diff --git a/.mill-jvm-opts b/.mill-jvm-opts
new file mode 100644
index 000000000..aa011f683
--- /dev/null
+++ b/.mill-jvm-opts
@@ -0,0 +1 @@
+-Xss10m
diff --git a/bench/src-jvm/Main.scala b/bench/src-jvm/Main.scala
index 43aa16bc1..e9a4e3419 100644
--- a/bench/src-jvm/Main.scala
+++ b/bench/src-jvm/Main.scala
@@ -2,6 +2,7 @@ package upickle
 
 import upack.MsgPackWriter
 
+import scala.annotation.nowarn
 
 object Main{
   import ADTs.ADT0
@@ -53,6 +54,7 @@ object Main{
       println()
     }
   }
+  @nowarn("cat=deprecation")
   def benchParsingRendering(duration: Int, bytes: Boolean, strings: Boolean, streams: Boolean, msgpack: Boolean) = {
     import java.nio.file.{Files, Paths}
     import collection.JavaConverters._
@@ -126,7 +128,7 @@ object Main{
 
   def circeJsonAst(duration: Int) = {
     Common.bench0[String, io.circe.Json](duration, Common.benchmarkSampleJson)(
-      io.circe.parser.parse(_).right.get,
+      io.circe.parser.parse(_).getOrElse(???),
       _.toString()
     )
   }
@@ -139,7 +141,7 @@ object Main{
 
   def argonautJsonAst(duration: Int) = {
     Common.bench0[String, argonaut.Json](duration, Common.benchmarkSampleJson)(
-      argonaut.Parse.parse(_).right.get,
+      argonaut.Parse.parse(_).getOrElse(???),
       _.toString()
     )
   }
diff --git a/bench/src/NonNative.scala b/bench/src/NonNative.scala
index 32b1299da..0181db0d6 100644
--- a/bench/src/NonNative.scala
+++ b/bench/src/NonNative.scala
@@ -32,7 +32,7 @@ object NonNative{
     implicit def _w9: Encoder[ADT0] = deriveEncoder
 
     Common.bench[String](duration)(
-      decode[Seq[Data]](_).right.get,
+      decode[Seq[Data]](_).getOrElse(???),
       implicitly[Encoder[Seq[Data]]].apply(_).toString()
     )
 
@@ -92,7 +92,7 @@ object NonNative{
     implicit lazy val _w9: Encoder[ADT0] = deriveEncoder
 
     Common.bench[String](duration)(
-      decode[Seq[Data]](_).right.get,
+      decode[Seq[Data]](_).getOrElse(???),
       implicitly[Encoder[Seq[Data]]].apply(_).toString()
     )
   }
diff --git a/build.sc b/build.sc
index 65d64fc07..0642d5243 100644
--- a/build.sc
+++ b/build.sc
@@ -32,12 +32,7 @@ val scalaJVMVersions = scala2JVMVersions ++ Seq(scala3) ++ dottyCustomVersion
 val scalaJSVersions = scalaJVMVersions.map((_, scalaJS))
 val scalaNativeVersions = scalaJVMVersions.map((_, scalaNative))
 
-trait CommonModule extends ScalaModule {
-  override def scalacOptions = T{
-    super.scalacOptions() ++ {
-      if (scalaVersion() == scala212) Seq("-opt:l:method") else Nil
-    }
-  }
+trait CommonBaseModule extends ScalaModule {
   def platformSegment: String
 
   override def sources = T.sources{
@@ -54,6 +49,19 @@ trait CommonModule extends ScalaModule {
   }
 }
 
+trait CommonModule extends CommonBaseModule {
+  override def scalacOptions = T{
+    super.scalacOptions() ++ {
+      if (scalaVersion() == scala212) Seq("-opt:l:method") else Nil
+    } ++ Seq(
+      "-unchecked",
+      "-deprecation",
+      "-encoding", "utf8",
+      "-feature",
+      "-Xfatal-warnings"
+    )
+  }
+}
 
 trait CommonPublishModule extends CommonModule with PublishModule with Mima with CrossScalaModule {
 
@@ -100,7 +108,7 @@ trait CommonPublishModule extends CommonModule with PublishModule with Mima with
   }
 }
 
-trait CommonTestModule extends CommonModule with TestModule.Utest{
+trait CommonTestModule extends CommonBaseModule with TestModule.Utest{
   override def ivyDeps = Agg(ivy"com.lihaoyi::utest::0.8.1") ++ (
     if (isScala3(scalaVersion())) Agg.empty[mill.scalalib.Dep]
     else Agg(ivy"com.lihaoyi:::acyclic:$acyclic")
@@ -111,6 +119,11 @@ trait CommonTestModule extends CommonModule with TestModule.Utest{
     os.makeDir.all(javadocDir)
     mill.modules.Jvm.createJar(Agg(javadocDir))(outDir)
   }
+  override def scalacOptions = super.scalacOptions() ++
+    (if (isScala3(scalaVersion())) Seq(
+      "-Ximplicit-search-limit",
+      "200000"
+    ) else Seq.empty[String])
 }
 
 trait CommonJvmModule extends CommonPublishModule{
@@ -373,12 +386,6 @@ trait UpickleModule extends CommonPublishModule{
     ivy"org.scala-lang:scala-compiler:${scalaVersion()}"
   )
   else Agg.empty[Dep]
-  override def scalacOptions = super.scalacOptions() ++ Seq(
-    "-unchecked",
-    "-deprecation",
-    "-encoding", "utf8",
-    "-feature",
-  )
 }
 
 
@@ -387,7 +394,7 @@ object upickle extends Module{
   class JvmModule(val crossScalaVersion: String) extends UpickleModule with CommonJvmModule{
     override def moduleDeps = Seq(ujson.jvm(), upack.jvm(), implicits.jvm())
 
-    object test extends Tests with CommonModule{
+    object test extends Tests with CommonBaseModule{
       override def moduleDeps = {
         (super.moduleDeps :+ core.jvm().test) ++ (
           if (isDotty) Nil else Seq(
diff --git a/core/src-2.12/upickle/core/compat/package.scala b/core/src-2.12/upickle/core/compat/package.scala
index ccd7db71d..e60f65331 100644
--- a/core/src-2.12/upickle/core/compat/package.scala
+++ b/core/src-2.12/upickle/core/compat/package.scala
@@ -27,6 +27,10 @@ package object compat {
    */
   type Factory[-A, +C] = CanBuildFrom[Nothing, A, C]
 
+  type IterableOnce[T] = TraversableOnce[T]
+
+  def toIterator[T](iterable: IterableOnce[T]): IterableOnce[T] = iterable
+
   implicit class FactoryOps[-A, +C](private val factory: Factory[A, C]) {
 
     /**
diff --git a/core/src-2.13+/upickle/core/compat/package.scala b/core/src-2.13+/upickle/core/compat/package.scala
index d735a4920..3ae3f87f2 100644
--- a/core/src-2.13+/upickle/core/compat/package.scala
+++ b/core/src-2.13+/upickle/core/compat/package.scala
@@ -4,4 +4,6 @@ package object compat {
 
   type Factory[-A, +C] = collection.Factory[A, C]
 
+  def toIterator[T](iterable: IterableOnce[T]) = iterable.iterator
+
 }
diff --git a/core/src/upickle/core/LinkedHashMap.scala b/core/src/upickle/core/LinkedHashMap.scala
index 73ed4797c..40395c8a6 100644
--- a/core/src/upickle/core/LinkedHashMap.scala
+++ b/core/src/upickle/core/LinkedHashMap.scala
@@ -46,9 +46,9 @@ object LinkedHashMap {
   def apply[K, V](): LinkedHashMap[K, V] =
     new LinkedHashMap[K, V](new ju.LinkedHashMap[K, V])
 
-  def apply[K, V](items: TraversableOnce[(K, V)]): LinkedHashMap[K, V] = {
+  def apply[K, V](items: IterableOnce[(K, V)]): LinkedHashMap[K, V] = {
     val map = LinkedHashMap[K, V]()
-    for ((key, value) <- items) {
+    toIterator(items).foreach { case (key, value) =>
       map._put(key, value)
     }
     map
diff --git a/core/src/upickle/core/TraceVisitor.scala b/core/src/upickle/core/TraceVisitor.scala
index 0fee2f9c4..e5ccc5857 100644
--- a/core/src/upickle/core/TraceVisitor.scala
+++ b/core/src/upickle/core/TraceVisitor.scala
@@ -3,7 +3,7 @@ package upickle.core
 import upickle.core.TraceVisitor._
 
 import scala.annotation.tailrec
-import scala.util.control.NoStackTrace
+import scala.util.control.{NonFatal, NoStackTrace}
 
 /**
   * Adds a JSON Path to exceptions thrown by the delegate Visitor.
@@ -19,7 +19,7 @@ object TraceVisitor {
     else{
       val wrapper = new upickle.core.TraceVisitor.Wrapper[T, J]()
       try f(wrapper.visitor(v))
-      catch{case e => throw new upickle.core.TraceVisitor.TraceException(wrapper.lastHasPath.toString, e) }
+      catch{case NonFatal(e) => throw new upickle.core.TraceVisitor.TraceException(wrapper.lastHasPath.toString, e) }
     }
   }
 
@@ -122,7 +122,7 @@ class TraceVisitor[T, J](
         objVisitor.visitEnd(index)
       }
 
-      override def pathComponent: Option[String] = Option(key).map("'" + _.replaceAllLiterally("'", "\\'") + "'")
+      override def pathComponent: Option[String] = Option(key).map("'" + _.replace("'", "\\'") + "'")
 
       override def parent: Option[HasPath] = Some(TraceVisitor.this.parentPath)
     }
diff --git a/implicits/src-2/upickle/implicits/MacroImplicits.scala b/implicits/src-2/upickle/implicits/MacroImplicits.scala
index e33559e65..92ac78f93 100644
--- a/implicits/src-2/upickle/implicits/MacroImplicits.scala
+++ b/implicits/src-2/upickle/implicits/MacroImplicits.scala
@@ -1,6 +1,7 @@
 package upickle.implicits
 
 import language.experimental.macros
+import scala.language.higherKinds
 
 /**
   * Stupid hacks to work around scalac not forwarding macro type params properly
diff --git a/implicits/src-2/upickle/implicits/internal/Macros.scala b/implicits/src-2/upickle/implicits/internal/Macros.scala
index 5423d1675..2a28288c8 100644
--- a/implicits/src-2/upickle/implicits/internal/Macros.scala
+++ b/implicits/src-2/upickle/implicits/internal/Macros.scala
@@ -1,6 +1,6 @@
 package upickle.implicits.internal
 
-import scala.annotation.StaticAnnotation
+import scala.annotation.{nowarn, StaticAnnotation}
 import scala.language.experimental.macros
 import compat._
 import acyclic.file
@@ -15,6 +15,7 @@ import language.existentials
  * directly, since they are called implicitly when trying to read/write
  * types you don't have a Reader/Writer in scope for.
  */
+@nowarn("cat=deprecation")
 object Macros {
 
   trait DeriveDefaults[M[_]] {
diff --git a/implicits/src-3/upickle/implicits/macros.scala b/implicits/src-3/upickle/implicits/macros.scala
index 7e3039c3c..da4fdde64 100644
--- a/implicits/src-3/upickle/implicits/macros.scala
+++ b/implicits/src-3/upickle/implicits/macros.scala
@@ -218,7 +218,7 @@ def defineEnumVisitorsImpl[T0, T <: Tuple](prefix: Expr[Any], macroX: String)(us
 
   def handleType(tpe: TypeRepr, name: String, skipTrait: Boolean): Option[(ValDef, Symbol)] = {
 
-    val AppliedType(typePrefix, List(arg)) = tpe
+    val AppliedType(typePrefix, List(arg)) = tpe: @unchecked
 
     if (skipTrait && arg.typeSymbol.flags.is(Flags.Trait)) None
     else {
@@ -231,7 +231,7 @@ def defineEnumVisitorsImpl[T0, T <: Tuple](prefix: Expr[Any], macroX: String)(us
       )
 
       val macroCall = TypeApply(
-        Select(prefix.asTerm, prefix.asTerm.tpe.typeSymbol.memberMethod(macroX).head),
+        Select(prefix.asTerm, prefix.asTerm.tpe.typeSymbol.methodMember(macroX).head),
         List(TypeTree.of(using arg.asType))
       )
 
diff --git a/implicits/src/upickle/implicits/Readers.scala b/implicits/src/upickle/implicits/Readers.scala
index 9cbbd7fca..03a62bb51 100644
--- a/implicits/src/upickle/implicits/Readers.scala
+++ b/implicits/src/upickle/implicits/Readers.scala
@@ -8,6 +8,7 @@ import upickle.core._
 import upickle.core.compat._
 import scala.collection.mutable
 import scala.concurrent.duration.{Duration, FiniteDuration}
+import scala.language.higherKinds
 import scala.reflect.ClassTag
 
 trait Readers extends upickle.core.Types
@@ -57,8 +58,8 @@ trait Readers extends upickle.core.Types
     override def expectedMsg = "expected number"
     override def visitString(s: CharSequence, index: Int) = visitFloat64String(s.toString, index)
     override def visitInt32(d: Int, index: Int) = d
-    override def visitInt64(d: Long, index: Int) = d
-    override def visitUInt64(d: Long, index: Int) = d
+    override def visitInt64(d: Long, index: Int) = d.toDouble
+    override def visitUInt64(d: Long, index: Int) = d.toDouble
     override def visitFloat32(d: Float, index: Int) = d
     override def visitFloat64(d: Double, index: Int) = d
     override def visitFloat64StringParts(s: CharSequence, decIndex: Int, expIndex: Int, index: Int) = {
diff --git a/implicits/src/upickle/implicits/Writers.scala b/implicits/src/upickle/implicits/Writers.scala
index fe002fe0f..bacce9ccc 100644
--- a/implicits/src/upickle/implicits/Writers.scala
+++ b/implicits/src/upickle/implicits/Writers.scala
@@ -5,6 +5,7 @@ import java.util.UUID
 import upickle.core.{ Visitor, Annotator }
 
 import scala.concurrent.duration.{Duration, FiniteDuration}
+import scala.language.higherKinds
 
 trait Writers extends upickle.core.Types
   with TupleReadWriters
diff --git a/ujson/play/src/ujson/play/PlayJson.scala b/ujson/play/src/ujson/play/PlayJson.scala
index ae71c646a..3a99140e6 100644
--- a/ujson/play/src/ujson/play/PlayJson.scala
+++ b/ujson/play/src/ujson/play/PlayJson.scala
@@ -9,6 +9,8 @@ import scala.collection.mutable.ArrayBuffer
 object PlayJson extends ujson.AstTransformer[JsValue] {
   def transform[T](j: JsValue, f: Visitor[_, T]): T = j match{
     case JsArray(xs) => transformArray(f, xs)
+    case JsFalse => f.visitFalse(-1)
+    case JsTrue => f.visitTrue(-1)
     case JsBoolean(b) => if (b) f.visitTrue(-1) else f.visitFalse(-1)
     case JsNull => f.visitNull(-1)
     case JsNumber(d) => f.visitFloat64String(d.toString, -1)
diff --git a/ujson/src-js/ujson/WebJson.scala b/ujson/src-js/ujson/WebJson.scala
index 4fc0cfcff..d9dc19b1c 100644
--- a/ujson/src-js/ujson/WebJson.scala
+++ b/ujson/src-js/ujson/WebJson.scala
@@ -12,7 +12,7 @@ object WebJson extends ujson.Transformer[js.Any]{
       case true => f.visitTrue(-1)
       case false => f.visitFalse(-1)
       case null => f.visitNull(-1)
-      case s: js.Array[js.Any] =>
+      case s: js.Array[js.Any] @unchecked =>
         val ctx = f.visitArray(-1, -1).narrow
         for(i <- s) ctx.visitValue(transform(i, ctx.subVisitor), -1)
         ctx.visitEnd(-1)
diff --git a/ujson/src/ujson/AstTransformer.scala b/ujson/src/ujson/AstTransformer.scala
index 63d41e7b2..336287f86 100644
--- a/ujson/src/ujson/AstTransformer.scala
+++ b/ujson/src/ujson/AstTransformer.scala
@@ -3,6 +3,8 @@ import upickle.core._
 
 import upickle.core.compat._
 
+import scala.language.higherKinds
+
 trait AstTransformer[I] extends Transformer[I] with JsVisitor[I, I]{
   def apply(t: Readable): I = t.transform(this)
 
@@ -32,7 +34,7 @@ trait AstTransformer[I] extends Transformer[I] with JsVisitor[I, I]{
 
     def visitValue(v: I, index: Int): Unit = vs += (key -> v)
 
-    def visitEnd(index: Int) = build(vs.result)
+    def visitEnd(index: Int) = build(vs.result())
   }
   class AstArrVisitor[T[_]](build: T[I] => I)
                            (implicit factory: Factory[I, T[I]]) extends ArrVisitor[I, I]{
diff --git a/ujson/src/ujson/JsVisitor.scala b/ujson/src/ujson/JsVisitor.scala
index 773ea2a15..f49bed4f7 100644
--- a/ujson/src/ujson/JsVisitor.scala
+++ b/ujson/src/ujson/JsVisitor.scala
@@ -30,11 +30,11 @@ trait JsVisitor[-T, +J] extends Visitor[T, J]{
   def visitInt32(i: Int, index: Int): J = visitFloat64(i, index)
   def visitInt64(i: Long, index: Int): J = {
     if (math.abs(i) > math.pow(2, 53) || i == -9223372036854775808L) visitString(i.toString, index)
-    else visitFloat64(i, index)
+    else visitFloat64(i.toDouble, index)
   }
   def visitUInt64(i: Long, index: Int): J = {
     if (i > math.pow(2, 53) || i < 0) visitString(java.lang.Long.toUnsignedString(i), index)
-    else visitFloat64(i, index)
+    else visitFloat64(i.toDouble, index)
   }
 
   def visitFloat64String(s: String, index: Int): J = {
diff --git a/ujson/src/ujson/Readable.scala b/ujson/src/ujson/Readable.scala
index cd9ccd35e..cbe090e5d 100644
--- a/ujson/src/ujson/Readable.scala
+++ b/ujson/src/ujson/Readable.scala
@@ -3,6 +3,9 @@ package ujson
 import java.nio.ByteBuffer
 import java.nio.channels.ReadableByteChannel
 import upickle.core.{Visitor, ObjArrVisitor}
+
+import scala.language.implicitConversions
+
 trait Readable {
   def transform[T](f: Visitor[_, T]): T
 }
diff --git a/ujson/src/ujson/Value.scala b/ujson/src/ujson/Value.scala
index 29355666b..e812da6cc 100644
--- a/ujson/src/ujson/Value.scala
+++ b/ujson/src/ujson/Value.scala
@@ -3,6 +3,7 @@ package ujson
 import upickle.core.{LinkedHashMap, ObjArrVisitor, ParseUtils, Visitor}
 import upickle.core.compat._
 
+import scala.language.implicitConversions
 import scala.collection.mutable
 
 sealed trait Value extends Readable with geny.Writable{
@@ -139,36 +140,10 @@ object Value extends AstTransformer[Value]{
     }
   }
 
-  @deprecated("use ujson.Str")
-  val Str = ujson.Str
-  @deprecated("use ujson.Str")
-  type Str = ujson.Str
-  @deprecated("use ujson.Obj")
-  val Obj = ujson.Obj
-  @deprecated("use ujson.Obj")
-  type Obj = ujson.Obj
-  @deprecated("use ujson.Arr")
-  val Arr = ujson.Arr
-  @deprecated("use ujson.Arr")
-  type Arr = ujson.Arr
-  @deprecated("use ujson.Num")
-  val Num = ujson.Num
-  @deprecated("use ujson.Num")
-  type Num = ujson.Num
-  @deprecated("use ujson.Bool")
-  val Bool = ujson.Bool
-  @deprecated("use ujson.Bool")
-  type Bool = ujson.Bool
-  @deprecated("use ujson.True")
-  val True = ujson.True
-  @deprecated("use ujson.False")
-  val False = ujson.False
-  @deprecated("use ujson.Null")
-  val Null = ujson.Null
-  implicit def JsonableSeq[T](items: TraversableOnce[T])
-                             (implicit f: T => Value): Arr = Arr.from(items.map(f))
-  implicit def JsonableDict[T](items: TraversableOnce[(String, T)])
-                              (implicit f: T => Value): Obj = Obj.from(items.map(x => (x._1, f(x._2))))
+  implicit def JsonableSeq[T](items: IterableOnce[T])
+                             (implicit f: T => Value): Arr = Arr.from(toIterator(items).map(f))
+  implicit def JsonableDict[T](items: IterableOnce[(String, T)])
+                              (implicit f: T => Value): Obj = Obj.from(toIterator(items).map(x => (x._1, f(x._2))))
   implicit def JsonableBoolean(i: Boolean): Bool = if (i) ujson.True else ujson.False
   implicit def JsonableByte(i: Byte): Num = Num(i)
   implicit def JsonableShort(i: Short): Num = Num(i)
@@ -206,7 +181,7 @@ object Value extends AstTransformer[Value]{
   override def visitFloat64StringParts(s: CharSequence, decIndex: Int, expIndex: Int, index: Int) = {
     ujson.Num(
       if (decIndex != -1 || expIndex != -1) s.toString.toDouble
-      else ParseUtils.parseIntegralNum(s, decIndex, expIndex, index)
+      else ParseUtils.parseIntegralNum(s, decIndex, expIndex, index).toDouble
     )
   }
 
@@ -229,7 +204,7 @@ object Value extends AstTransformer[Value]{
 case class Str(value: String) extends Value
 case class Obj(value: LinkedHashMap[String, Value]) extends Value
 object Obj{
-  implicit def from(items: TraversableOnce[(String, Value)]): Obj = {
+  implicit def from(items: IterableOnce[(String, Value)]): Obj = {
     Obj(LinkedHashMap(items))
   }
 
@@ -249,9 +224,9 @@ object Obj{
 case class Arr(value: mutable.ArrayBuffer[Value]) extends Value
 
 object Arr{
-  implicit def from[T](items: TraversableOnce[T])(implicit conv: T => Value): Arr = {
+  implicit def from[T](items: IterableOnce[T])(implicit conv: T => Value): Arr = {
     val buf = new mutable.ArrayBuffer[Value]()
-    items.foreach{ item =>
+    toIterator(items).foreach{ item =>
       buf += (conv(item): Value)
     }
     Arr(buf)
diff --git a/ujson/templates/ElemParser.scala b/ujson/templates/ElemParser.scala
index d739f7f23..30afcd85a 100644
--- a/ujson/templates/ElemParser.scala
+++ b/ujson/templates/ElemParser.scala
@@ -1,10 +1,11 @@
 package ujson
+
 import java.io.StringWriter
 
 import upickle.core.{Abort, AbortException, ObjArrVisitor, ObjVisitor, Visitor}
+import java.nio.CharBuffer
 import java.nio.charset.Charset
 
-
 import scala.annotation.{switch, tailrec}
 
 /**
@@ -77,7 +78,7 @@ abstract class ElemParser[J] extends upickle.core.BufferingElemParser{
     upickle.core.RenderUtils.escapeElem(
       new upickle.core.CharBuilder(),
       out,
-      new ArrayCharSequence(Array(elemOps.toInt(getElemSafe(i)).toChar)),
+      CharBuffer.wrap(Array(elemOps.toInt(getElemSafe(i)).toChar)),
       unicode = false,
       true
     )
diff --git a/ujson/test/src-2/ujson/Scala2JsonTests.scala b/ujson/test/src-2/ujson/Scala2JsonTests.scala
index 0395059a4..9dc1851f8 100644
--- a/ujson/test/src-2/ujson/Scala2JsonTests.scala
+++ b/ujson/test/src-2/ujson/Scala2JsonTests.scala
@@ -7,14 +7,14 @@ import utest._
 object Scala2JsonTests extends TestSuite {
   val tests = Tests {
     test {
-      val unparsed = """"\\\uCAFE""""
+      val unparsed = """"\\쫾""""
       val fromString = ujson.read(unparsed)
       val fromBytes = ujson.read(unparsed.getBytes)
       assert(fromString == fromBytes)
     }
 
     test {
-      val unparsed = """"\/\\\"\uCAFE\uBABE\uAB98\uFCDE\ubcda\uef4A\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?""""
+      val unparsed = """"\/\\\"쫾몾ꮘﳞ볚\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?""""
       val fromString = ujson.read(unparsed)
       val fromBytes = ujson.read(unparsed.getBytes)
       assert(fromString == fromBytes)
diff --git a/ujson/test/src/ujson/JsonTests.scala b/ujson/test/src/ujson/JsonTests.scala
index ffe8c8e41..827fe0122 100644
--- a/ujson/test/src/ujson/JsonTests.scala
+++ b/ujson/test/src/ujson/JsonTests.scala
@@ -35,7 +35,7 @@ object JsonTests extends TestSuite {
         |        "digit": "0123456789",
         |        "0123456789": "digit",
         |        "special": "`1~!@#$%^&*()_+-={':[,]}|;.</>?",
-        |        "hex": "\u0123\u4567\u89AB\uCDEF\uabcd\uef4A",
+        |        "hex": "ģ䕧覫췯ꯍ",
         |        "true": true,
         |        "false": false,
         |        "null": null,
@@ -51,8 +51,8 @@ object JsonTests extends TestSuite {
         |
         |4 , 5        ,          6           ,7        ],"compact":[1,2,3,4,5,6,7],
         |        "jsontext": "{\"object with 1 member\":[\"array with 1 element\"]}",
-        |        "quotes": "&#34; \u005Cu0022 %22 0x22 034 &#x22;",
-        |        "\/\\\"\uCAFE\uBABE\uAB98\uFCDE\ubcda\uef4A\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?"
+        |        "quotes": "&#34; \""".stripMargin + "\\" + """u0022 %22 0x22 034 &#x22;",
+        |        "\/\\\"쫾몾ꮘﳞ볚\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?"
         |: "A key can be any string"
         |    },
         |    0.5 ,98.6
diff --git a/ujson/test/src/ujson/SmallJsonTests.scala b/ujson/test/src/ujson/SmallJsonTests.scala
index bc5597fd2..11fd25eaa 100644
--- a/ujson/test/src/ujson/SmallJsonTests.scala
+++ b/ujson/test/src/ujson/SmallJsonTests.scala
@@ -49,7 +49,7 @@ object SmallJsonTests extends TestSuite {
           |      "members_50": 4,
           |      "activity_expert_groups": "none",
           |      "other_code_of_conduct": null,
-          |      "head_office_town": "K\u00f8benhavn V"
+          |      "head_office_town": "København V"
           |    }
           |
           |
diff --git a/upack/src/upack/Readable.scala b/upack/src/upack/Readable.scala
index 5364dbf8a..7defafd54 100644
--- a/upack/src/upack/Readable.scala
+++ b/upack/src/upack/Readable.scala
@@ -2,6 +2,8 @@ package upack
 
 import upickle.core.Visitor
 
+import scala.language.implicitConversions
+
 trait Readable {
   def transform[T](f: Visitor[_, T]): T
 }
diff --git a/upickle/test/src/upickle/JsonTests.scala b/upickle/test/src/upickle/JsonTests.scala
index bc7cc6684..7c667e744 100644
--- a/upickle/test/src/upickle/JsonTests.scala
+++ b/upickle/test/src/upickle/JsonTests.scala
@@ -33,7 +33,7 @@ object JsonTests extends TestSuite {
         |        "digit": "0123456789",
         |        "0123456789": "digit",
         |        "special": "`1~!@#$%^&*()_+-={':[,]}|;.</>?",
-        |        "hex": "\u0123\u4567\u89AB\uCDEF\uabcd\uef4A",
+        |        "hex": "ģ䕧覫췯ꯍ",
         |        "true": true,
         |        "false": false,
         |        "null": null,
@@ -49,8 +49,8 @@ object JsonTests extends TestSuite {
         |
         |4 , 5        ,          6           ,7        ],"compact":[1,2,3,4,5,6,7],
         |        "jsontext": "{\"object with 1 member\":[\"array with 1 element\"]}",
-        |        "quotes": "&#34; \u005Cu0022 %22 0x22 034 &#x22;",
-        |        "\/\\\"\uCAFE\uBABE\uAB98\uFCDE\ubcda\uef4A\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?"
+        |        "quotes": "&#34; \""".stripMargin + "\\" + """u0022 %22 0x22 034 &#x22;",
+        |        "\/\\\"쫾몾ꮘﳞ볚\b\f\n\r\t`1~!@#$%^&*()_+-=[]{}|;:',./<>?"
         |: "A key can be any string"
         |    },
         |    0.5 ,98.6
@@ -155,7 +155,7 @@ object JsonTests extends TestSuite {
           |      "members_50": 4,
           |      "activity_expert_groups": "none",
           |      "other_code_of_conduct": null,
-          |      "head_office_town": "K\u00f8benhavn V"
+          |      "head_office_town": "København V"
           |    }
           |
           |
diff --git a/upickle/test/src/upickle/LegacyTests.scala b/upickle/test/src/upickle/LegacyTests.scala
index bf4c057ff..48768294e 100644
--- a/upickle/test/src/upickle/LegacyTests.scala
+++ b/upickle/test/src/upickle/LegacyTests.scala
@@ -37,7 +37,7 @@ object LegacyTests extends TestSuite {
       )
       val chunks = for (i <- 1 to 18) yield {
         val rhs = if (i % 2 == 1) "1" else "\"1\""
-        val lhs = '"' + s"t$i" + '"'
+        val lhs = s""""t$i""""
         s"$lhs:$rhs"
       }
 
diff --git a/upickle/test/src/upickle/MacroTests.scala b/upickle/test/src/upickle/MacroTests.scala
index 8b84290d3..e06a3b14b 100644
--- a/upickle/test/src/upickle/MacroTests.scala
+++ b/upickle/test/src/upickle/MacroTests.scala
@@ -30,7 +30,7 @@ object Custom {
 
   abstract class ThingBaseCompanion[T <: ThingBase](f: (Int, String) => T){
     implicit val thing2Writer: RW[T] = upickle.default.readwriter[String].bimap[T](
-      t => t.i + " " + t.s,
+      t => s"${t.i} ${t.s}",
       str => {
         val Array(i, s) = str.toString.split(" ", 2)
         f(i.toInt, s)
@@ -169,7 +169,7 @@ object MacroTests extends TestSuite {
         )
         val chunks = for (i <- 1 to 18) yield {
           val rhs = if (i % 2 == 1) "1" else "\"1\""
-          val lhs = '"' + s"t$i" + '"'
+          val lhs = s""""t$i""""
           s"$lhs:$rhs"
         }
 
diff --git a/upickle/test/src/upickle/example/ExampleTests.scala b/upickle/test/src/upickle/example/ExampleTests.scala
index 0566afedd..1079fe536 100644
--- a/upickle/test/src/upickle/example/ExampleTests.scala
+++ b/upickle/test/src/upickle/example/ExampleTests.scala
@@ -70,7 +70,7 @@ object Custom2{
   class CustomThing2(val i: Int, val s: String)
   object CustomThing2 {
     implicit val rw: RW[CustomThing2] = upickle.default.readwriter[String].bimap[CustomThing2](
-      x => x.i + " " + x.s,
+      x => s"${x.i} ${x.s}",
       str => {
         val Array(i, s) = str.split(" ", 2)
         new CustomThing2(i.toInt, s)
@@ -293,8 +293,8 @@ object ExampleTests extends TestSuite {
             s.replaceAll("([A-Z])","#$1").split('#').map(_.toLowerCase).mkString("_")
           }
           def snakeToCamel(s: String) = {
-            val res = s.split("_", -1).map(x => x(0).toUpper + x.drop(1)).mkString
-            s(0).toLower + res.drop(1)
+            val res = s.split("_", -1).map(x => s"${x(0).toUpper}${x.drop(1)}").mkString
+            s"${s(0).toLower}${res.drop(1)}"
           }
 
           override def objectAttributeKeyReadMap(s: CharSequence) =
diff --git a/upickle/test/src/upickle/example/OptionsAsNullTests.scala b/upickle/test/src/upickle/example/OptionsAsNullTests.scala
index 0d5bb2764..7d35f51cf 100644
--- a/upickle/test/src/upickle/example/OptionsAsNullTests.scala
+++ b/upickle/test/src/upickle/example/OptionsAsNullTests.scala
@@ -82,7 +82,7 @@ object OptionsAsNullTests extends TestSuite {
 
         object CustomThing2 {
           implicit val rw: OptionPickler.ReadWriter[CustomThing2] = /*upickle.default*/ OptionPickler.readwriter[String].bimap[CustomThing2](
-            x => x.i + " " + x.s,
+            x => s"${x.i} ${x.s}",
             str => {
               val Array(i, s) = str.split(" ", 2)
               new CustomThing2(i.toInt, s)