diff --git a/compiler/src/dotty/tools/backend/jvm/CollectEntryPoints.scala b/compiler/src/dotty/tools/backend/jvm/CollectEntryPoints.scala
index 2ee1b6011ec7..80a0cf0f694e 100644
--- a/compiler/src/dotty/tools/backend/jvm/CollectEntryPoints.scala
+++ b/compiler/src/dotty/tools/backend/jvm/CollectEntryPoints.scala
@@ -68,7 +68,7 @@ object CollectEntryPoints{
     def fail(msg: String, pos: Position = sym.pos) = {
       ctx.warning(          sym.name +
         s" has a main method with parameter type Array[String], but ${toDenot(sym).fullName} will not be a runnable program.\n  Reason: $msg",
-        sourcePos(sym.pos)
+        pos
         // TODO: make this next claim true, if possible
         //   by generating valid main methods as static in module classes
         //   not sure what the jvm allows here
diff --git a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
index 51fa15706c08..8345d45f6832 100644
--- a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
+++ b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
@@ -15,6 +15,7 @@ import scala.reflect.internal.util.WeakHashSet
 import scala.reflect.io.{AbstractFile, Directory, PlainDirectory}
 import scala.tools.asm.{AnnotationVisitor, ClassVisitor, FieldVisitor, MethodVisitor}
 import scala.tools.nsc.backend.jvm.{BCodeHelpers, BackendInterface}
+import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.dotc.core._
 import Periods._
 import SymDenotations._
@@ -481,13 +482,13 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
 
 
 
-  implicit def positionHelper(a: Position): PositionHelper = new PositionHelper {
+  implicit class positionHelper(a: Position) extends PositionHelper {
     def isDefined: Boolean = a.exists
-    def line: Int = sourcePos(a).line + 1
+    def line: Int = SourcePosition(a).line + 1
     def finalPosition: Position = a
   }
 
-  implicit def constantHelper(a: Constant): ConstantHelper = new ConstantHelper {
+  implicit class constantHelper(a: Constant) extends ConstantHelper {
     def booleanValue: Boolean = a.booleanValue
     def longValue: Long = a.longValue
     def byteValue: Byte = a.byteValue
@@ -503,8 +504,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     def charValue: Char = a.charValue
   }
 
-
-  implicit def treeHelper(a: Tree): TreeHelper = new TreeHelper {
+  implicit class treeHelper(a: Tree) extends TreeHelper {
     def symbol: Symbol = a.symbol
 
     def pos: Position = a.pos
@@ -517,7 +517,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   }
 
 
-  implicit def annotHelper(a: Annotation): AnnotationHelper = new AnnotationHelper {
+  implicit class annotHelper(a: Annotation) extends AnnotationHelper {
     def atp: Type = a.tree.tpe
 
     def assocs: List[(Name, Tree)] = assocsFromApply(a.tree)
@@ -538,7 +538,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   }
 
 
-  implicit def nameHelper(n: Name): NameHelper = new NameHelper {
+  implicit class nameHelper(n: Name) extends NameHelper {
     def toTypeName: Name = n.toTypeName
     def isTypeName: Boolean = n.isTypeName
     def toTermName: Name = n.toTermName
@@ -551,7 +551,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   }
 
 
-  implicit def symHelper(sym: Symbol): SymbolHelper = new SymbolHelper {
+  implicit class symHelper(sym: Symbol) extends SymbolHelper {
     // names
     def fullName(sep: Char): String = sym.showFullName
     def fullName: String = sym.showFullName
@@ -778,7 +778,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   }
 
 
-  implicit def typeHelper(tp: Type): TypeHelper = new TypeHelper {
+  implicit class typeHelper(tp: Type) extends TypeHelper {
     def member(string: Name): Symbol = tp.member(string.toTermName).symbol
 
     def isFinalType: Boolean = tp.typeSymbol is Flags.Final //in scalac checks for type parameters. Why? Aren't they gone by backend?
diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index b5be894402fa..53e8b4560676 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -118,7 +118,7 @@ object desugar {
         vparamss = (setterParam :: Nil) :: Nil,
         tpt = TypeTree(defn.UnitType),
         rhs = setterRhs
-      ).withMods((mods | Accessor) &~ CaseAccessor) // rhs gets filled in later, when field is generated and getter has parameters
+      ).withMods((mods | Accessor) &~ (CaseAccessor | Implicit)) // rhs gets filled in later, when field is generated and getter has parameters
       Thicket(vdef, setter)
     }
     else vdef
diff --git a/compiler/src/dotty/tools/dotc/core/Decorators.scala b/compiler/src/dotty/tools/dotc/core/Decorators.scala
index a105741f5b6d..1d85ef9509b0 100644
--- a/compiler/src/dotty/tools/dotc/core/Decorators.scala
+++ b/compiler/src/dotty/tools/dotc/core/Decorators.scala
@@ -4,10 +4,8 @@ package core
 import annotation.tailrec
 import Symbols._
 import Contexts._, Names._, Phases._, printing.Texts._, printing.Printer, printing.Showable
-import util.Positions.Position, util.SourcePosition
 import collection.mutable.ListBuffer
 import dotty.tools.dotc.transform.TreeTransforms._
-import ast.tpd._
 import scala.language.implicitConversions
 import printing.Formatting._
 
@@ -149,16 +147,6 @@ object Decorators {
     }
   }
 
-  implicit def sourcePos(pos: Position)(implicit ctx: Context): SourcePosition = {
-    def recur(inlinedCalls: List[Tree], pos: Position): SourcePosition = inlinedCalls match {
-      case inlinedCall :: rest =>
-        sourceFile(inlinedCall).atPos(pos).withOuter(recur(rest, inlinedCall.pos))
-      case empty =>
-        ctx.source.atPos(pos)
-    }
-    recur(enclosingInlineds, pos)
-  }
-
   implicit class StringInterpolators(val sc: StringContext) extends AnyVal {
 
     /** General purpose string formatting */
diff --git a/compiler/src/dotty/tools/dotc/transform/CollectEntryPoints.scala b/compiler/src/dotty/tools/dotc/transform/CollectEntryPoints.scala
index 331fce46ac16..cc376b29ce94 100644
--- a/compiler/src/dotty/tools/dotc/transform/CollectEntryPoints.scala
+++ b/compiler/src/dotty/tools/dotc/transform/CollectEntryPoints.scala
@@ -49,7 +49,7 @@ class CollectEntryPoints extends MiniPhaseTransform {
     def fail(msg: String, pos: Position = sym.pos) = {
       ctx.warning(sym.name +
         s" has a main method with parameter type Array[String], but ${sym.fullName} will not be a runnable program.\n  Reason: $msg",
-        sourcePos(sym.pos)
+        pos
         // TODO: make this next claim true, if possible
         //   by generating valid main methods as static in module classes
         //   not sure what the jvm allows here
diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index 1ca7eb107c61..741c2ba13005 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -501,10 +501,22 @@ trait Checking {
   /** Check that a non-implicit parameter making up the first parameter section of an
    *  implicit conversion is not a singleton type.
    */
-  def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = vparamss match {
+  def checkImplicitConversion(sym: Symbol, vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = vparamss match {
     case (vparam :: Nil) :: _ if !(vparam.symbol is Implicit) =>
       if (vparam.tpt.tpe.isInstanceOf[SingletonType])
         ctx.error(s"implicit conversion may not have a parameter of singleton type", vparam.tpt.pos)
+      if (!ctx.isAfterTyper && sym.owner.isClass && !sym.is(AccessFlags)) {
+        val fromTpe = vparam.tpt.tpe
+        val toTpe = sym.info.finalResultType
+        val fromClasses = fromTpe.classSymbols
+        val toClasses = toTpe.classSymbols
+        def isLocal(cls: ClassSymbol) =
+          ctx.owner.topLevelClass.associatedFile == cls.associatedFile
+        if (!fromClasses.exists(isLocal) && !toClasses.forall(isLocal))
+          ctx.errorOrMigrationWarning(
+              em"""implicit conversion must be defined in same compilation unit as its source ${fromClasses.mkString(" or ")}
+                  |or its target ${toClasses.mkString(" and ")}""", sym.pos)
+      }
     case _ =>
   }
 
@@ -633,7 +645,7 @@ trait NoChecking extends Checking {
   override def checkValue(tree: Tree, proto: Type)(implicit ctx: Context): tree.type = tree
   override def checkStable(tp: Type, pos: Position)(implicit ctx: Context): Unit = ()
   override def checkClassType(tp: Type, pos: Position, traitReq: Boolean, stablePrefixReq: Boolean)(implicit ctx: Context): Type = tp
-  override def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = ()
+  override def checkImplicitConversion(sym: Symbol, vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = ()
   override def checkFeasible(tp: Type, pos: Position, where: => String = "")(implicit ctx: Context): Type = tp
   override def checkInlineConformant(tree: Tree, what: => String)(implicit ctx: Context) = ()
   override def checkNoDoubleDefs(cls: Symbol)(implicit ctx: Context): Unit = ()
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 57e3c1b880a5..4537adb4aba3 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1219,7 +1219,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     completeAnnotations(ddef, sym)
     val tparams1 = tparams mapconserve (typed(_).asInstanceOf[TypeDef])
     val vparamss1 = vparamss nestedMapconserve (typed(_).asInstanceOf[ValDef])
-    if (sym is Implicit) checkImplicitParamsNotSingletons(vparamss1)
+    if (sym is Implicit) checkImplicitConversion(sym, vparamss1)
     var tpt1 = checkSimpleKinded(typedType(tpt))
 
     var rhsCtx = ctx
diff --git a/compiler/src/dotty/tools/dotc/util/SourcePosition.scala b/compiler/src/dotty/tools/dotc/util/SourcePosition.scala
index be5c469952be..c11001a10a72 100644
--- a/compiler/src/dotty/tools/dotc/util/SourcePosition.scala
+++ b/compiler/src/dotty/tools/dotc/util/SourcePosition.scala
@@ -51,6 +51,21 @@ extends interfaces.SourcePosition {
     else s"(no source file, offset = ${pos.point})"
 }
 
+object SourcePosition {
+  import core.Contexts.Context
+  import ast.tpd.{Tree, sourceFile, enclosingInlineds}
+
+  implicit def apply(pos: Position)(implicit ctx: Context): SourcePosition = {
+    def recur(inlinedCalls: List[Tree], pos: Position): SourcePosition = inlinedCalls match {
+      case inlinedCall :: rest =>
+        sourceFile(inlinedCall).atPos(pos).withOuter(recur(rest, inlinedCall.pos))
+      case empty =>
+        ctx.source.atPos(pos)
+    }
+    recur(enclosingInlineds, pos)
+  }
+}
+
 /** A sentinel for a non-existing source position */
 @sharable object NoSourcePosition extends SourcePosition(NoSource, NoPosition) {
   override def toString = "?"
diff --git a/compiler/src/dotty/tools/io/Jar.scala b/compiler/src/dotty/tools/io/Jar.scala
index 42efc7e0623d..a021ed0a8768 100644
--- a/compiler/src/dotty/tools/io/Jar.scala
+++ b/compiler/src/dotty/tools/io/Jar.scala
@@ -34,6 +34,8 @@ import scala.language.{postfixOps, implicitConversions}
 // static Attributes.Name   SPECIFICATION_VERSION
 
 class Jar(file: File) extends Iterable[JarEntry] {
+  import Jar.enrichManifest
+
   def this(jfile: JFile) = this(File(jfile))
   def this(path: String) = this(File(path))
 
@@ -133,6 +135,8 @@ object Jar {
     def apply(manifest: JManifest): WManifest = new WManifest(manifest)
     implicit def unenrichManifest(x: WManifest): JManifest = x.underlying
   }
+  implicit def enrichManifest(m: JManifest): WManifest = WManifest(m)
+
   class WManifest(manifest: JManifest) {
     for ((k, v) <- initialMainAttrs)
       this(k) = v
diff --git a/compiler/src/dotty/tools/io/package.scala b/compiler/src/dotty/tools/io/package.scala
index 1c0e0b5c4471..8f62aaf9e597 100644
--- a/compiler/src/dotty/tools/io/package.scala
+++ b/compiler/src/dotty/tools/io/package.scala
@@ -30,7 +30,6 @@ package object io {
   type JManifest = java.util.jar.Manifest
   type JFile = java.io.File
 
-  implicit def enrichManifest(m: JManifest): Jar.WManifest = Jar.WManifest(m)
   private lazy val daemonThreadPool = DaemonThreadFactory.newPool()
 
   def runnable(body: => Unit): Runnable       = new Runnable { override def run() = body }
diff --git a/compiler/test/dotty/tools/dotc/parsing/ModifiersParsingTest.scala b/compiler/test/dotty/tools/dotc/parsing/ModifiersParsingTest.scala
index 806e1af46421..16916d3e25f2 100644
--- a/compiler/test/dotty/tools/dotc/parsing/ModifiersParsingTest.scala
+++ b/compiler/test/dotty/tools/dotc/parsing/ModifiersParsingTest.scala
@@ -16,7 +16,7 @@ import core.Flags
 object ModifiersParsingTest {
   implicit val ctx: Context = (new ContextBase).initialCtx
 
-  implicit def parse(code: String): Tree = {
+  private implicit def parse(code: String): Tree = {
     val (_, stats) = new Parser(new SourceFile("<meta>", code.toCharArray)).templateStatSeq()
     stats match { case List(stat) => stat; case stats => Thicket(stats) }
   }
diff --git a/library/src/scala/reflect/Selectable.scala b/library/src/scala/reflect/Selectable.scala
index 0dbdbc293d1b..2d1c2e4c3eaa 100644
--- a/library/src/scala/reflect/Selectable.scala
+++ b/library/src/scala/reflect/Selectable.scala
@@ -66,7 +66,7 @@ class Selectable(val receiver: Any) extends AnyVal with scala.Selectable {
 }
 
 object Selectable {
-  implicit def reflectiveSelectable(receiver: Any): scala.Selectable = receiver match {
+  implicit def reflectiveSelectable(receiver: Any): Selectable = receiver match {
     case receiver: scala.Selectable => receiver
     case _                          => new Selectable(receiver)
   }
diff --git a/tests/neg/function-arity.scala b/tests/neg/function-arity.scala
index 5e0cb1058f98..58f03683c3df 100644
--- a/tests/neg/function-arity.scala
+++ b/tests/neg/function-arity.scala
@@ -23,6 +23,6 @@ object Test {
   foo((a: Int, b: String) => a + b) // error: none of the overloaded alternatives of method foo match arguments (Int, Int)
 }
 object jasonComment {
-  implicit def i2s(i: Int): String = i.toString
+  private implicit def i2s(i: Int): String = i.toString
   ((x: String, y: String) => 42) : (((Int, Int)) => String) // error
 }
diff --git a/tests/neg/implicitDefs.scala b/tests/neg/implicitDefs.scala
index 3bfe604341a3..24bc622f85d9 100644
--- a/tests/neg/implicitDefs.scala
+++ b/tests/neg/implicitDefs.scala
@@ -7,7 +7,7 @@ object implicitDefs {
 
   implicit val x = 2 // error: type of implicit definition needs to be given explicitly
   implicit def y(x: Int) = 3 // error: result type of implicit definition needs to be given explicitly
-  implicit def z(a: x.type): String = "" // error: implicit conversion may not have a parameter of singleton type
+  implicit def z(a: x.type): String = "" // error: implicit conversion may not have a parameter of singleton type // error: implicit must be defined in same compilation unit
 
   def foo(implicit x: String) = 1
 
diff --git a/tests/neg/zoo.scala b/tests/neg/zoo.scala
index 1674548e842c..fe0af30efa39 100644
--- a/tests/neg/zoo.scala
+++ b/tests/neg/zoo.scala
@@ -40,5 +40,5 @@ def newLion: Lion = new {
 }
 val milka = newCow
 val leo = newLion
-leo.eats(milka)  // error: no projector found
+leo.eats(milka)  // error found: Test.Lion(Test.leo) required: Selectable
 }
diff --git a/tests/new/package-implicit/ActorRef.scala b/tests/pos-scala2/package-implicit/ActorRef.scala
similarity index 100%
rename from tests/new/package-implicit/ActorRef.scala
rename to tests/pos-scala2/package-implicit/ActorRef.scala
diff --git a/tests/new/package-implicit/DataFlow.scala b/tests/pos-scala2/package-implicit/DataFlow.scala
similarity index 100%
rename from tests/new/package-implicit/DataFlow.scala
rename to tests/pos-scala2/package-implicit/DataFlow.scala
diff --git a/tests/new/package-implicit/package.scala b/tests/pos-scala2/package-implicit/package.scala
similarity index 100%
rename from tests/new/package-implicit/package.scala
rename to tests/pos-scala2/package-implicit/package.scala
diff --git a/tests/pos/t2429.scala b/tests/pos-scala2/t2429.scala
similarity index 100%
rename from tests/pos/t2429.scala
rename to tests/pos-scala2/t2429.scala
diff --git a/tests/pos/t0438.scala b/tests/pos/t0438.scala
index 1615e3da7074..4340eb79a7c7 100644
--- a/tests/pos/t0438.scala
+++ b/tests/pos/t0438.scala
@@ -1,5 +1,5 @@
 class Foo {
-  implicit def pair2fun2[A, B, C](f: (A, B) => C): ((A, B)) => C =
+  private implicit def pair2fun2[A, B, C](f: (A, B) => C): ((A, B)) => C =
      {p: (A, B) => f(p._1, p._2) }
 
   def foo(f: ((Int, Int)) => Int) = f
diff --git a/tests/pos/typealiases.scala b/tests/pos/typealiases.scala
index 93d1dce4dc31..e296bcb29fe1 100644
--- a/tests/pos/typealiases.scala
+++ b/tests/pos/typealiases.scala
@@ -14,7 +14,7 @@ trait Test[T] {
 object main extends Test[Int] {
   val pair1 = (1,1)
 
-  implicit def topair(x: Int): Tuple2[Int, Int] = (x,x)
+  private implicit def topair(x: Int): Tuple2[Int, Int] = (x,x)
   val pair2: MyPair[Int] = 1
   val x: Short = 1
 }
diff --git a/tests/run/t3346g.check b/tests/run-scala2/t3346g.check
similarity index 100%
rename from tests/run/t3346g.check
rename to tests/run-scala2/t3346g.check
diff --git a/tests/run/t3346g.scala b/tests/run-scala2/t3346g.scala
similarity index 100%
rename from tests/run/t3346g.scala
rename to tests/run-scala2/t3346g.scala
diff --git a/tests/run/t3346h.check b/tests/run-scala2/t3346h.check
similarity index 100%
rename from tests/run/t3346h.check
rename to tests/run-scala2/t3346h.check
diff --git a/tests/run/t3346h.scala b/tests/run-scala2/t3346h.scala
similarity index 100%
rename from tests/run/t3346h.scala
rename to tests/run-scala2/t3346h.scala
diff --git a/tests/run/t8280.scala b/tests/run-scala2/t8280.scala
similarity index 100%
rename from tests/run/t8280.scala
rename to tests/run-scala2/t8280.scala
diff --git a/tests/run/implicits.check b/tests/run/implicits.check
index 010571589cec..4ac46c960066 100644
--- a/tests/run/implicits.check
+++ b/tests/run/implicits.check
@@ -1,2 +1,2 @@
-OK
-[2]
+Str(OK)
+Str([2])
diff --git a/tests/run/implicits.scala b/tests/run/implicits.scala
index 60a361a44d08..b7878d16f6bb 100644
--- a/tests/run/implicits.scala
+++ b/tests/run/implicits.scala
@@ -1,8 +1,9 @@
 import scala.language.implicitConversions
 
+case class Str(str: String)
 object A {
   object B {
-    implicit def int2string(x: Int): String = "["+x.toString+"]"
+    implicit def int2string(x: Int): Str = Str("["+x.toString+"]")
   }
 }
 
@@ -12,8 +13,8 @@ class C(x: String) {
   }
 
   object Inner {
-    val s: String = x
-    implicit def Inner2String(x: Inner): String = s
+    val s: Str = Str(x)
+    implicit def Inner2Str(x: Inner): Str = s
   }
 }
 
@@ -21,9 +22,9 @@ object Test extends dotty.runtime.LegacyApp {
   import A.B._
   val c = new C("OK")
   val i = new c.Inner
-  val s: String = i
+  val s: Str = i
   Console.println(s)
-  Console.println(2: String)
+  Console.println(2: Str)
 }
 
 object TestPriority {
diff --git a/tests/run/tcpoly_parseridioms.scala b/tests/run/tcpoly_parseridioms.scala
index e86af0caa90a..89a024a90e0c 100644
--- a/tests/run/tcpoly_parseridioms.scala
+++ b/tests/run/tcpoly_parseridioms.scala
@@ -90,9 +90,9 @@ trait ParserIdioms extends Parsers with Idioms {
     def pure[a](x: a): Parser[a] = success(x)
   }
 
-  implicit def parserIdiomFun[s, t](fun: s=>t): IdiomaticFunction[Parser, ParserIdiom.type, s, t] =
+  protected implicit def parserIdiomFun[s, t](fun: s=>t): IdiomaticFunction[Parser, ParserIdiom.type, s, t] =
     new IdiomaticFunction[Parser, ParserIdiom.type, s, t](ParserIdiom, fun)
-  implicit def parserIdiomTgt[s](tgt: s): IdiomaticTarget[Parser, ParserIdiom.type, s] =
+  protected implicit def parserIdiomTgt[s](tgt: s): IdiomaticTarget[Parser, ParserIdiom.type, s] =
     new IdiomaticTarget[Parser, ParserIdiom.type, s](ParserIdiom, tgt)
 
   trait Expr
@@ -103,8 +103,8 @@ trait ParserIdioms extends Parsers with Idioms {
   // TODO: how can parserIdiom(curry2(_)) be omitted?
   def expr: Parser[Expr] = parserIdiomFun(curry2(Plus)) <| num <> num |>
 
-  implicit def curry2[s,t,u](fun: (s, t)=>u)(a: s)(b: t): u = fun(a, b)
-  implicit def curry3[r,s,t,u](fun: (r,s, t)=>u)(a: r)(b: s)(c: t): u = fun(a, b, c)
+  protected implicit def curry2[s,t,u](fun: (s, t)=>u)(a: s)(b: t): u = fun(a, b)
+  protected implicit def curry3[r,s,t,u](fun: (r,s, t)=>u)(a: r)(b: s)(c: t): u = fun(a, b, c)
 }
 
 object Test extends ParserIdioms with App {