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/* NSC -- new Scala compiler
* Copyright 2005-2011 LAMP/EPFL
* @author Martin Odersky
*/

//todo: rewrite or disllow new T where T is a mixin (currently: <init> not a member of T)
//todo: use inherited type info also for vars and values
//todo: disallow C#D in superclass
//todo: treat :::= correctly

package scala.tools.nsc
package typechecker

import annotation.tailrec
import scala.collection.{ mutable, immutable }
import mutable.{ LinkedHashMap, ListBuffer }
import scala.util.matching.Regex
import symtab.Flags._
import util.Statistics._

/** This trait provides methods to find various kinds of implicits.
*
* @author Martin Odersky
* @version 1.0
*/
trait Implicits {
  self: Analyzer =>

  import global._
  import definitions._
  import typeDebug.{ ptTree, ptBlock, ptLine }
  import global.typer.{ printTyping, deindentTyping, indentTyping, printInference }

  /** Search for an implicit value. See the comment on `result` at the end of class `ImplicitSearch`
* for more info how the search is conducted.
* @param tree The tree for which the implicit needs to be inserted.
* (the inference might instantiate some of the undetermined
* type parameters of that tree.
* @param pt The expected type of the implicit.
* @param reportAmbiguous Should ambiguous implicit errors be reported?
* False iff we search for a view to find out
* whether one type is coercible to another.
* @param isView We are looking for a view
* @param context The current context
* @return A search result
*/
  def inferImplicit(tree: Tree, pt: Type, reportAmbiguous: Boolean, isView: Boolean, context: Context): SearchResult = {
    printInference("[inferImplicit%s] pt = %s".format(
      if (isView) " view" else "", pt)
    )
    printTyping(
      ptBlock("infer implicit" + (if (isView) " view" else ""),
        "tree" -> tree,
        "pt" -> pt,
        "undetparams" -> context.outer.undetparams
      )
    )
    indentTyping()

    val rawTypeStart = startCounter(rawTypeImpl)
    val findMemberStart = startCounter(findMemberImpl)
    val subtypeStart = startCounter(subtypeImpl)
    val start = startTimer(implicitNanos)
    if (printInfers && !tree.isEmpty && !context.undetparams.isEmpty)
      printTyping("typing implicit: %s %s".format(tree, context.undetparamsString))

    val result = new ImplicitSearch(tree, pt, isView, context.makeImplicit(reportAmbiguous)).bestImplicit
    printInference("[inferImplicit] result: " + result)
    context.undetparams = context.undetparams filterNot result.subst.from.contains

    stopTimer(implicitNanos, start)
    stopCounter(rawTypeImpl, rawTypeStart)
    stopCounter(findMemberImpl, findMemberStart)
    stopCounter(subtypeImpl, subtypeStart)
    deindentTyping()
    printTyping("Implicit search yielded: "+ result)
    result
  }

  private final val sizeLimit = 50000
  private type Infos = List[ImplicitInfo]
  private type Infoss = List[List[ImplicitInfo]]
  private type InfoMap = LinkedHashMap[Symbol, List[ImplicitInfo]] // A map from class symbols to their associated implicits
  private val implicitsCache = new LinkedHashMap[Type, Infoss]
  private val infoMapCache = new LinkedHashMap[Symbol, InfoMap]
  private val improvesCache = perRunCaches.newMap[(ImplicitInfo, ImplicitInfo), Boolean]()

  def resetImplicits() {
    implicitsCache.clear()
    infoMapCache.clear()
    improvesCache.clear()
  }

  private val ManifestSymbols = Set(PartialManifestClass, FullManifestClass, OptManifestClass)

  /** Map all type params in given list to WildcardType
* @param tparams The list of type parameters to map
* @param tp The type in which to do the mapping
*/
  private def tparamsToWildcards(tparams: List[Symbol], tp: Type) =
    if (tparams.isEmpty) tp
    else tp.instantiateTypeParams(tparams, tparams map (_ => WildcardType))

  /* Map a polytype to one in which all type parameters and argument-dependent types are replaced by wildcards.
* Consider `implicit def b(implicit x: A): x.T = error("")`. We need to approximate DebruijnIndex types
* when checking whether `b` is a valid implicit, as we haven't even searched a value for the implicit arg `x`,
* so we have to approximate (otherwise it is excluded a priori).
*/
  private def depoly(tp: Type): Type = tp match {
    case PolyType(tparams, restpe) => tparamsToWildcards(tparams, ApproximateDependentMap(restpe))
    case _ => ApproximateDependentMap(tp)
  }

  /** The result of an implicit search
* @param tree The tree representing the implicit
* @param subst A substituter that represents the undetermined type parameters
* that were instantiated by the winning implicit.
*/
  class SearchResult(val tree: Tree, val subst: TreeTypeSubstituter) {
    override def toString = "SearchResult(%s, %s)".format(tree,
      if (subst.isEmpty) "" else subst)
  }

  lazy val SearchFailure = new SearchResult(EmptyTree, EmptyTreeTypeSubstituter)

  /** A class that records an available implicit
* @param name The name of the implicit
* @param pre The prefix type of the implicit
* @param sym The symbol of the implicit
*/
  class ImplicitInfo(val name: Name, val pre: Type, val sym: Symbol) {
    private var tpeCache: Type = null

    /** Computes member type of implicit from prefix `pre` (cached). */
    def tpe: Type = {
      if (tpeCache eq null) tpeCache = pre.memberType(sym)
      tpeCache
    }

    def isCyclicOrErroneous =
      try containsError(tpe)
      catch { case _: CyclicReference => true }

    var useCountArg: Int = 0
    var useCountView: Int = 0

    /** Does type `tp` contain an Error type as parameter or result?
*/
    private def containsError(tp: Type): Boolean = tp match {
      case PolyType(tparams, restpe) =>
        containsError(restpe)
      case NullaryMethodType(restpe) =>
        containsError(restpe)
      case MethodType(params, restpe) =>
        params.exists(_.tpe.isError) || containsError(restpe)
      case _ =>
        tp.isError
    }

    /** Todo reconcile with definition of stability given in Types.scala */
    private def isStable(tp: Type): Boolean = tp match {
     case TypeRef(pre, sym, _) =>
       sym.isPackageClass ||
       sym.isModuleClass && isStable(pre) /*||
sym.isAliasType && isStable(tp.normalize)*/
     case _ => tp.isStable
    }
    def isStablePrefix = isStable(pre)

    override def equals(other: Any) = other match {
      case that: ImplicitInfo =>
          this.name == that.name &&
          this.pre =:= that.pre &&
          this.sym == that.sym
      case _ => false
    }
    override def hashCode = name.## + pre.## + sym.##
    override def toString = name + ": " + tpe
  }

  /** A sentinel indicating no implicit was found */
  val NoImplicitInfo = new ImplicitInfo(null, NoType, NoSymbol) {
    // equals used to be implemented in ImplicitInfo with an `if(this eq NoImplicitInfo)`
    // overriding the equals here seems cleaner and benchmarks show no difference in performance
    override def equals(other: Any) = other match { case that: AnyRef => that eq this case _ => false }
    override def hashCode = 1
  }

  /** A constructor for types ?{ name: tp }, used in infer view to member
* searches.
*/
  def memberWildcardType(name: Name, tp: Type) = {
    val result = refinedType(List(WildcardType), NoSymbol)
    var psym = name match {
      case x: TypeName => result.typeSymbol.newAbstractType(NoPosition, x)
      case x: TermName => result.typeSymbol.newValue(NoPosition, x)
    }
    psym setInfo tp
    result.decls enter psym
    result
  }

  /** An extractor for types of the form ? { name: ? }
*/
  object HasMember {
    private val hasMemberCache = perRunCaches.newMap[Name, Type]()
    def apply(name: Name): Type = hasMemberCache.getOrElseUpdate(name, memberWildcardType(name, WildcardType))
    def unapply(pt: Type): Option[Name] = pt match {
      case RefinedType(List(WildcardType), Scope(sym)) if sym.tpe == WildcardType => Some(sym.name)
      case _ => None
    }
  }

  /** An extractor for types of the form ? { name: (? >: argtpe <: Any*)restp }
*/
  object HasMethodMatching {
    def apply(name: Name, argtpes: List[Type], restpe: Type): Type = {
      def templateArgType(argtpe: Type) =
        new BoundedWildcardType(TypeBounds(argtpe, AnyClass.tpe))
      val dummyMethod = new TermSymbol(NoSymbol, NoPosition, "typer$dummy")
      val mtpe = MethodType(dummyMethod.newSyntheticValueParams(argtpes map templateArgType), restpe)
      memberWildcardType(name, mtpe)
    }
    def unapply(pt: Type): Option[(Name, List[Type], Type)] = pt match {
      case RefinedType(List(WildcardType), decls) =>
        decls.toList match {
          case List(sym) =>
            sym.tpe match {
              case MethodType(params, restpe)
              if (params forall (_.tpe.isInstanceOf[BoundedWildcardType])) =>
                Some((sym.name, params map (_.tpe.bounds.lo), restpe))
              case _ => None
            }
          case _ => None
        }
      case _ => None
    }
  }

  /** An extractor for unary function types arg => res
*/
  object Function1 {
    val Sym = FunctionClass(1)
    def unapply(tp: Type) = tp match {
      case TypeRef(_, Sym, arg1 :: arg2 :: _) => Some((arg1, arg2))
      case _ => None
    }
  }

  /** A class that sets up an implicit search. For more info, see comments for `inferImplicit`.
* @param tree The tree for which the implicit needs to be inserted.
* @param pt The original expected type of the implicit.
* @param isView We are looking for a view
* @param context0 The context used for the implicit search
*/
  class ImplicitSearch(tree: Tree, pt: Type, isView: Boolean, context0: Context) extends Typer(context0) {
      printTyping(
        ptBlock("new ImplicitSearch",
          "tree" -> tree,
          "pt" -> pt,
          "isView" -> isView,
          "context0" -> context0,
          "undetparams" -> context.outer.undetparams
        )
      )
// assert(tree.isEmpty || tree.pos.isDefined, tree)

    import infer._
    /** Is implicit info `info1` better than implicit info `info2`?
*/
    def improves(info1: ImplicitInfo, info2: ImplicitInfo) = {
      incCounter(improvesCount)
      (info2 == NoImplicitInfo) ||
      (info1 != NoImplicitInfo) && {
        if (info1.sym.isStatic && info2.sym.isStatic) {
          improvesCache get (info1, info2) match {
            case Some(b) => incCounter(improvesCachedCount); b
            case None =>
              val result = isStrictlyMoreSpecific(info1.tpe, info2.tpe, info1.sym, info2.sym)
              improvesCache((info1, info2)) = result
              result
          }
        } else isStrictlyMoreSpecific(info1.tpe, info2.tpe, info1.sym, info2.sym)
      }
    }
    def isPlausiblyCompatible(tp: Type, pt: Type) = checkCompatibility(fast = true, tp, pt)
    def normSubType(tp: Type, pt: Type) = checkCompatibility(fast = false, tp, pt)

    /** Does type `dtor` dominate type `dted`?
* This is the case if the stripped cores `dtor1` and `dted1` of both types are
* the same wrt `=:=`, or if they overlap and the complexity of `dtor1` is higher
* than the complexity of `dted1`.
* The _stripped core_ of a type is the type where
* - all refinements and annotations are dropped,
* - all universal and existential quantification is eliminated
* by replacing variables by their upper bounds,
* - all remaining free type parameters in the type are replaced by WildcardType.
* The _complexity_ of a stripped core type corresponds roughly to the number of
* nodes in its ast, except that singleton types are widened before taking the complexity.
* Two types overlap if they have the same type symbol, or
* if one or both are intersection types with a pair of overlapiing parent types.
*/
    private def dominates(dtor: Type, dted: Type): Boolean = {
      def core(tp: Type): Type = tp.normalize match {
        case RefinedType(parents, defs) => intersectionType(parents map core, tp.typeSymbol.owner)
        case AnnotatedType(annots, tp, selfsym) => core(tp)
        case ExistentialType(tparams, result) => core(result).subst(tparams, tparams map (t => core(t.info.bounds.hi)))
        case PolyType(tparams, result) => core(result).subst(tparams, tparams map (t => core(t.info.bounds.hi)))
        case _ => tp
      }
      def stripped(tp: Type): Type = {
        deriveTypeWithWildcards(freeTypeParametersNoSkolems.collect(tp))(tp)
      }
      def sum(xs: List[Int]) = (0 /: xs)(_ + _)
      def complexity(tp: Type): Int = tp.normalize match {
        case NoPrefix =>
          0
        case SingleType(pre, sym) =>
          if (sym.isPackage) 0 else complexity(tp.normalize.widen)
        case TypeRef(pre, sym, args) =>
          complexity(pre) + sum(args map complexity) + 1
        case RefinedType(parents, _) =>
          sum(parents map complexity) + 1
        case _ =>
          1
      }
      def overlaps(tp1: Type, tp2: Type): Boolean = (tp1, tp2) match {
        case (RefinedType(parents, _), _) => parents exists (overlaps(_, tp2))
        case (_, RefinedType(parents, _)) => parents exists (overlaps(tp1, _))
        case _ => tp1.typeSymbol == tp2.typeSymbol
      }
      val dtor1 = stripped(core(dtor))
      val dted1 = stripped(core(dted))
      overlaps(dtor1, dted1) && (dtor1 =:= dted1 || complexity(dtor1) > complexity(dted1))
    }

    incCounter(implicitSearchCount)

    /** Issues an error signalling ambiguous implicits */
    private def ambiguousImplicitError(info1: ImplicitInfo, info2: ImplicitInfo,
                               pre1: String, pre2: String, trailer: String) =
      if (!info1.tpe.isErroneous && !info2.tpe.isErroneous) {
        val coreMsg =
          pre1+" "+info1.sym.fullLocationString+" of type "+info1.tpe+"\n "+
          pre2+" "+info2.sym.fullLocationString+" of type "+info2.tpe+"\n "+
          trailer
        error(tree.pos,
          if (isView) {
            val found = pt.typeArgs(0)
            val req = pt.typeArgs(1)
            def defaultExplanation =
              "Note that implicit conversions are not applicable because they are ambiguous:\n "+
              coreMsg+"are possible conversion functions from "+ found+" to "+req

            def explanation = {
              val sym = found.typeSymbol
              // Explain some common situations a bit more clearly.
              if (AnyRefClass.tpe <:< req) {
                if (sym == AnyClass || sym == UnitClass) {
                  "Note: " + sym.name + " is not implicitly converted to AnyRef. You can safely\n" +
                  "pattern match `x: AnyRef` or cast `x.asInstanceOf[AnyRef]` to do so."
                }
                else boxedClass get sym match {
                  case Some(boxed) =>
                    "Note: an implicit exists from " + sym.fullName + " => " + boxed.fullName + ", but\n" +
                    "methods inherited from Object are rendered ambiguous. This is to avoid\n" +
                    "a blanket implicit which would convert any " + sym.fullName + " to any AnyRef.\n" +
                    "You may wish to use a type ascription: `x: " + boxed.fullName + "`."
                  case _ =>
                    defaultExplanation
                }
              }
              else defaultExplanation
            }

            typeErrorMsg(found, req) + "\n" + explanation
          }
          else {
            "ambiguous implicit values:\n "+coreMsg + "match expected type "+pt
          })
        }

    /** The type parameters to instantiate */
    val undetParams = if (isView) List() else context.outer.undetparams

    /** The expected type with all undetermined type parameters replaced with wildcards. */
    def approximate(tp: Type) = deriveTypeWithWildcards(undetParams)(tp)
    val wildPt = approximate(pt)

    /** Try to construct a typed tree from given implicit info with given
* expected type.
* Detect infinite search trees for implicits.
*
* @param info The given implicit info describing the implicit definition
* @pre `info.tpe` does not contain an error
*/
    private def typedImplicit(info: ImplicitInfo, ptChecked: Boolean): SearchResult = {
      printInference("[typedImplicit] " + info)
      (context.openImplicits find { case (tp, sym) => sym == tree.symbol && dominates(pt, tp)}) match {
         case Some(pending) =>
           // println("Pending implicit "+pending+" dominates "+pt+"/"+undetParams) //@MDEBUG
           throw DivergentImplicit
         case None =>
           try {
             context.openImplicits = (pt, tree.symbol) :: context.openImplicits
             // println(" "*context.openImplicits.length+"typed implicit "+info+" for "+pt) //@MDEBUG
             typedImplicit0(info, ptChecked)
           } catch {
             case ex: DivergentImplicit =>
               // println("DivergentImplicit for pt:"+ pt +", open implicits:"+context.openImplicits) //@MDEBUG
               if (context.openImplicits.tail.isEmpty) {
                 if (!(pt.isErroneous))
                   context.unit.error(
                     tree.pos, "diverging implicit expansion for type "+pt+"\nstarting with "+
                     info.sym.fullLocationString)
                 SearchFailure
               } else {
                 throw DivergentImplicit
               }
           } finally {
             context.openImplicits = context.openImplicits.tail
           }
       }
    }

    /** Does type `tp` match expected type `pt`
* This is the case if either `pt` is a unary function type with a
* HasMethodMatching type as result, and `tp` is a unary function
* or method type whose result type has a method whose name and type
* correspond to the HasMethodMatching type,
* or otherwise if `tp` is compatible with `pt`.
* This method is performance critical: 5-8% of typechecking time.
*/
    private def matchesPt(tp: Type, pt: Type, undet: List[Symbol]): Boolean = {
      val start = startTimer(matchesPtNanos)
      val result = normSubType(tp, pt) || isView && {
        pt match {
          case TypeRef(_, Function1.Sym, args) =>
            matchesPtView(tp, args.head, args.tail.head, undet)
          case _ =>
            false
        }
      }
      stopTimer(matchesPtNanos, start)
      result
    }
    private def matchesPt(info: ImplicitInfo): Boolean = (
      info.isStablePrefix && matchesPt(depoly(info.tpe), wildPt, Nil)
    )

    private def matchesPtView(tp: Type, ptarg: Type, ptres: Type, undet: List[Symbol]): Boolean = tp match {
      case MethodType(p :: _, restpe) if p.isImplicit => matchesPtView(restpe, ptarg, ptres, undet)
      case MethodType(p :: Nil, restpe) => matchesArgRes(p.tpe, restpe, ptarg, ptres, undet)
      case ExistentialType(_, qtpe) => matchesPtView(normalize(qtpe), ptarg, ptres, undet)
      case Function1(arg1, res1) => matchesArgRes(arg1, res1, ptarg, ptres, undet)
      case _ => false
    }

    private def matchesArgRes(tparg: Type, tpres: Type, ptarg: Type, ptres: Type, undet: List[Symbol]): Boolean =
     (ptarg weak_<:< tparg) && {
       ptres match {
         case HasMethodMatching(name, argtpes, restpe) =>
           (tpres.member(name) filter (m =>
             isApplicableSafe(undet, m.tpe, argtpes, restpe))) != NoSymbol
         case _ =>
           tpres <:< ptres
       }
     }

    /** Capturing the overlap between isPlausiblyCompatible and normSubType.
* This is a faithful translation of the code which was there, but it
* seems likely the methods are intended to be even more similar than
* they are: perhaps someone more familiar with the intentional distinctions
* can examine the now much smaller concrete implementations below.
*/
    private def checkCompatibility(fast: Boolean, tp0: Type, pt0: Type): Boolean = {
      @tailrec def loop(tp: Type, pt: Type): Boolean = tp match {
        case mt @ MethodType(params, restpe) =>
          if (mt.isImplicit)
            loop(restpe, pt)
          else pt match {
            case tr @ TypeRef(pre, sym, args) =>
              if (sym.isAliasType) loop(tp, pt.normalize)
              else if (sym.isAbstractType) loop(tp, pt.bounds.lo)
              else {
                val len = args.length - 1
                hasLength(params, len) &&
                sym == FunctionClass(len) && {
                  var ps = params
                  var as = args
                  if (fast) {
                    while (ps.nonEmpty && as.nonEmpty) {
                      if (!isPlausiblySubType(as.head, ps.head.tpe))
                        return false
                      ps = ps.tail
                      as = as.tail
                    }
                  } else {
                    while (ps.nonEmpty && as.nonEmpty) {
                      if (!(as.head <:< ps.head.tpe))
                        return false
                      ps = ps.tail
                      as = as.tail
                    }
                  }
                  ps.isEmpty && as.nonEmpty && {
                    val lastArg = as.head
                    as.tail.isEmpty && loop(restpe, lastArg)
                  }
                }
              }

            case _ => if (fast) false else tp <:< pt
          }
        case NullaryMethodType(restpe) => loop(restpe, pt)
        case PolyType(_, restpe) => loop(restpe, pt)
        case ExistentialType(_, qtpe) => if (fast) loop(qtpe, pt) else normalize(tp) <:< pt // is !fast case needed??
        case _ => if (fast) isPlausiblySubType(tp, pt) else tp <:< pt
      }
      loop(tp0, pt0)
    }

    /** This expresses more cleanly in the negative: there's a linear path
* to a final true or false.
*/
    private def isPlausiblySubType(tp1: Type, tp2: Type) = !isImpossibleSubType(tp1, tp2)
    private def isImpossibleSubType(tp1: Type, tp2: Type) = tp1.normalize.widen match {
      case tr1 @ TypeRef(_, sym1, _) =>
        // We can only rule out a subtype relationship if the left hand
        // side is a class, else we may not know enough.
        sym1.isClass && (tp2.normalize.widen match {
          case TypeRef(_, sym2, _) =>
             sym2.isClass && !(sym1 isWeakSubClass sym2)
          case RefinedType(parents, decls) =>
            decls.nonEmpty &&
            tr1.member(decls.head.name) == NoSymbol
          case _ => false
        })
      case _ => false
    }

    private def typedImplicit0(info: ImplicitInfo, ptChecked: Boolean): SearchResult = {
      incCounter(plausiblyCompatibleImplicits)
      printTyping(
        ptBlock("typedImplicit0",
          "info.name" -> info.name,
          "ptChecked" -> ptChecked,
          "pt" -> wildPt,
          "orig" -> ptBlock("info",
            "undetParams" -> undetParams,
            "info.pre" -> info.pre
          ).replaceAll("\\n", "\n ")
        )
      )

      if (ptChecked || matchesPt(info))
        typedImplicit1(info)
      else
        SearchFailure
    }

    private def typedImplicit1(info: ImplicitInfo): SearchResult = {
      incCounter(matchingImplicits)

      val itree = atPos(tree.pos.focus) {
        if (info.pre == NoPrefix) Ident(info.name)
        else Select(gen.mkAttributedQualifier(info.pre), info.name)
      }
      printTyping("typedImplicit1 %s, pt=%s, from implicit %s:%s".format(
        typeDebug.ptTree(itree), wildPt, info.name, info.tpe)
      )

      def fail(reason: String): SearchResult = {
        if (settings.XlogImplicits.value)
          inform(itree+" is not a valid implicit value for "+pt+" because:\n"+reason)
        SearchFailure
      }
      try {
        val itree1 =
          if (isView) {
            val arg1 :: arg2 :: _ = pt.typeArgs
            typed1(
              atPos(itree.pos)(Apply(itree, List(Ident("<argument>") setType approximate(arg1)))),
              EXPRmode,
              approximate(arg2)
            )
          }
          else
            typed1(itree, EXPRmode, wildPt)

        incCounter(typedImplicits)

        printTyping("typed implicit %s:%s, pt=%s".format(itree1, itree1.tpe, wildPt))
        val itree2 = if (isView) (itree1: @unchecked) match { case Apply(fun, _) => fun }
                     else adapt(itree1, EXPRmode, wildPt)

        printTyping("adapted implicit %s:%s to %s".format(
          itree1.symbol, itree2.tpe, wildPt)
        )

        def hasMatchingSymbol(tree: Tree): Boolean = (tree.symbol == info.sym) || {
          tree match {
            case Apply(fun, _) => hasMatchingSymbol(fun)
            case TypeApply(fun, _) => hasMatchingSymbol(fun)
            case Select(pre, nme.apply) => pre.symbol == info.sym
            case _ => false
          }
        }

        if (itree2.tpe.isError)
          SearchFailure
        else if (!hasMatchingSymbol(itree1))
          fail("candidate implicit %s is shadowed by other implicit %s".format(
            info.sym.fullLocationString, itree1.symbol.fullLocationString))
        else {
          val tvars = undetParams map freshVar

          if (matchesPt(itree2.tpe, pt.instantiateTypeParams(undetParams, tvars), undetParams)) {
            printInference(
              ptBlock("matchesPt",
                "itree1" -> itree1,
                "tvars" -> tvars,
                "undetParams" -> undetParams
              )
            )

            if (tvars.nonEmpty)
              printTyping(ptLine("" + info.sym, "tvars" -> tvars, "tvars.constr" -> tvars.map(_.constr)))

            val targs = solvedTypes(tvars, undetParams, undetParams map varianceInType(pt),
                                    false, lubDepth(List(itree2.tpe, pt)))

            // #2421: check that we correctly instantiated type parameters outside of the implicit tree:
            checkBounds(itree2.pos, NoPrefix, NoSymbol, undetParams, targs, "inferred ")

            // filter out failures from type inference, don't want to remove them from undetParams!
            // we must be conservative in leaving type params in undetparams
            // prototype == WildcardType: want to remove all inferred Nothings
            val AdjustedTypeArgs(okParams, okArgs) = adjustTypeArgs(undetParams, tvars, targs)
            val subst: TreeTypeSubstituter =
              if (okParams.isEmpty) EmptyTreeTypeSubstituter
              else {
                val subst = new TreeTypeSubstituter(okParams, okArgs)
                subst traverse itree2
                subst
              }

            // #2421b: since type inference (which may have been
            // performed during implicit search) does not check whether
            // inferred arguments meet the bounds of the corresponding
            // parameter (see note in solvedTypes), must check again
            // here:
            // TODO: I would prefer to just call typed instead of
            // duplicating the code here, but this is probably a
            // hotspot (and you can't just call typed, need to force
            // re-typecheck)
            // TODO: the return tree is ignored. This seems to make
            // no difference, but it's bad practice regardless.
            itree2 match {
              case TypeApply(fun, args) => typedTypeApply(itree2, EXPRmode, fun, args)
              case Apply(TypeApply(fun, args), _) => typedTypeApply(itree2, EXPRmode, fun, args) // t2421c
              case t => t
            }
            val result = new SearchResult(itree2, subst)
            incCounter(foundImplicits)
            printInference("[typedImplicit1] SearchResult: " + result)
            result
          }
          else fail("incompatible: %s does not match expected type %s".format(
            itree2.tpe, pt.instantiateTypeParams(undetParams, tvars)))
        }
      }
      catch {
        case ex: TypeError => fail(ex.getMessage())
      }
    }

    // #3453: in addition to the implicit symbols that may shadow the implicit with
    // name `name`, this method tests whether there's a non-implicit symbol with name
    // `name` in scope. Inspired by logic in typedIdent.
    private def nonImplicitSynonymInScope(name: Name) = {
      // the implicit ones are handled by the `shadowed` set above
      context.scope.lookupEntry(name) match {
        case x: ScopeEntry => reallyExists(x.sym) && !x.sym.isImplicit
        case _ => false
      }
    }

    /** Should implicit definition symbol `sym` be considered for applicability testing?
* This is the case if one of the following holds:
* - the symbol's type is initialized
* - the symbol comes from a classfile
* - the symbol comes from a different sourcefile than the current one
* - the symbol and the accessed symbol's definitions come before, and do not contain the closest enclosing definition, // see #3373
* - the symbol's definition is a val, var, or def with an explicit result type
* The aim of this method is to prevent premature cyclic reference errors
* by computing the types of only those implicits for which one of these
* conditions is true.
*/
    def isValid(sym: Symbol) = {
      def hasExplicitResultType(sym: Symbol) = {
        def hasExplicitRT(tree: Tree) = tree match {
          case x: ValOrDefDef => !x.tpt.isEmpty
          case _ => false
        }
        sym.rawInfo match {
          case tc: TypeCompleter => hasExplicitRT(tc.tree)
          case PolyType(_, tc: TypeCompleter) => hasExplicitRT(tc.tree)
          case _ => true
        }
      }
      def comesBefore(sym: Symbol, owner: Symbol) = {
        val ownerPos = owner.pos.pointOrElse(Int.MaxValue)
        sym.pos.pointOrElse(0) < ownerPos && (
          if (sym hasAccessorFlag) {
            val symAcc = sym.accessed // #3373
            symAcc.pos.pointOrElse(0) < ownerPos &&
            !(owner.ownerChain exists (o => (o eq sym) || (o eq symAcc))) // probably faster to iterate only once, don't feel like duplicating hasTransOwner for this case
          } else !(owner hasTransOwner sym)) // faster than owner.ownerChain contains sym
      }

      sym.isInitialized ||
      sym.sourceFile == null ||
      (sym.sourceFile ne context.unit.source.file) ||
      hasExplicitResultType(sym) ||
      comesBefore(sym, context.owner)
    }

    /** Prune ImplicitInfos down to either all the eligible ones or the best one.
*
* @param iss list of list of infos
* @param shadowed set in which to record names that are shadowed by implicit infos
* If it is null, no shadowing.
*/
    class ImplicitComputation(iss: Infoss, shadowed: util.HashSet[Name]) {
      private var best: SearchResult = SearchFailure
      private def isShadowed(name: Name) = (
           (shadowed != null)
        && (shadowed(name) || nonImplicitSynonymInScope(name))
      )
      private def isIneligible(info: ImplicitInfo) = (
           info.isCyclicOrErroneous
        || isView && isPredefMemberNamed(info.sym, nme.conforms)
        || isShadowed(info.name)
      )

      /** True if a given ImplicitInfo (already known isValid) is eligible.
*/
      def survives(info: ImplicitInfo) = (
           !isIneligible(info) // cyclic, erroneous, shadowed, or specially excluded
        && isPlausiblyCompatible(info.tpe, wildPt) // optimization to avoid matchesPt
        && matchesPt(info) // stable and matches expected type
      )
      /** The implicits that are not valid because they come later in the source and
* lack an explicit result type. Used for error diagnostics only.
*/
      val invalidImplicits = new ListBuffer[Symbol]

      /** Tests for validity and updates invalidImplicits by side effect when false.
*/
      private def checkValid(sym: Symbol) = isValid(sym) || { invalidImplicits += sym ; false }

      /** Preventing a divergent implicit from terminating implicit search,
* so that if there is a best candidate it can still be selected.
*/
      private var divergence = false
      private val MaxDiverges = 1 // not sure if this should be > 1
      private val divergenceHandler = util.Exceptional.expiringHandler(MaxDiverges) {
        case x: DivergentImplicit =>
          divergence = true
          log("discarding divergent implicit during implicit search")
          SearchFailure
      }

      /** Sorted list of eligible implicits.
*/
      val eligible = {
        val matches = iss flatMap { is =>
          val result = is filter (info => checkValid(info.sym) && survives(info))
          if (shadowed ne null)
            shadowed addEntries (is map (_.name))

          result
        }

        // most frequent one first
        matches sortBy (x => if (isView) -x.useCountView else -x.useCountArg)
      }
      def eligibleString = {
        val args = List(
          "search" -> pt,
          "target" -> tree,
          "isView" -> isView
        ) ++ eligible.map("eligible" -> _)

        ptBlock("Implicit search in " + context, args: _*)
      }
      printInference(eligibleString)

      /** Faster implicit search. Overall idea:
* - prune aggressively
* - find the most likely one
* - if it matches, forget about all others it improves upon
*/
      @tailrec private def rankImplicits(pending: Infos, acc: Infos): Infos = pending match {
        case Nil => acc
        case i :: is =>
          def tryImplicitInfo(i: ImplicitInfo) =
            try typedImplicit(i, true)
            catch divergenceHandler

          tryImplicitInfo(i) match {
            case SearchFailure => rankImplicits(is, acc)
            case newBest =>
              best = newBest
              val newPending = undoLog undo {
                is filterNot (alt => alt == i || {
                  try improves(i, alt)
                  catch { case e: CyclicReference => true }
                })
              }
              rankImplicits(newPending, i :: acc)
          }
      }

      /** Returns all eligible ImplicitInfos and their SearchResults in a map.
*/
      def findAll() = eligible map (info => (info, typedImplicit(info, false))) toMap

      /** Returns the SearchResult of the best match.
*/
      def findBest(): SearchResult = {
        // After calling rankImplicits, the least frequent matching one is first and
        // earlier elems may improve on later ones, but not the other way.
        // So if there is any element not improved upon by the first it is an error.
        rankImplicits(eligible, Nil) match {
          case Nil => ()
          case chosen :: rest =>
            rest find (alt => !improves(chosen, alt)) match {
              case Some(competing) =>
                ambiguousImplicitError(chosen, competing, "both", "and", "")
              case _ =>
                if (isView) chosen.useCountView += 1
                else chosen.useCountArg += 1
            }
        }

        if (best == SearchFailure) {
          /** If there is no winner, and we witnessed and caught divergence,
* now we can throw it for the error message.
*/
          if (divergence)
            throw DivergentImplicit

          if (invalidImplicits.nonEmpty)
            setAddendum(tree.pos, () =>
              "\n Note: implicit "+invalidImplicits.head+" is not applicable here"+
              " because it comes after the application point and it lacks an explicit result type")
        }

        best
      }
    }

    /** Computes from a list of lists of implicit infos a map which takes
* infos which are applicable for given expected type `pt` to their attributed trees.
*
* @param iss The given list of lists of implicit infos
* @param isLocal Is implicit definition visible without prefix?
* If this is the case then symbols in preceding lists shadow
* symbols of the same name in succeeding lists.
* @return map from infos to search results
*/
    def applicableInfos(iss: Infoss, isLocal: Boolean): Map[ImplicitInfo, SearchResult] = {
      val start = startCounter(subtypeAppInfos)
      val computation = new ImplicitComputation(iss, if (isLocal) util.HashSet[Name](512) else null) { }
      val applicable = computation.findAll()

      stopCounter(subtypeAppInfos, start)
      applicable
    }

    /** Search list of implicit info lists for one matching prototype `pt`.
* If found return a search result with a tree from found implicit info
* which is typed with expected type `pt`. Otherwise return SearchFailure.
*
* @param implicitInfoss The given list of lists of implicit infos
* @param isLocal Is implicit definition visible without prefix?
* If this is the case then symbols in preceding lists shadow
* symbols of the same name in succeeding lists.
*/
    def searchImplicit(implicitInfoss: Infoss, isLocal: Boolean): SearchResult =
      if (implicitInfoss.forall(_.isEmpty)) SearchFailure
      else new ImplicitComputation(implicitInfoss, if (isLocal) util.HashSet[Name](128) else null) findBest()

    /** Produce an implicict info map, i.e. a map from the class symbols C of all parts of this type to
* the implicit infos in the companion objects of these class symbols C.
* The parts of a type is the smallest set of types that contains
* - the type itself
* - the parts of its immediate components (prefix and argument)
* - the parts of its base types
* - for alias types and abstract types, we take instead the parts
* - of their upper bounds.
* @return For those parts that refer to classes with companion objects that
* can be accessed with unambiguous stable prefixes, the implicits infos
* which are members of these companion objects.
*/
    private def companionImplicitMap(tp: Type): InfoMap = {

      /** Populate implicit info map by traversing all parts of type `tp`.
* Parameters as for `getParts`.
*/
      def getClassParts(tp: Type)(implicit infoMap: InfoMap, seen: mutable.Set[Type], pending: Set[Symbol]) = tp match {
        case TypeRef(pre, sym, args) =>
          infoMap get sym match {
            case Some(infos1) =>
              if (infos1.nonEmpty && !(pre =:= infos1.head.pre.prefix)) {
                println("amb prefix: "+pre+"#"+sym+" "+infos1.head.pre.prefix+"#"+sym)
                infoMap(sym) = List() // ambiguous prefix - ignore implicit members
              }
            case None =>
              if (pre.isStable) {
                val companion = sym.companionModule
                companion.moduleClass match {
                  case mc: ModuleClassSymbol =>
                    val infos =
                      for (im <- mc.implicitMembers) yield new ImplicitInfo(im.name, singleType(pre, companion), im)
                    if (infos.nonEmpty)
                      infoMap += (sym -> infos)
                  case _ =>
                }
              }
              val bts = tp.baseTypeSeq
              var i = 1
              while (i < bts.length) {
                getParts(bts(i))
                i += 1
              }
              getParts(pre)
            }
      }

      /** Populate implicit info map by traversing all parts of type `tp`.
* This method is performance critical.
* @param tp The type for which we want to traverse parts
* @param infoMap The infoMap in which implicit infos corresponding to parts are stored
* @param seen The types that were already visited previously when collecting parts for the given infoMap
* @param pending The set of static symbols for which we are currently trying to collect their parts
* in order to cache them in infoMapCache
*/
      def getParts(tp: Type)(implicit infoMap: InfoMap, seen: mutable.Set[Type], pending: Set[Symbol]) {
        if (seen(tp))
          return
        seen += tp
        tp match {
          case TypeRef(pre, sym, args) =>
            if (sym.isClass) {
              if (!((sym.name == tpnme.REFINE_CLASS_NAME) ||
                    (sym.name startsWith tpnme.ANON_CLASS_NAME) ||
                    (sym.name == tpnme.ROOT))) {
                if (sym.isStatic && !(pending contains sym))
                  infoMap ++= {
                    infoMapCache get sym match {
                      case Some(imap) => imap
                      case None =>
                        val result = new InfoMap
                        getClassParts(sym.tpe)(result, new mutable.HashSet(), pending + sym)
                        infoMapCache(sym) = result
                        result
                    }
                  }
                else
                  getClassParts(tp)
                args foreach (getParts(_))
              }
            } else if (sym.isAliasType) {
              getParts(tp.normalize)
            } else if (sym.isAbstractType) {
              getParts(tp.bounds.hi)
            }
          case ThisType(_) =>
            getParts(tp.widen)
          case _: SingletonType =>
            getParts(tp.widen)
          case HasMethodMatching(_, argtpes, restpe) =>
            for (tp <- argtpes) getParts(tp)
            getParts(restpe)
          case RefinedType(ps, _) =>
            for (p <- ps) getParts(p)
          case AnnotatedType(_, t, _) =>
            getParts(t)
          case ExistentialType(_, t) =>
            getParts(t)
          case PolyType(_, t) =>
            getParts(t)
          case _ =>
        }
      }

      val infoMap = new InfoMap
      getParts(tp)(infoMap, new mutable.HashSet(), Set())
      printInference(
        ptBlock("companionImplicitMap " + tp, infoMap.toSeq.map({ case (k, v) => ("" + k, v.mkString(", ")) }): _*)
      )
      infoMap
    }

    /** The parts of a type is the smallest set of types that contains
* - the type itself
* - the parts of its immediate components (prefix and argument)
* - the parts of its base types
* - for alias types and abstract types, we take instead the parts
* - of their upper bounds.
* @return For those parts that refer to classes with companion objects that
* can be accessed with unambiguous stable prefixes, the implicits infos
* which are members of these companion objects.

private def companionImplicits(tp: Type): Infoss = {
val partMap = new LinkedHashMap[Symbol, Type]
val seen = mutable.HashSet[Type]() // cycle detection

/** Enter all parts of `tp` into `parts` set.
* This method is performance critical: about 2-4% of all type checking is spent here
*/
def getParts(tp: Type) {
if (seen(tp))
return
seen += tp
tp match {
case TypeRef(pre, sym, args) =>
if (sym.isClass) {
if (!((sym.name == tpnme.REFINE_CLASS_NAME) ||
(sym.name startsWith tpnme.ANON_CLASS_NAME) ||
(sym.name == tpnme.ROOT)))
partMap get sym match {
case Some(pre1) =>
if (!(pre =:= pre1)) partMap(sym) = NoType // ambiguous prefix - ignore implicit members
case None =>
if (pre.isStable) partMap(sym) = pre
val bts = tp.baseTypeSeq
var i = 1
while (i < bts.length) {
getParts(bts(i))
i += 1
}
getParts(pre)
args foreach getParts
}
} else if (sym.isAliasType) {
getParts(tp.normalize)
} else if (sym.isAbstractType) {
getParts(tp.bounds.hi)
}
case ThisType(_) =>
getParts(tp.widen)
case _: SingletonType =>
getParts(tp.widen)
case RefinedType(ps, _) =>
for (p <- ps) getParts(p)
case AnnotatedType(_, t, _) =>
getParts(t)
case ExistentialType(_, t) =>
getParts(t)
case PolyType(_, t) =>
getParts(t)
case _ =>
}
}

getParts(tp)

val buf = new ListBuffer[Infos]
for ((clazz, pre) <- partMap) {
if (pre != NoType) {
val companion = clazz.companionModule
companion.moduleClass match {
case mc: ModuleClassSymbol =>
buf += (mc.implicitMembers map (im =>
new ImplicitInfo(im.name, singleType(pre, companion), im)))
case _ =>
}
}
}
//println("companion implicits of "+tp+" = "+buf.toList) // DEBUG
buf.toList
}

*/

    /** The implicits made available by type `pt`.
* These are all implicits found in companion objects of classes C
* such that some part of `tp` has C as one of its superclasses.
*/
    private def implicitsOfExpectedType: Infoss = implicitsCache get pt match {
      case Some(implicitInfoss) =>
        incCounter(implicitCacheHits)
        implicitInfoss
      case None =>
        incCounter(implicitCacheMisses)
        val start = startTimer(subtypeETNanos)
// val implicitInfoss = companionImplicits(pt)
        val implicitInfoss1 = companionImplicitMap(pt).valuesIterator.toList
// val is1 = implicitInfoss.flatten.toSet
// val is2 = implicitInfoss1.flatten.toSet
// for (i <- is1)
// if (!(is2 contains i)) println("!!! implicit infos of "+pt+" differ, new does not contain "+i+",\nold: "+implicitInfoss+",\nnew: "+implicitInfoss1)
// for (i <- is2)
// if (!(is1 contains i)) println("!!! implicit infos of "+pt+" differ, old does not contain "+i+",\nold: "+implicitInfoss+",\nnew: "+implicitInfoss1)
        stopTimer(subtypeETNanos, start)
        implicitsCache(pt) = implicitInfoss1
        if (implicitsCache.size >= sizeLimit)
          implicitsCache -= implicitsCache.keysIterator.next
        implicitInfoss1
    }

    /** Creates a tree that calls the relevant factory method in object
* reflect.Manifest for type 'tp'. An EmptyTree is returned if
* no manifest is found. todo: make this instantiate take type params as well?
*/
    private def manifestOfType(tp: Type, full: Boolean): SearchResult = {

      /** Creates a tree that calls the factory method called constructor in object reflect.Manifest */
      def manifestFactoryCall(constructor: String, tparg: Type, args: Tree*): Tree =
        if (args contains EmptyTree) EmptyTree
        else typedPos(tree.pos.focus)(gen.mkManifestFactoryCall(full, constructor, tparg, args.toList))

      /** Creates a tree representing one of the singleton manifests.*/
      def findSingletonManifest(name: String) = typedPos(tree.pos.focus) {
        Select(gen.mkAttributedRef(FullManifestModule), name)
      }

      /** Re-wraps a type in a manifest before calling inferImplicit on the result */
      def findManifest(tp: Type, manifestClass: Symbol = if (full) FullManifestClass else PartialManifestClass) =
        inferImplicit(tree, appliedType(manifestClass.typeConstructor, List(tp)), true, false, context).tree

      def findSubManifest(tp: Type) = findManifest(tp, if (full) FullManifestClass else OptManifestClass)
      def mot(tp0: Type, from: List[Symbol], to: List[Type]): SearchResult = {
        implicit def wrapResult(tree: Tree): SearchResult =
          if (tree == EmptyTree) SearchFailure else new SearchResult(tree, if (from.isEmpty) EmptyTreeTypeSubstituter else new TreeTypeSubstituter(from, to))

        val tp1 = tp0.normalize
        tp1 match {
          case ThisType(_) | SingleType(_, _) =>
            // can't generate a reference to a value that's abstracted over by an existential
            if (containsExistential(tp1)) EmptyTree
            else manifestFactoryCall("singleType", tp, gen.mkAttributedQualifier(tp1))
          case ConstantType(value) =>
            manifestOfType(tp1.deconst, full)
          case TypeRef(pre, sym, args) =>
            if (isValueClass(sym) || isPhantomClass(sym)) {
              findSingletonManifest(sym.name.toString)
            } else if (sym == ObjectClass || sym == AnyRefClass) {
              findSingletonManifest("Object")
            } else if (sym == RepeatedParamClass || sym == ByNameParamClass) {
              EmptyTree
            } else if (sym == ArrayClass && args.length == 1) {
              manifestFactoryCall("arrayType", args.head, findManifest(args.head))
            } else if (sym.isClass) {
              val classarg0 = gen.mkClassOf(tp1)
              val classarg = tp match {
                case _: ExistentialType => gen.mkCast(classarg0, ClassType(tp))
                case _ => classarg0
              }
              val suffix = classarg :: (args map findSubManifest)
              manifestFactoryCall(
                "classType", tp,
                (if ((pre eq NoPrefix) || pre.typeSymbol.isStaticOwner) suffix
                 else findSubManifest(pre) :: suffix): _*)
            } else if (sym.isExistentiallyBound && full) {
              manifestFactoryCall("wildcardType", tp,
                                  findManifest(tp.bounds.lo), findManifest(tp.bounds.hi))
            }
            // looking for a manifest of a type parameter that hasn't been inferred by now,
            // can't do much, but let's not fail
            else if (undetParams contains sym) {
              // #3859: need to include the mapping from sym -> NothingClass.tpe in the SearchResult
              mot(NothingClass.tpe, sym :: from, NothingClass.tpe :: to)
            } else {
              // a manifest should have been found by normal searchImplicit
              EmptyTree
            }
          case RefinedType(parents, decls) => // !!! not yet: if !full || decls.isEmpty =>
            // refinement is not generated yet
            if (hasLength(parents, 1)) findManifest(parents.head)
            else if (full) manifestFactoryCall("intersectionType", tp, parents map findSubManifest: _*)
            else mot(erasure.intersectionDominator(parents), from, to)
          case ExistentialType(tparams, result) =>
            mot(tp1.skolemizeExistential, from, to)
          case _ =>
            EmptyTree
/* !!! the following is almost right, but we have to splice nested manifest
* !!! types into this type. This requires a substantial extension of
* !!! reifiers.
val reifier = new liftcode.Reifier()
val rtree = reifier.reifyTopLevel(tp1)
manifestFactoryCall("apply", tp, rtree)
*/
          }
      }

      mot(tp, Nil, Nil)
    }

    def wrapResult(tree: Tree): SearchResult =
      if (tree == EmptyTree) SearchFailure else new SearchResult(tree, EmptyTreeTypeSubstituter)

    /** The manifest corresponding to type `pt`, provided `pt` is an instance of Manifest.
*/
    private def implicitManifestOrOfExpectedType(pt: Type): SearchResult = pt.dealias match {
      case TypeRef(_, sym, args) if ManifestSymbols(sym) =>
        manifestOfType(args.head, sym == FullManifestClass) match {
          case SearchFailure if sym == OptManifestClass => wrapResult(gen.mkAttributedRef(NoManifest))
          case result => result
        }
      case tp@TypeRef(_, sym, _) if sym.isAbstractType =>
        implicitManifestOrOfExpectedType(tp.bounds.lo) // #3977: use tp (==pt.dealias), not pt (if pt is a type alias, pt.bounds.lo == pt)
      case _ =>
        searchImplicit(implicitsOfExpectedType, false)
        // shouldn't we pass `pt` to `implicitsOfExpectedType`, or is the recursive case
        // for an abstract type really only meant for manifests?
    }

    /** The result of the implicit search:
* First search implicits visible in current context.
* If that fails, search implicits in expected type `pt`.
* If that fails, and `pt` is an instance of Manifest, try to construct a manifest.
* If all fails return SearchFailure
*/
    def bestImplicit: SearchResult = {
      val failstart = startTimer(inscopeFailNanos)
      val succstart = startTimer(inscopeSucceedNanos)

      var result = searchImplicit(context.implicitss, true)

      if (result == SearchFailure) {
        stopTimer(inscopeFailNanos, failstart)
      } else {
        stopTimer(inscopeSucceedNanos, succstart)
        incCounter(inscopeImplicitHits)
      }
      if (result == SearchFailure) {
        val failstart = startTimer(oftypeFailNanos)
        val succstart = startTimer(oftypeSucceedNanos)

        result = implicitManifestOrOfExpectedType(pt)

        if (result == SearchFailure) {
          stopTimer(oftypeFailNanos, failstart)
        } else {
          stopTimer(oftypeSucceedNanos, succstart)
          incCounter(oftypeImplicitHits)
        }
      }

      if (result == SearchFailure && settings.debug.value)
        log("no implicits found for "+pt+" "+pt.typeSymbol.info.baseClasses+" "+implicitsOfExpectedType)

      result
    }

    def allImplicits: List[SearchResult] = {
      def search(iss: Infoss, isLocal: Boolean) = applicableInfos(iss, isLocal).values
      (search(context.implicitss, true) ++ search(implicitsOfExpectedType, false)).toList.filter(_.tree ne EmptyTree)
    }
  }

  object ImplicitNotFoundMsg {
    def unapply(sym: Symbol): Option[(Message)] = sym.implicitNotFoundMsg map (m => (new Message(sym, m)))
    // check the message's syntax: should be a string literal that may contain occurrences of the string "${X}",
    // where `X` refers to a type parameter of `sym`
    def check(sym: Symbol): Option[String] =
      sym.getAnnotation(ImplicitNotFoundClass).flatMap(_.stringArg(0) match {
        case Some(m) => new Message(sym, m) validate
        case None => Some("Missing argument `msg` on implicitNotFound annotation.")
      })


    class Message(sym: Symbol, msg: String) {
      // http://dcsobral.blogspot.com/2010/01/string-interpolation-in-scala-with.html
      private def interpolate(text: String, vars: Map[String, String]) = {
        """\$\{([^}]+)\}""".r.replaceAllIn(text, (_: Regex.Match) match {
          case Regex.Groups(v) => java.util.regex.Matcher.quoteReplacement(vars.getOrElse(v, "")) // #3915: need to quote replacement string since it may include $'s (such as the interpreter's $iw)
        })}

      private lazy val typeParamNames: List[String] = sym.typeParams.map(_.decodedName)

      def format(paramName: Name, paramTp: Type): String = format(paramTp.typeArgs map (_.toString))
      def format(typeArgs: List[String]): String =
        interpolate(msg, Map((typeParamNames zip typeArgs): _*)) // TODO: give access to the name and type of the implicit argument, etc?

      def validate: Option[String] = {
        import scala.util.matching.Regex; import collection.breakOut
        // is there a shorter way to avoid the intermediate toList?
        val refs = """\$\{([^}]+)\}""".r.findAllIn(msg).matchData.map(_ group 1).toSet
        val decls = typeParamNames.toSet

        (refs &~ decls) match {
          case s if s isEmpty => None
          case unboundNames =>
            val singular = unboundNames.size == 1
            Some("The type parameter"+( if(singular) " " else "s " )+ unboundNames.mkString(", ") +
                  " referenced in the message of the @implicitNotFound annotation "+( if(singular) "is" else "are" )+
                  " not defined by "+ sym +".")
        }
      }
    }
  }
}
class DivergentImplicit extends Exception
object DivergentImplicit extends DivergentImplicit
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