef   => List(Select(br), Zero)
+            case br: BuildRef =>
+              List(Select(br): ScopeAxis[ResolvedReference], Zero: ScopeAxis[ResolvedReference])
           }
         expandDelegateScopes(resolvedProj)(projAxes)
     }
@@ -369,13 +372,15 @@ object Scope {
 
   private val zeroL = List(Zero)
   def withZeroAxis[T](base: ScopeAxis[T]): Seq[ScopeAxis[T]] =
-    if (base.isSelect) List(base, Zero)
+    if (base.isSelect) List(base, Zero: ScopeAxis[T])
     else zeroL
 
   def withGlobalScope(base: Scope): Seq[Scope] =
     if (base == GlobalScope) GlobalScope :: Nil else base :: GlobalScope :: Nil
   def withRawBuilds(ps: Seq[ScopeAxis[ProjectRef]]): Seq[ScopeAxis[ResolvedReference]] =
-    ps ++ (ps flatMap rawBuild).distinct :+ Zero
+    (ps: Seq[ScopeAxis[ResolvedReference]]) ++
+      ((ps flatMap rawBuild).distinct: Seq[ScopeAxis[ResolvedReference]]) :+
+      (Zero: ScopeAxis[ResolvedReference])
 
   def rawBuild(ps: ScopeAxis[ProjectRef]): Seq[ScopeAxis[BuildRef]] = ps match {
     case Select(ref) => Select(BuildRef(ref.build)) :: Nil; case _ => Nil
@@ -421,8 +426,8 @@ object Scope {
   def topologicalSort[T](node: T, appendZero: Boolean)(
       dependencies: T => Seq[T]
   ): Seq[ScopeAxis[T]] = {
-    val o = Dag.topologicalSortUnchecked(node)(dependencies).map(Select.apply)
-    if (appendZero) o ::: Zero :: Nil
+    val o = Dag.topologicalSortUnchecked(node)(dependencies).map(x => Select(x): ScopeAxis[T])
+    if (appendZero) o ::: (Zero: ScopeAxis[T]) :: Nil
     else o
   }
   def globalProjectDelegates(scope: Scope): Seq[Scope] =
diff --git a/main-settings/src/main/scala/sbt/ScopeAxis.scala b/main-settings/src/main/scala/sbt/ScopeAxis.scala
index 11cfc8389..6b494a73b 100644
--- a/main-settings/src/main/scala/sbt/ScopeAxis.scala
+++ b/main-settings/src/main/scala/sbt/ScopeAxis.scala
@@ -7,7 +7,7 @@
 
 package sbt
 
-import sbt.internal.util.Types.some
+import sbt.internal.util.Util._
 
 sealed trait ScopeAxis[+S] {
   def foldStrict[T](f: S => T, ifZero: T, ifThis: T): T = fold(f, ifZero, ifThis)
@@ -16,8 +16,9 @@ sealed trait ScopeAxis[+S] {
     case Zero      => ifZero
     case Select(s) => f(s)
   }
-  def toOption: Option[S] = foldStrict(some.fn, None, None)
-  def map[T](f: S => T): ScopeAxis[T] = foldStrict(s => Select(f(s)), Zero, This)
+  def toOption: Option[S] = foldStrict(Option(_), none, none)
+  def map[T](f: S => T): ScopeAxis[T] =
+    foldStrict(s => Select(f(s)): ScopeAxis[T], Zero: ScopeAxis[T], This: ScopeAxis[T])
   def isSelect: Boolean = false
 }
 
diff --git a/main-settings/src/main/scala/sbt/ScopeMask.scala b/main-settings/src/main/scala/sbt/ScopeMask.scala
index dedec0c00..056fdc385 100644
--- a/main-settings/src/main/scala/sbt/ScopeMask.scala
+++ b/main-settings/src/main/scala/sbt/ScopeMask.scala
@@ -16,11 +16,19 @@ final case class ScopeMask(
 ) {
   def concatShow(p: String, c: String, t: String, sep: String, x: String): String = {
     val sb = new StringBuilder
-    if (project) sb.append(p)
-    if (config) sb.append(c)
-    if (task) sb.append(t)
+    if (project) {
+      sb.append(p); ()
+    }
+    if (config) {
+      sb.append(c); ()
+    }
+    if (task) {
+      sb.append(t); ()
+    }
     sb.append(sep)
-    if (extra) sb.append(x)
+    if (extra) {
+      sb.append(x); ()
+    }
     sb.toString
   }
 }
diff --git a/protocol/src/main/scala/sbt/protocol/ClientSocket.scala b/protocol/src/main/scala/sbt/protocol/ClientSocket.scala
index bc05fe58f..d22905a08 100644
--- a/protocol/src/main/scala/sbt/protocol/ClientSocket.scala
+++ b/protocol/src/main/scala/sbt/protocol/ClientSocket.scala
@@ -35,8 +35,8 @@ object ClientSocket {
     }
     val sk = uri.getScheme match {
       case "local" if isWindows =>
-        new Win32NamedPipeSocket("""\\.\pipe\""" + uri.getSchemeSpecificPart)
-      case "local" => new UnixDomainSocket(uri.getSchemeSpecificPart)
+        (new Win32NamedPipeSocket("""\\.\pipe\""" + uri.getSchemeSpecificPart): Socket)
+      case "local" => (new UnixDomainSocket(uri.getSchemeSpecificPart): Socket)
       case "tcp"   => new Socket(InetAddress.getByName(uri.getHost), uri.getPort)
       case _       => sys.error(s"Unsupported uri: $uri")
     }
diff --git a/run/src/main/scala/sbt/Fork.scala b/run/src/main/scala/sbt/Fork.scala
index 734e8609e..9745c28a6 100644
--- a/run/src/main/scala/sbt/Fork.scala
+++ b/run/src/main/scala/sbt/Fork.scala
@@ -13,6 +13,7 @@ import java.lang.ProcessBuilder.Redirect
 import scala.sys.process.Process
 import OutputStrategy._
 import sbt.internal.util.Util
+import Util.{ AnyOps, none }
 
 import java.lang.{ ProcessBuilder => JProcessBuilder }
 
@@ -52,11 +53,13 @@ final class Fork(val commandName: String, val runnerClass: Option[String]) {
     val jpb = new JProcessBuilder(command.toArray: _*)
     workingDirectory foreach (jpb directory _)
     environment foreach { case (k, v) => jpb.environment.put(k, v) }
-    if (connectInput)
+    if (connectInput) {
       jpb.redirectInput(Redirect.INHERIT)
+      ()
+    }
     val process = Process(jpb)
 
-    outputStrategy.getOrElse(StdoutOutput) match {
+    outputStrategy.getOrElse(StdoutOutput: OutputStrategy) match {
       case StdoutOutput => process.run(connectInput = false)
       case out: BufferedOutput =>
         out.logger.buffer { process.run(out.logger, connectInput = false) }
@@ -70,9 +73,9 @@ final class Fork(val commandName: String, val runnerClass: Option[String]) {
       arguments: Seq[String]
   ): Seq[String] = {
     val boot =
-      if (bootJars.isEmpty) None
+      if (bootJars.isEmpty) none[String]
       else
-        Some("-Xbootclasspath/a:" + bootJars.map(_.getAbsolutePath).mkString(File.pathSeparator))
+        ("-Xbootclasspath/a:" + bootJars.map(_.getAbsolutePath).mkString(File.pathSeparator)).some
     jvmOptions ++ boot.toList ++ runnerClass.toList ++ arguments
   }
 }
diff --git a/run/src/main/scala/sbt/SelectMainClass.scala b/run/src/main/scala/sbt/SelectMainClass.scala
index 03774ccd1..7de45e9ca 100644
--- a/run/src/main/scala/sbt/SelectMainClass.scala
+++ b/run/src/main/scala/sbt/SelectMainClass.scala
@@ -7,6 +7,8 @@
 
 package sbt
 
+import sbt.internal.util.Util.{ AnyOps, none }
+
 object SelectMainClass {
   // Some(SimpleReader.readLine _)
   def apply(
@@ -28,18 +30,18 @@ object SelectMainClass {
     }
   }
   private def trim(s: Option[String]) = s.getOrElse("")
-  private def toInt(s: String, size: Int) =
+  private def toInt(s: String, size: Int): Option[Int] =
     try {
       val i = s.toInt
       if (i > 0 && i <= size)
-        Some(i - 1)
+        (i - 1).some
       else {
         println("Number out of range: was " + i + ", expected number between 1 and " + size)
-        None
+        none
       }
     } catch {
       case nfe: NumberFormatException =>
         println("Invalid number: " + nfe.toString)
-        None
+        none
     }
 }
diff --git a/run/src/main/scala/sbt/TrapExit.scala b/run/src/main/scala/sbt/TrapExit.scala
index ea41e7259..5c80e1be0 100644
--- a/run/src/main/scala/sbt/TrapExit.scala
+++ b/run/src/main/scala/sbt/TrapExit.scala
@@ -18,6 +18,7 @@ import java.util.function.Supplier
 
 import sbt.util.Logger
 import sbt.util.InterfaceUtil
+import sbt.internal.util.Util.{ AnyOps, none }
 import TrapExit._
 
 /**
@@ -294,8 +295,8 @@ private final class TrapExit(delegateManager: SecurityManager) extends SecurityM
     private[this] def setExceptionHandler(t: Thread): Unit = {
       val group = t.getThreadGroup
       val previousHandler = t.getUncaughtExceptionHandler match {
-        case null | `group` | (_: LoggingExceptionHandler) => None
-        case x                                             => Some(x) // delegate to a custom handler only
+        case null | `group` | (_: LoggingExceptionHandler) => none[Thread.UncaughtExceptionHandler]
+        case x                                             => x.some // delegate to a custom handler only
       }
       t.setUncaughtExceptionHandler(new LoggingExceptionHandler(log, previousHandler))
     }
diff --git a/run/src/test/scala/sbt/ForkTest.scala b/run/src/test/scala/sbt/ForkTest.scala
index eed1b4b98..a3a66d47b 100644
--- a/run/src/test/scala/sbt/ForkTest.scala
+++ b/run/src/test/scala/sbt/ForkTest.scala
@@ -15,6 +15,7 @@ import java.io.File
 import sbt.internal.TestLogger
 import sbt.io.{ IO, Path }
 import OutputStrategy._
+import sbt.internal.util.Util._
 
 object ForkTest extends Properties("Fork") {
 
@@ -25,7 +26,7 @@ object ForkTest extends Properties("Fork") {
    */
   final val MaximumClasspathLength = 100000
 
-  lazy val genOptionName = frequency((9, Some("-cp")), (9, Some("-classpath")), (1, None))
+  lazy val genOptionName = frequency((9, "-cp".some), (9, "-classpath".some), (1, none))
   lazy val pathElement = nonEmptyListOf(alphaNumChar).map(_.mkString)
   lazy val path = nonEmptyListOf(pathElement).map(_.mkString(File.separator))
   lazy val genRelClasspath = nonEmptyListOf(path)
diff --git a/tasks-standard/src/main/scala/sbt/Action.scala b/tasks-standard/src/main/scala/sbt/Action.scala
index 914bb6701..9be95676e 100644
--- a/tasks-standard/src/main/scala/sbt/Action.scala
+++ b/tasks-standard/src/main/scala/sbt/Action.scala
@@ -63,9 +63,14 @@ final case class Task[T](info: Info[T], work: Action[T]) {
   override def hashCode = info.hashCode
 
   def tag(tags: Tag*): Task[T] = tagw(tags.map(t => (t, 1)): _*)
-  def tagw(tags: (Tag, Int)*): Task[T] =
-    copy(info = info.set(tagsKey, info.get(tagsKey).getOrElse(Map.empty) ++ tags))
-  def tags: TagMap = info get tagsKey getOrElse Map.empty
+  def tagw(tags: (Tag, Int)*): Task[T] = {
+    val tgs: TagMap = info.get(tagsKey).getOrElse(TagMap.empty)
+    val value = tags.foldLeft(tgs)((acc, tag) => acc + tag)
+    val nextInfo = info.set(tagsKey, value)
+    copy(info = nextInfo)
+  }
+
+  def tags: TagMap = info get tagsKey getOrElse TagMap.empty
 }
 
 /**
@@ -83,7 +88,7 @@ final case class Info[T](
   def setName(n: String) = set(Name, n)
   def setDescription(d: String) = set(Description, d)
   def set[A](key: AttributeKey[A], value: A) = copy(attributes = this.attributes.put(key, value))
-  def get[A](key: AttributeKey[A]) = attributes.get(key)
+  def get[A](key: AttributeKey[A]): Option[A] = attributes.get(key)
   def postTransform(f: (T, AttributeMap) => AttributeMap) = copy(post = (t: T) => f(t, post(t)))
 
   override def toString = if (attributes.isEmpty) "_" else attributes.toString
diff --git a/tasks-standard/src/main/scala/sbt/std/Streams.scala b/tasks-standard/src/main/scala/sbt/std/Streams.scala
index b71c8567c..c014f7ad9 100644
--- a/tasks-standard/src/main/scala/sbt/std/Streams.scala
+++ b/tasks-standard/src/main/scala/sbt/std/Streams.scala
@@ -13,6 +13,7 @@ import java.util.concurrent.ConcurrentHashMap
 
 import sbt.internal.io.DeferredWriter
 import sbt.internal.util.ManagedLogger
+import sbt.internal.util.Util.{ nil }
 import sbt.io.IO
 import sbt.io.syntax._
 import sbt.util._
@@ -141,7 +142,7 @@ object Streams {
   ): Streams[Key] = new Streams[Key] {
 
     def apply(a: Key): ManagedStreams[Key] = new ManagedStreams[Key] {
-      private[this] var opened: List[Closeable] = Nil
+      private[this] var opened: List[Closeable] = nil
       private[this] var closed = false
 
       def getInput(a: Key, sid: String = default): Input =
@@ -199,7 +200,7 @@ object Streams {
           ()
         }
         val t = f(file)
-        opened ::= t
+        opened ::= (t: Closeable)
         t
       }
 
diff --git a/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala b/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala
index 5eba31888..d3fd9c700 100644
--- a/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala
+++ b/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala
@@ -171,7 +171,8 @@ trait TaskExtra {
           }
       )
     def ||[T >: S](alt: Task[T]): Task[T] = flatMapR {
-      case Value(v) => task(v); case Inc(_) => alt
+      case Value(v) => task(v: T)
+      case Inc(_)   => alt
     }
     def &&[T](alt: Task[T]): Task[T] = flatMap(_ => alt)
   }
diff --git a/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala b/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
index 588ceefbc..e4db63e2b 100644
--- a/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
+++ b/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
@@ -84,6 +84,7 @@ object ConcurrentRestrictions {
   val All = Tag("all")
 
   type TagMap = Map[Tag, Int]
+  val TagMap = Map.empty[Tag, Int]
 
   /**
    * Implements concurrency restrictions on tasks based on Tags.
@@ -129,7 +130,10 @@ object ConcurrentRestrictions {
       warn: String => Unit
   ): (CompletionService[A, R], () => Unit) = {
     val pool = Executors.newCachedThreadPool()
-    (completionService[A, R](pool, tags, warn), () => { pool.shutdownNow(); () })
+    (completionService[A, R](pool, tags, warn), () => {
+      pool.shutdownNow()
+      ()
+    })
   }
 
   /**
@@ -166,26 +170,31 @@ object ConcurrentRestrictions {
         if (tags valid newState) {
           tagState = newState
           submitValid(node, work)
+          ()
         } else {
           if (running == 0) errorAddingToIdle()
           pending.add(new Enqueue(node, work))
+          ()
         }
         ()
       }
-      private[this] def submitValid(node: A, work: () => R) = {
+      private[this] def submitValid(node: A, work: () => R): Unit = {
         running += 1
         val wrappedWork = () =>
           try work()
           finally cleanup(node)
         CompletionService.submit(wrappedWork, jservice)
+        ()
       }
       private[this] def cleanup(node: A): Unit = synchronized {
         running -= 1
         tagState = tags.remove(tagState, node)
-        if (!tags.valid(tagState))
+        if (!tags.valid(tagState)) {
           warn(
             "Invalid restriction: removing a completed node from a valid system must result in a valid system."
           )
+          ()
+        }
         submitValid(new LinkedList)
       }
       private[this] def errorAddingToIdle() =
@@ -205,8 +214,11 @@ object ConcurrentRestrictions {
           if (tags.valid(newState)) {
             tagState = newState
             submitValid(next.node, next.work)
-          } else
+            ()
+          } else {
             tried.add(next)
+            ()
+          }
           submitValid(tried)
         }
 
diff --git a/tasks/src/main/scala/sbt/Execute.scala b/tasks/src/main/scala/sbt/Execute.scala
index 627aa16cd..7057d0f6d 100644
--- a/tasks/src/main/scala/sbt/Execute.scala
+++ b/tasks/src/main/scala/sbt/Execute.scala
@@ -12,6 +12,7 @@ import java.util.concurrent.ExecutionException
 import sbt.internal.util.ErrorHandling.wideConvert
 import sbt.internal.util.{ DelegatingPMap, IDSet, PMap, RMap, ~> }
 import sbt.internal.util.Types._
+import sbt.internal.util.Util.nilSeq
 import Execute._
 
 import scala.annotation.tailrec
@@ -167,7 +168,7 @@ private[sbt] final class Execute[F[_] <: AnyRef](
     results(node) = result
     state(node) = Done
     progress.afterCompleted(node, result)
-    remove(reverse, node) foreach { dep =>
+    remove(reverse.asInstanceOf[Map[F[A], Iterable[F[_]]]], node) foreach { dep =>
       notifyDone(node, dep)
     }
     callers.remove(node).toList.flatten.foreach { c =>
@@ -196,7 +197,7 @@ private[sbt] final class Execute[F[_] <: AnyRef](
     val f = forward(dependent)
     f -= node
     if (f.isEmpty) {
-      remove(forward, dependent)
+      remove[F[_], IDSet[F[_]]](forward, dependent)
       ready(dependent)
     }
   }
@@ -323,7 +324,7 @@ private[sbt] final class Execute[F[_] <: AnyRef](
   def runBefore(node: F[_]): Seq[F[_]] = getSeq(triggers.runBefore, node)
   def triggeredBy(node: F[_]): Seq[F[_]] = getSeq(triggers.injectFor, node)
   def getSeq(map: collection.Map[F[_], Seq[F[_]]], node: F[_]): Seq[F[_]] =
-    map.getOrElse(node, Nil)
+    map.getOrElse(node, nilSeq[F[_]])
 
   // Contracts
 
diff --git a/testing/src/main/scala/sbt/TestReportListener.scala b/testing/src/main/scala/sbt/TestReportListener.scala
index d33467a73..08df6c415 100644
--- a/testing/src/main/scala/sbt/TestReportListener.scala
+++ b/testing/src/main/scala/sbt/TestReportListener.scala
@@ -77,10 +77,10 @@ final class SuiteResult(
   def +(other: SuiteResult): SuiteResult = {
     val combinedTestResult =
       (result, other.result) match {
-        case (TestResult.Passed, TestResult.Passed) => TestResult.Passed
-        case (_, TestResult.Error)                  => TestResult.Error
-        case (TestResult.Error, _)                  => TestResult.Error
-        case _                                      => TestResult.Failed
+        case (TestResult.Passed, TestResult.Passed) => TestResult.Passed: TestResult
+        case (_, TestResult.Error)                  => TestResult.Error: TestResult
+        case (TestResult.Error, _)                  => TestResult.Error: TestResult
+        case _                                      => TestResult.Failed: TestResult
       }
     new SuiteResult(
       combinedTestResult,
diff --git a/testing/src/main/scala/sbt/internal/testing/TestLogger.scala b/testing/src/main/scala/sbt/internal/testing/TestLogger.scala
index 0b7f5e2ba..62da7f6d8 100644
--- a/testing/src/main/scala/sbt/internal/testing/TestLogger.scala
+++ b/testing/src/main/scala/sbt/internal/testing/TestLogger.scala
@@ -93,16 +93,19 @@ object TestLogger {
     }
 
   private[sbt] def toTestItemEvent(event: TestEvent): TestItemEvent =
-    TestItemEvent(event.result, event.detail.toVector map { d =>
-      TestItemDetail(
-        d.fullyQualifiedName,
-        d.status,
-        d.duration match {
-          case -1 => None
-          case x  => Some(x)
-        }
-      )
-    })
+    TestItemEvent(
+      event.result,
+      event.detail.toVector map { d =>
+        TestItemDetail(
+          d.fullyQualifiedName,
+          d.status,
+          d.duration match {
+            case -1 => (None: Option[Long]) // util.Util is not in classpath
+            case x  => (Some(x): Option[Long])
+          }
+        )
+      }
+    )
 }
 final class TestLogging(
     val global: TLogger,
@@ -129,8 +132,9 @@ class TestLogger(val logging: TestLogging) extends TestsListener {
     log.error(s"Could not run test $name: $t")
     managed.logEvent(
       Level.Info,
-      EndTestGroupErrorEvent(name, (t.getMessage +: t.getStackTrace).mkString("\n"))
+      EndTestGroupErrorEvent(name, (t.getMessage + t.getStackTrace.toString).mkString("\n"))
     )
+    ()
   }
 
   def doComplete(finalResult: TestResult): Unit =
diff --git a/zinc-lm-integration/src/test/scala/sbt/internal/inc/IvyBridgeProviderSpecification.scala b/zinc-lm-integration/src/test/scala/sbt/internal/inc/IvyBridgeProviderSpecification.scala
index 14e3a0640..75d48496e 100644
--- a/zinc-lm-integration/src/test/scala/sbt/internal/inc/IvyBridgeProviderSpecification.scala
+++ b/zinc-lm-integration/src/test/scala/sbt/internal/inc/IvyBridgeProviderSpecification.scala
@@ -30,12 +30,12 @@ abstract class IvyBridgeProviderSpecification extends FlatSpec with Matchers {
   def secondaryCacheDirectory: File = file("target").getAbsoluteFile / "zinc-components"
 
   val resolvers = Array(
-    ZincComponentCompiler.LocalResolver,
-    Resolver.mavenCentral,
+    ZincComponentCompiler.LocalResolver: Resolver,
+    Resolver.mavenCentral: Resolver,
     MavenRepository(
       "scala-integration",
       "https://scala-ci.typesafe.com/artifactory/scala-integration/"
-    ),
+    ): Resolver,
   )
 
   private def ivyConfiguration(log: Logger) =