Scala example source code file (TypeDiagnostics.scala)

This example Scala source code file (TypeDiagnostics.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

collection, compiler, list, mutable, nil, nsc, position, seq, set, string, symbol, tree, type, typediag

The TypeDiagnostics.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author  Paul Phillips
 */

package scala.tools.nsc
package typechecker

import scala.collection.mutable
import scala.collection.mutable.ListBuffer
import scala.util.control.Exception.ultimately
import symtab.Flags._
import PartialFunction._

/** An interface to enable higher configurability of diagnostic messages
 *  regarding type errors.  This is barely a beginning as error messages are
 *  distributed far and wide across the codebase.  The plan is to partition
 *  error messages into some broad groups and provide some mechanism for
 *  being more or less verbose on a selective basis.  Possible groups include
 *  such examples as
 *
 *    arity errors
 *    kind errors
 *    variance errors
 *    ambiguity errors
 *    volatility/stability errors
 *    implementation restrictions
 *
 *  And more, and there is plenty of overlap, so it'll be a process.
 *
 *  @author Paul Phillips
 *  @version 1.0
 */
trait TypeDiagnostics {
  self: Analyzer =>

  import global._
  import definitions._

  /** For errors which are artifacts of the implementation: such messages
   *  indicate that the restriction may be lifted in the future.
   */
  def restrictionWarning(pos: Position, unit: CompilationUnit, msg: String): Unit =
    unit.warning(pos, "Implementation restriction: " + msg)
  def restrictionError(pos: Position, unit: CompilationUnit, msg: String): Unit =
    unit.error(pos, "Implementation restriction: " + msg)

  /** A map of Positions to addendums - if an error involves a position in
   *  the map, the addendum should also be printed.
   */
  private val addendums = perRunCaches.newMap[Position, () => String]()
  private var isTyperInPattern = false

  /** Devising new ways of communicating error info out of
   *  desperation to work on error messages.  This is used
   *  by typedPattern to wrap its business so we can generate
   *  a sensible error message when things go south.
   */
  def typingInPattern[T](body: => T): T = {
    val saved = isTyperInPattern
    isTyperInPattern = true
    try body
    finally isTyperInPattern = saved
  }

  def setAddendum(pos: Position, msg: () => String) =
    if (pos != NoPosition)
      addendums(pos) = msg

  def withAddendum(pos: Position) = (_: String) + addendums.getOrElse(pos, () => "")()

  def decodeWithKind(name: Name, owner: Symbol): String = {
    val prefix = (
      if (name.isTypeName) "type "
      else if (owner.isPackageClass) "object "
      else "value "
    )
    prefix + name.decode
  }

  /** Does the positioned line assigned to t1 precede that of t2?
   */
  def posPrecedes(p1: Position, p2: Position) = p1.isDefined && p2.isDefined && p1.line < p2.line
  def linePrecedes(t1: Tree, t2: Tree) = posPrecedes(t1.pos, t2.pos)

  private object DealiasedType extends TypeMap {
    def apply(tp: Type): Type = tp match {
      // Avoid "explaining" that String is really java.lang.String,
      // while still dealiasing types from non-default namespaces.
      case TypeRef(pre, sym, args) if sym.isAliasType && !sym.isInDefaultNamespace =>
        mapOver(tp.dealias)
      case _ =>
        mapOver(tp)
    }
  }

  /** An explanatory note to be added to error messages
   *  when there's a problem with abstract var defs */
  def abstractVarMessage(sym: Symbol): String =
    if (underlyingSymbol(sym).isVariable)
      "\n(Note that variables need to be initialized to be defined)"
    else ""

  private def methodTypeErrorString(tp: Type) = tp match {
    case mt @ MethodType(params, resultType)  =>
      def forString = params map (_.defString)

       forString.mkString("(", ",", ")") + resultType
    case x                                    => x.toString
  }

  /**
   * [a, b, c] => "(a, b, c)"
   * [a, B]    => "(param1, param2)"
   * [a, B, c] => "(param1, ..., param2)"
   */
  final def exampleTuplePattern(names: List[Name]): String = {
    val arity = names.length
    val varPatterNames: Option[List[String]] = sequence(names map {
      case name if nme.isVariableName(name) => Some(name.decode)
      case _                                => None
    })
    def parenthesize(a: String) = s"($a)"
    def genericParams = (Seq("param1") ++ (if (arity > 2) Seq("...") else Nil) ++ Seq(s"param$arity"))
    parenthesize(varPatterNames.getOrElse(genericParams).mkString(", "))
  }

  def alternatives(tree: Tree): List[Type] = tree.tpe match {
    case OverloadedType(pre, alternatives)  => alternatives map pre.memberType
    case _                                  => Nil
  }
  def alternativesString(tree: Tree) =
    alternatives(tree) map (x => "  " + methodTypeErrorString(x)) mkString ("", " <and>\n", "\n")

  /** The symbol which the given accessor represents (possibly in part).
   *  This is used for error messages, where we want to speak in terms
   *  of the actual declaration or definition, not in terms of the generated setters
   *  and getters.
   */
  def underlyingSymbol(member: Symbol): Symbol =
    if (!member.hasAccessorFlag) member
    else if (!member.isDeferred) member.accessed
    else {
      val getter = if (member.isSetter) member.getter(member.owner) else member
      val flags  = if (getter.setter(member.owner) != NoSymbol) DEFERRED.toLong | MUTABLE else DEFERRED

      getter.owner.newValue(getter.name.toTermName, getter.pos, flags) setInfo getter.tpe.resultType
    }

  def treeSymTypeMsg(tree: Tree): String = {
    val sym               = tree.symbol
    def hasParams         = tree.tpe.paramSectionCount > 0
    def preResultString   = if (hasParams) ": " else " of type "

    def patternMessage    = "pattern " + tree.tpe.finalResultType + valueParamsString(tree.tpe)
    def exprMessage       = "expression of type " + tree.tpe
    def overloadedMessage = s"overloaded method $sym with alternatives:\n" + alternativesString(tree)
    def moduleMessage     = "" + sym
    def defaultMessage    = moduleMessage + preResultString + tree.tpe
    def applyMessage      = defaultMessage + tree.symbol.locationString

    if (!tree.hasExistingSymbol) {
      if (isTyperInPattern) patternMessage
      else exprMessage
    }
    else if (sym.isOverloaded) overloadedMessage
    else if (sym.isModule) moduleMessage
    else if (sym.name == nme.apply) applyMessage
    else defaultMessage
  }

  def disambiguate(ss: List[String]) = ss match {
    case Nil      => Nil
    case s :: ss  => s :: (ss map { case `s` => "(some other)"+s ; case x => x })
  }

  // todo: use also for other error messages
  def existentialContext(tp: Type) = tp.skolemsExceptMethodTypeParams match {
    case Nil  => ""
    case xs   => " where " + (disambiguate(xs map (_.existentialToString)) mkString ", ")
  }

  def explainAlias(tp: Type) = {
    // Don't automatically normalize standard aliases; they still will be
    // expanded if necessary to disambiguate simple identifiers.
    val deepDealias = DealiasedType(tp)
    if (tp eq deepDealias) "" else {
      // A sanity check against expansion being identical to original.
      val s = "" + deepDealias
      if (s == "" + tp) ""
      else "\n    (which expands to)  " + s
    }
  }

  /** Look through the base types of the found type for any which
   *  might have been valid subtypes if given conformant type arguments.
   *  Examine those for situations where the type error would have been
   *  eliminated if the variance were different.  In such cases, append
   *  an additional explanatory message.
   *
   *  TODO: handle type aliases better.
   */
  def explainVariance(found: Type, req: Type): String = {
    found.baseTypeSeq.toList foreach { tp =>
      if (tp.typeSymbol isSubClass req.typeSymbol) {
        val foundArgs = tp.typeArgs
        val reqArgs   = req.typeArgs
        val params    = req.typeConstructor.typeParams

        if (foundArgs.nonEmpty && foundArgs.length == reqArgs.length) {
          val relationships = (foundArgs, reqArgs, params).zipped map {
            case (arg, reqArg, param) =>
              def mkMsg(isSubtype: Boolean) = {
                val op      = if (isSubtype) "<:" else ">:"
                val suggest = if (isSubtype) "+" else "-"
                val reqsym  = req.typeSymbol
                def isJava  = reqsym.isJavaDefined
                def isScala = reqsym hasTransOwner ScalaPackageClass

                val explainFound = "%s %s %s%s, but ".format(
                  arg, op, reqArg,
                  // If the message involves a type from the base type sequence rather than the
                  // actual found type, we need to explain why we're talking about it.  Less brute
                  // force measures than comparing normalized Strings were producing error messages
                  // like "and java.util.ArrayList[String] <: java.util.ArrayList[String]" but there
                  // should be a cleaner way to do this.
                  if (found.dealiasWiden.toString == tp.dealiasWiden.toString) ""
                  else " (and %s <: %s)".format(found, tp)
                )
                val explainDef = {
                  val prepend = if (isJava) "Java-defined " else ""
                  "%s%s is %s in %s.".format(prepend, reqsym, param.variance, param)
                }
                // Don't suggest they change the class declaration if it's somewhere
                // under scala.* or defined in a java class, because attempting either
                // would be fruitless.
                val suggestChange = "\nYou may wish to " + (
                  if (isScala || isJava)
                    "investigate a wildcard type such as `_ %s %s`. (SLS 3.2.10)".format(op, reqArg)
                  else
                    "define %s as %s%s instead. (SLS 4.5)".format(param.name, suggest, param.name)
                )

                Some("Note: " + explainFound + explainDef + suggestChange)
              }
              // In these cases the arg is OK and needs no explanation.
              val conforms = (
                   (arg =:= reqArg)
                || ((arg <:< reqArg) && param.isCovariant)
                || ((reqArg <:< arg) && param.isContravariant)
              )
              val invariant = param.variance.isInvariant

              if (conforms)                             Some("")
              else if ((arg <:< reqArg) && invariant)   mkMsg(isSubtype = true)   // covariant relationship
              else if ((reqArg <:< arg) && invariant)   mkMsg(isSubtype = false)  // contravariant relationship
              else None // we assume in other cases our ham-fisted advice will merely serve to confuse
          }
          val messages = relationships.flatten
          // the condition verifies no type argument came back None
          if (messages.size == foundArgs.size)
            return messages filterNot (_ == "") mkString ("\n", "\n", "")
        }
      }
    }
    ""    // no elaborable variance situation found
  }

  // For found/required errors where AnyRef would have sufficed:
  // explain in greater detail.
  def explainAnyVsAnyRef(found: Type, req: Type): String = {
    if (AnyRefTpe <:< req) notAnyRefMessage(found) else ""
  }

  // TODO - figure out how to avoid doing any work at all
  // when the message will never be seen.  I though context.reportErrors
  // being false would do that, but if I return "<suppressed>" under
  // that condition, I see it.
  def foundReqMsg(found: Type, req: Type): String = {
    def baseMessage = (
      ";\n found   : " + found.toLongString + existentialContext(found) + explainAlias(found) +
       "\n required: " + req + existentialContext(req) + explainAlias(req)
    )
    (   withDisambiguation(Nil, found, req)(baseMessage)
      + explainVariance(found, req)
      + explainAnyVsAnyRef(found, req)
    )
  }

  def typePatternAdvice(sym: Symbol, ptSym: Symbol) = {
    val clazz = if (sym.isModuleClass) sym.companionClass else sym
    val caseString =
      if (clazz.isCaseClass && (clazz isSubClass ptSym))
        ( clazz.caseFieldAccessors
          map (_ => "_")    // could use the actual param names here
          mkString (s"`case ${clazz.name}(", ",", ")`")
        )
      else
        "`case _: " + (clazz.typeParams match {
          case Nil  => "" + clazz.name
          case xs   => xs map (_ => "_") mkString (clazz.name + "[", ",", "]")
        })+ "`"

    if (!clazz.exists) ""
    else "\nNote: if you intended to match against the class, try "+ caseString
  }

  case class TypeDiag(tp: Type, sym: Symbol) extends Ordered[TypeDiag] {
    // save the name because it will be mutated until it has been
    // distinguished from the other types in the same error message
    private val savedName = sym.name
    def restoreName()     = sym.name = savedName
    def modifyName(f: String => String) = sym setName newTypeName(f(sym.name.toString))

    /** Prepend java.lang, scala., or Predef. if this type originated
     *  in one of those.
     */
    def qualifyDefaultNamespaces() = {
      val intersect = Set(trueOwner, aliasOwner) intersect UnqualifiedOwners
      if (intersect.nonEmpty) preQualify()
    }

    // functions to manipulate the name
    def preQualify()   = modifyName(trueOwner.fullName + "." + _)
    def postQualify()  = modifyName(_ + "(in " + trueOwner + ")")
    def typeQualify()  = if (sym.isTypeParameterOrSkolem) postQualify()
    def nameQualify()  = if (trueOwner.isPackageClass) preQualify() else postQualify()

    def trueOwner  = tp.typeSymbol.effectiveOwner
    def aliasOwner = tp.typeSymbolDirect.effectiveOwner

    def sym_==(other: TypeDiag)     = tp.typeSymbol == other.tp.typeSymbol
    def owner_==(other: TypeDiag)   = trueOwner == other.trueOwner
    def string_==(other: TypeDiag)  = tp.toString == other.tp.toString
    def name_==(other: TypeDiag)    = sym.name == other.sym.name

    def compare(other: TypeDiag) =
      if (this == other) 0
      else if (sym isLess other.sym) -1
      else 1

    override def toString = {
      """
      |tp = %s
      |tp.typeSymbol = %s
      |tp.typeSymbol.owner = %s
      |tp.typeSymbolDirect = %s
      |tp.typeSymbolDirect.owner = %s
      """.stripMargin.format(
        tp, tp.typeSymbol, tp.typeSymbol.owner, tp.typeSymbolDirect, tp.typeSymbolDirect.owner
      )
    }
  }
  /** This is tricky stuff - we need to traverse types deeply to
   *  explain name ambiguities, which may occur anywhere.  However
   *  when lub explosions come through it knocks us into an n^2
   *  disaster, see SI-5580.  This is trying to perform the initial
   *  filtering of possibly ambiguous types in a sufficiently
   *  aggressive way that the state space won't explode.
   */
  private def typeDiags(locals: List[Symbol], types0: Type*): List[TypeDiag] = {
    val types   = types0.toList
    // If two different type diag instances are seen for a given
    // key (either the string representation of a type, or the simple
    // name of a symbol) then keep them for disambiguation.
    val strings = mutable.Map[String, Set[TypeDiag]]() withDefaultValue Set()
    val names   = mutable.Map[Name, Set[TypeDiag]]() withDefaultValue Set()

    val localsSet = locals.toSet

    def record(t: Type, sym: Symbol) = {
      if (!localsSet(sym)) {
        val diag = TypeDiag(t, sym)
        strings("" + t) += diag
        names(sym.name) += diag
      }
    }
    for (tpe <- types ; t <- tpe) {
      t match {
        case ConstantType(_)    => record(t, t.underlying.typeSymbol)
        case TypeRef(_, sym, _) => record(t, sym)
        case _                  => ()
      }
    }

    val collisions = strings.values ++ names.values filter (_.size > 1)
    collisions.flatten.toList
  }

  /** The distinct pairs from an ordered list. */
  private def pairs[T <: Ordered[T]](xs: Seq[T]): Seq[(T, T)] = {
    for (el1 <- xs ; el2 <- xs ; if el1 < el2) yield
      ((el1, el2))
  }

  /** Given any number of types, alters the name information in the symbols
   *  until they can be distinguished from one another: then executes the given
   *  code.  The names are restored and the result is returned.
   */
  def withDisambiguation[T](locals: List[Symbol], types: Type*)(op: => T): T = {
    val typeRefs = typeDiags(locals, types: _*)
    val toCheck  = pairs(typeRefs) filterNot { case (td1, td2) => td1 sym_== td2 }

    ultimately(typeRefs foreach (_.restoreName())) {
      for ((td1, td2) <- toCheck) {
        val tds = List(td1, td2)

        // If the types print identically, qualify them:
        //   a) If the dealiased owner is a package, the full path
        //   b) Otherwise, append (in <owner>)
        if (td1 string_== td2)
          tds foreach (_.nameQualify())

        // If they have the same simple name, and either of them is in the
        // scala package or predef, qualify with scala so it is not confusing why
        // e.g. java.util.Iterator and Iterator are different types.
        if (td1 name_== td2)
          tds foreach (_.qualifyDefaultNamespaces())

        // If they still print identically:
        //   a) If they are type parameters with different owners, append (in <owner>)
        //   b) Failing that, the best we can do is append "(some other)" to the latter.
        if (td1 string_== td2) {
          if (td1 owner_== td2)
            td2.modifyName("(some other)" + _)
          else
            tds foreach (_.typeQualify())
        }
      }
      // performing the actual operation
      op
    }
  }

  trait TyperDiagnostics {
    self: Typer =>

    private def contextError(context0: Analyzer#Context, pos: Position, msg: String) = context0.error(pos, msg)
    private def contextError(context0: Analyzer#Context, pos: Position, err: Throwable) = context0.error(pos, err)
    private def contextWarning(pos: Position, msg: String) = context.unit.warning(pos, msg)

    def permanentlyHiddenWarning(pos: Position, hidden: Name, defn: Symbol) =
      contextWarning(pos, "imported `%s' is permanently hidden by definition of %s".format(hidden, defn.fullLocationString))

    object checkUnused {
      val ignoreNames = Set[TermName]("readResolve", "readObject", "writeObject", "writeReplace")

      class UnusedPrivates extends Traverser {
        val defnTrees = ListBuffer[MemberDef]()
        val targets   = mutable.Set[Symbol]()
        val setVars   = mutable.Set[Symbol]()
        val treeTypes = mutable.Set[Type]()

        def defnSymbols = defnTrees.toList map (_.symbol)
        def localVars   = defnSymbols filter (t => t.isLocalToBlock && t.isVar)

        def qualifiesTerm(sym: Symbol) = (
             (sym.isModule || sym.isMethod || sym.isPrivateLocal || sym.isLocalToBlock)
          && !nme.isLocalName(sym.name)
          && !sym.isParameter
          && !sym.isParamAccessor       // could improve this, but it's a pain
          && !sym.isEarlyInitialized    // lots of false positives in the way these are encoded
          && !(sym.isGetter && sym.accessed.isEarlyInitialized)
        )
        def qualifiesType(sym: Symbol) = !sym.isDefinedInPackage
        def qualifies(sym: Symbol) = (
             (sym ne null)
          && (sym.isTerm && qualifiesTerm(sym) || sym.isType && qualifiesType(sym))
        )

        override def traverse(t: Tree): Unit = {
          t match {
            case t: MemberDef if qualifies(t.symbol)   => defnTrees += t
            case t: RefTree if t.symbol ne null        => targets += t.symbol
            case Assign(lhs, _) if lhs.symbol != null  => setVars += lhs.symbol
            case _                                     =>
          }
          // Only record type references which don't originate within the
          // definition of the class being referenced.
          if (t.tpe ne null) {
            for (tp <- t.tpe ; if !treeTypes(tp) && !currentOwner.ownerChain.contains(tp.typeSymbol)) {
              tp match {
                case NoType | NoPrefix    =>
                case NullaryMethodType(_) =>
                case MethodType(_, _)     =>
                case _                    =>
                  log(s"$tp referenced from $currentOwner")
                  treeTypes += tp
              }
            }
            // e.g. val a = new Foo ; new a.Bar ; don't let a be reported as unused.
            t.tpe.prefix foreach {
              case SingleType(_, sym) => targets += sym
              case _                 =>
            }
          }
          super.traverse(t)
        }
        def isUnusedType(m: Symbol): Boolean = (
              m.isType
          && !m.isTypeParameterOrSkolem // would be nice to improve this
          && (m.isPrivate || m.isLocalToBlock)
          && !(treeTypes.exists(tp => tp exists (t => t.typeSymbolDirect == m)))
        )
        def isUnusedTerm(m: Symbol): Boolean = (
             (m.isTerm)
          && (m.isPrivate || m.isLocalToBlock)
          && !targets(m)
          && !(m.name == nme.WILDCARD)              // e.g. val _ = foo
          && !ignoreNames(m.name.toTermName)        // serialization methods
          && !isConstantType(m.info.resultType)     // subject to constant inlining
          && !treeTypes.exists(_ contains m)        // e.g. val a = new Foo ; new a.Bar
        )
        def unusedTypes = defnTrees.toList filter (t => isUnusedType(t.symbol))
        def unusedTerms = defnTrees.toList filter (v => isUnusedTerm(v.symbol))
        // local vars which are never set, except those already returned in unused
        def unsetVars = localVars filter (v => !setVars(v) && !isUnusedTerm(v))
      }

      def apply(unit: CompilationUnit) = {
        val p = new UnusedPrivates
        p traverse unit.body
        val unused = p.unusedTerms
        unused foreach { defn: DefTree =>
          val sym             = defn.symbol
          val pos = (
            if (defn.pos.isDefined) defn.pos
            else if (sym.pos.isDefined) sym.pos
            else sym match {
              case sym: TermSymbol => sym.referenced.pos
              case _               => NoPosition
            }
          )
          val why = if (sym.isPrivate) "private" else "local"
          val what = (
            if (sym.isDefaultGetter) "default argument"
            else if (sym.isConstructor) "constructor"
            else if (sym.isVar || sym.isGetter && sym.accessed.isVar) "var"
            else if (sym.isVal || sym.isGetter && sym.accessed.isVal) "val"
            else if (sym.isSetter) "setter"
            else if (sym.isMethod) "method"
            else if (sym.isModule) "object"
            else "term"
          )
          unit.warning(pos, s"$why $what in ${sym.owner} is never used")
        }
        p.unsetVars foreach { v =>
          unit.warning(v.pos, s"local var ${v.name} in ${v.owner} is never set - it could be a val")
        }
        p.unusedTypes foreach { t =>
          val sym = t.symbol
          val why = if (sym.isPrivate) "private" else "local"
          unit.warning(t.pos, s"$why ${sym.fullLocationString} is never used")
        }
      }
    }

    object checkDead {
      private val exprStack: mutable.Stack[Symbol] = mutable.Stack(NoSymbol)
      // The method being applied to `tree` when `apply` is called.
      private def expr = exprStack.top

      private def exprOK =
        (expr != Object_synchronized) &&
        !(expr.isLabel && treeInfo.isSynthCaseSymbol(expr)) // it's okay to jump to matchEnd (or another case) with an argument of type nothing

      private def treeOK(tree: Tree) = {
        val isLabelDef = tree match { case _: LabelDef => true; case _ => false}
        tree.tpe != null && tree.tpe.typeSymbol == NothingClass && !isLabelDef
      }

      @inline def updateExpr[A](fn: Tree)(f: => A) = {
        if (fn.symbol != null && fn.symbol.isMethod && !fn.symbol.isConstructor) {
          exprStack push fn.symbol
          try f finally exprStack.pop()
        } else f
      }
      def apply(tree: Tree): Tree = {
        // Error suppression will squash some of these warnings unless we circumvent it.
        // It is presumed if you are using a -Y option you would really like to hear
        // the warnings you've requested.
        if (settings.warnDeadCode && context.unit.exists && treeOK(tree) && exprOK)
          context.warning(tree.pos, "dead code following this construct", force = true)
        tree
      }

      // The checkDead call from typedArg is more selective.
      def inMode(mode: Mode, tree: Tree): Tree = if (mode.typingMonoExprByValue) apply(tree) else tree
    }

    private def symWasOverloaded(sym: Symbol) = sym.owner.isClass && sym.owner.info.member(sym.name).isOverloaded
    private def cyclicAdjective(sym: Symbol)  = if (symWasOverloaded(sym)) "overloaded" else "recursive"

    /** Returns Some(msg) if the given tree is untyped apparently due
     *  to a cyclic reference, and None otherwise.
     */
    def cyclicReferenceMessage(sym: Symbol, tree: Tree) = condOpt(tree) {
      case ValDef(_, _, tpt, _) if tpt.tpe == null        => "recursive "+sym+" needs type"
      case DefDef(_, _, _, _, tpt, _) if tpt.tpe == null  => List(cyclicAdjective(sym), sym, "needs result type") mkString " "
      case Import(expr, selectors)                        =>
        ( "encountered unrecoverable cycle resolving import." +
          "\nNote: this is often due in part to a class depending on a definition nested within its companion." +
          "\nIf applicable, you may wish to try moving some members into another object."
        )
    }

    /** Report a type error.
     *
     *  @param pos    The position where to report the error
     *  @param ex     The exception that caused the error
     */
    def reportTypeError(context0: Context, pos: Position, ex: TypeError) {
      if (ex.pos == NoPosition) ex.pos = pos
      // TODO: should be replaced by throwErrors
      // but it seems that throwErrors excludes some of the errors that should actually be
      // buffered, causing TypeErrors to fly around again. This needs some more investigation.
      if (!context0.reportErrors) throw ex
      if (settings.debug) ex.printStackTrace()

      ex match {
        case CyclicReference(sym, info: TypeCompleter) =>
          if (context0.owner.isTermMacro) {
            // see comments to TypeSigError for an explanation of this special case
            throw ex
          } else {
            val pos = info.tree match {
              case Import(expr, _)  => expr.pos
              case _                => ex.pos
            }
            contextError(context0, pos, cyclicReferenceMessage(sym, info.tree) getOrElse ex.getMessage())

            if (sym == ObjectClass)
              throw new FatalError("cannot redefine root "+sym)
          }
        case _ =>
          contextError(context0, ex.pos, ex)
      }
    }
  }
}