Typer.scala

package dotty.tools
package dotc
package typer

import core._
import ast._
import Trees._
import Constants._
import StdNames._
import Scopes._
import Denotations._
import ProtoTypes._
import Contexts._
import Comments._
import Symbols._
import Types._
import SymDenotations._
import Annotations._
import Names._
import NameOps._
import Flags._
import Decorators._
import ErrorReporting._
import Checking._
import Inferencing._
import EtaExpansion.etaExpand
import dotty.tools.dotc.transform.Erasure.Boxing
import util.Positions._
import util.common._
import util.{SourcePosition, Property}
import collection.mutable
import annotation.tailrec
import Implicits._
import util.Stats.{track, record}
import config.Printers.{typr, gadts}
import rewrite.Rewrites.patch
import NavigateAST._
import transform.SymUtils._
import language.implicitConversions
import printing.SyntaxHighlighting._

object Typer {

  /** The precedence of bindings which determines which of several bindings will be
   *  accessed by an Ident.
   */
  object BindingPrec {
    val definition = 4
    val namedImport = 3
    val wildImport = 2
    val packageClause = 1
    val nothingBound = 0
    def isImportPrec(prec: Int) = prec == namedImport || prec == wildImport
  }

  /** Assert tree has a position, unless it is empty or a typed splice */
  def assertPositioned(tree: untpd.Tree)(implicit ctx: Context) =
    if (!tree.isEmpty && !tree.isInstanceOf[untpd.TypedSplice] && ctx.typerState.isGlobalCommittable)
      assert(tree.pos.exists, s"position not set for $tree # ${tree.uniqueId}")

  private val ExprOwner = new Property.Key[Symbol]
}

class Typer extends Namer with TypeAssigner with Applications with Implicits with Dynamic with Checking with Docstrings {

  import Typer._
  import tpd.{cpy => _, _}
  import untpd.cpy
  import Dynamic.isDynamicMethod
  import reporting.diagnostic.Message
  import reporting.diagnostic.messages._

  /** A temporary data item valid for a single typed ident:
   *  The set of all root import symbols that have been
   *  encountered as a qualifier of an import so far.
   *  Note: It would be more proper to move importedFromRoot into typedIdent.
   *  We should check that this has no performance degradation, however.
   */
  private var unimported: Set[Symbol] = Set()

  /** Temporary data item for single call to typed ident:
   *  This symbol would be found under Scala2 mode, but is not
   *  in dotty (because dotty conforms to spec section 2
   *  wrt to package member resolution but scalac doe not).
   */
  private var foundUnderScala2: Type = NoType

  def newLikeThis: Typer = new Typer

  /** Attribute an identifier consisting of a simple name or wildcard
   *
   *  @param tree      The tree representing the identifier.
   *  Transformations: (1) Prefix class members with this.
   *                   (2) Change imported symbols to selections.
   *                   (3) Change pattern Idents id (but not wildcards) to id @ _
   */
  def typedIdent(tree: untpd.Ident, pt: Type)(implicit ctx: Context): Tree = track("typedIdent") {
    val refctx = ctx
    val name = tree.name
    val noImports = ctx.mode.is(Mode.InPackageClauseName)

    /** Method is necessary because error messages need to bind to
     *  to typedIdent's context which is lost in nested calls to findRef
     */
    def error(msg: => Message, pos: Position) = ctx.error(msg, pos)

    /** Does this identifier appear as a constructor of a pattern? */
    def isPatternConstr =
      if (ctx.mode.isExpr && (ctx.outer.mode is Mode.Pattern))
        ctx.outer.tree match {
          case Apply(`tree`, _) => true
          case _ => false
        }
      else false

    /** A symbol qualifies if it really exists and is not a package class.
     *  In addition, if we are in a constructor of a pattern, we ignore all definitions
     *  which are methods and not accessors (note: if we don't do that
     *  case x :: xs in class List would return the :: method).
     *
     *  Package classes are part of their parent's scope, because otherwise
     *  we could not reload them via `_.member`. On the other hand, accessing a
     *  package as a type from source is always an error.
     */
    def qualifies(denot: Denotation): Boolean =
      reallyExists(denot) &&
        !(pt.isInstanceOf[UnapplySelectionProto] &&
          (denot.symbol is (Method, butNot = Accessor))) &&
        !(denot.symbol is PackageClass)

    /** Find the denotation of enclosing `name` in given context `ctx`.
     *  @param previous    A denotation that was found in a more deeply nested scope,
     *                     or else `NoDenotation` if nothing was found yet.
     *  @param prevPrec    The binding precedence of the previous denotation,
     *                     or else `nothingBound` if nothing was found yet.
     *  @param prevCtx     The context of the previous denotation,
     *                     or else `NoContext` if nothing was found yet.
     */
    def findRef(previous: Type, prevPrec: Int, prevCtx: Context)(implicit ctx: Context): Type = {
      import BindingPrec._

      /** A string which explains how something was bound; Depending on `prec` this is either
       *      imported by <tree>
       *  or  defined in <symbol>
       */
      def bindingString(prec: Int, whereFound: Context, qualifier: String = "") =
        if (prec == wildImport || prec == namedImport) {
          ex"""imported$qualifier by ${hl"${whereFound.importInfo.toString}"}"""
        } else
          ex"""defined$qualifier in ${hl"${whereFound.owner.toString}"}"""

      /** Check that any previously found result from an inner context
       *  does properly shadow the new one from an outer context.
       *  @param found     The newly found result
       *  @param newPrec   Its precedence
       *  @param scala2pkg Special mode where we check members of the same package, but defined
       *                   in different compilation units under Scala2. If set, and the
       *                   previous and new contexts do not have the same scope, we select
       *                   the previous (inner) definition. This models what scalac does.
       */
      def checkNewOrShadowed(found: Type, newPrec: Int, scala2pkg: Boolean = false)(implicit ctx: Context): Type =
        if (!previous.exists || ctx.typeComparer.isSameRef(previous, found)) found
        else if ((prevCtx.scope eq ctx.scope) &&
                 (newPrec == definition ||
                  newPrec == namedImport && prevPrec == wildImport)) {
          // special cases: definitions beat imports, and named imports beat
          // wildcard imports, provided both are in contexts with same scope
          found
        }
        else {
          if (!scala2pkg && !previous.isError && !found.isError) {
            error(
              ex"""|reference to `$name` is ambiguous
                   |it is both ${bindingString(newPrec, ctx, "")}
                   |and ${bindingString(prevPrec, prevCtx, " subsequently")}""",
              tree.pos)
          }
          previous
        }

      /** The type representing a named import with enclosing name when imported
       *  from given `site` and `selectors`.
       */
      def namedImportRef(imp: ImportInfo)(implicit ctx: Context): Type = {
        val Name = name.toTermName.decode
        def recur(selectors: List[untpd.Tree]): Type = selectors match {
          case selector :: rest =>
            def checkUnambiguous(found: Type) = {
              val other = recur(selectors.tail)
              if (other.exists && found.exists && (found != other))
                error(em"reference to `$name` is ambiguous; it is imported twice in ${ctx.tree}",
                      tree.pos)
              found
            }

            def selection(name: Name) =
              if (imp.sym.isCompleting) {
                ctx.warning(i"cyclic ${imp.sym}, ignored", tree.pos)
                NoType
              }
              else if (unimported.nonEmpty && unimported.contains(imp.site.termSymbol))
                NoType
              else {
                // Pass refctx so that any errors are reported in the context of the
                // reference instead of the
                checkUnambiguous(selectionType(imp.site, name, tree.pos)(refctx))
              }

            selector match {
              case Thicket(fromId :: Ident(Name) :: _) =>
                val Ident(from) = fromId
                selection(if (name.isTypeName) from.toTypeName else from)
              case Ident(Name) =>
                selection(name)
              case _ =>
                recur(rest)
            }
          case nil =>
            NoType
        }
        recur(imp.selectors)
      }

      /** The type representing a wildcard import with enclosing name when imported
       *  from given import info
       */
      def wildImportRef(imp: ImportInfo)(implicit ctx: Context): Type = {
        if (imp.isWildcardImport) {
          val pre = imp.site
          if (!unimported.contains(pre.termSymbol) &&
              !imp.excluded.contains(name.toTermName) &&
              name != nme.CONSTRUCTOR) {
            val denot = pre.member(name).accessibleFrom(pre)(refctx)
            if (reallyExists(denot)) return pre.select(name, denot)
          }
        }
        NoType
      }

      /** Is (some alternative of) the given predenotation `denot`
       *  defined in current compilation unit?
       */
      def isDefinedInCurrentUnit(denot: Denotation)(implicit ctx: Context): Boolean = denot match {
        case MultiDenotation(d1, d2) => isDefinedInCurrentUnit(d1) || isDefinedInCurrentUnit(d2)
        case denot: SingleDenotation => denot.symbol.sourceFile == ctx.source.file
      }

      /** Is `denot` the denotation of a self symbol? */
      def isSelfDenot(denot: Denotation)(implicit ctx: Context) = denot match {
        case denot: SymDenotation => denot is SelfName
        case _ => false
      }

      /** Would import of kind `prec` be not shadowed by a nested higher-precedence definition? */
      def isPossibleImport(prec: Int)(implicit ctx: Context) =
        !noImports &&
        (prevPrec < prec || prevPrec == prec && (prevCtx.scope eq ctx.scope))

      @tailrec def loop(implicit ctx: Context): Type = {
        if (ctx.scope == null) previous
        else {
          val outer = ctx.outer
          var result: Type = NoType

          // find definition
          if ((ctx.scope ne outer.scope) || (ctx.owner ne outer.owner)) {
            val defDenot = ctx.denotNamed(name)
            if (qualifies(defDenot)) {
              val curOwner = ctx.owner
              val found =
                if (isSelfDenot(defDenot)) curOwner.enclosingClass.thisType
                else curOwner.thisType.select(name, defDenot)
              if (!(curOwner is Package) || isDefinedInCurrentUnit(defDenot))
                result = checkNewOrShadowed(found, definition) // no need to go further out, we found highest prec entry
              else {
                if (ctx.scala2Mode && !foundUnderScala2.exists)
                  foundUnderScala2 = checkNewOrShadowed(found, definition, scala2pkg = true)
                if (defDenot.symbol is Package)
                  result = checkNewOrShadowed(previous orElse found, packageClause)
                else if (prevPrec < packageClause)
                  result = findRef(found, packageClause, ctx)(outer)
              }
            }
          }

          if (result.exists) result
          else {  // find import
            val curImport = ctx.importInfo
            def updateUnimported() =
              if (curImport.unimported.exists) unimported += curImport.unimported
            if (ctx.owner.is(Package) && curImport != null && curImport.isRootImport && previous.exists)
              previous // no more conflicts possible in this case
            else if (isPossibleImport(namedImport) && (curImport ne outer.importInfo)) {
              val namedImp = namedImportRef(curImport)
              if (namedImp.exists)
                findRef(checkNewOrShadowed(namedImp, namedImport), namedImport, ctx)(outer)
              else if (isPossibleImport(wildImport) && !curImport.sym.isCompleting) {
                val wildImp = wildImportRef(curImport)
                if (wildImp.exists)
                  findRef(checkNewOrShadowed(wildImp, wildImport), wildImport, ctx)(outer)
                else {
                  updateUnimported()
                  loop(outer)
                }
              }
              else {
                updateUnimported()
                loop(outer)
              }
            }
            else loop(outer)
          }
        }
      }

      loop
    }

    // begin typedIdent
    def kind = if (name.isTermName) "" else "type "
    typr.println(s"typed ident $kind$name in ${ctx.owner}")
    if (ctx.mode is Mode.Pattern) {
      if (name == nme.WILDCARD)
        return tree.withType(pt)
      if (isVarPattern(tree) && name.isTermName)
        return typed(desugar.patternVar(tree), pt)
    }

    val rawType = {
      val saved1 = unimported
      val saved2 = foundUnderScala2
      unimported = Set.empty
      foundUnderScala2 = NoType
      try {
        var found = findRef(NoType, BindingPrec.nothingBound, NoContext)
        if (foundUnderScala2.exists && !(foundUnderScala2 =:= found)) {
          ctx.migrationWarning(
            ex"""Name resolution will change.
              | currently selected                     : $foundUnderScala2
              | in the future, without -language:Scala2: $found""", tree.pos)
          found = foundUnderScala2
        }
        found
      }
      finally {
      	unimported = saved1
      	foundUnderScala2 = saved2
      }
    }

    val ownType =
      if (rawType.exists)
        ensureAccessible(rawType, superAccess = false, tree.pos)
      else
        errorType(new MissingIdent(tree, kind, name.show), tree.pos)

    val tree1 = ownType match {
      case ownType: NamedType if !prefixIsElidable(ownType) =>
        ref(ownType).withPos(tree.pos)
      case _ =>
        tree.withType(ownType)
    }

    checkValue(tree1, pt)
  }

  private def typedSelect(tree: untpd.Select, pt: Type, qual: Tree)(implicit ctx: Context): Select =
    healNonvariant(
      checkValue(assignType(cpy.Select(tree)(qual, tree.name), qual), pt),
      pt)

  /** Let `tree = p.n` where `p: T`. If tree's type is an unsafe instantiation
   *  (see TypeOps#asSeenFrom for how this can happen), rewrite the prefix `p`
   *  to `(p: <unknown skolem of type T>)` and try again with the new (stable)
   *  prefix. If the result has another unsafe instantiation, raise an error.
   */
  private def healNonvariant[T <: Tree](tree: T, pt: Type)(implicit ctx: Context): T  =
    if (ctx.unsafeNonvariant == ctx.runId && tree.tpe.widen.hasUnsafeNonvariant)
      tree match {
        case tree @ Select(qual, _) if !qual.tpe.isStable =>
          val alt = typedSelect(tree, pt, Typed(qual, TypeTree(SkolemType(qual.tpe.widen))))
          typr.println(i"healed type: ${tree.tpe} --> $alt")
          alt.asInstanceOf[T]
        case _ =>
          ctx.error(ex"unsafe instantiation of type ${tree.tpe}", tree.pos)
          tree
      }
    else tree

  def typedSelect(tree: untpd.Select, pt: Type)(implicit ctx: Context): Tree = track("typedSelect") {
    def typeSelectOnTerm(implicit ctx: Context): Tree = {
      val qual1 = typedExpr(tree.qualifier, selectionProto(tree.name, pt, this))
      if (tree.name.isTypeName) checkStable(qual1.tpe, qual1.pos)
      val select = typedSelect(tree, pt, qual1)
      if (select.tpe ne TryDynamicCallType) select
      else if (pt.isInstanceOf[PolyProto] || pt.isInstanceOf[FunProto] || pt == AssignProto) select
      else typedDynamicSelect(tree, Nil, pt)
    }

    def typeSelectOnType(qual: untpd.Tree)(implicit ctx: Context) =
      typedSelect(untpd.cpy.Select(tree)(qual, tree.name.toTypeName), pt)

    def tryJavaSelectOnType(implicit ctx: Context): Tree = tree.qualifier match {
      case Select(qual, name) => typeSelectOnType(untpd.Select(qual, name.toTypeName))
      case Ident(name)        => typeSelectOnType(untpd.Ident(name.toTypeName))
      case _                  => errorTree(tree, "cannot convert to type selection") // will never be printed due to fallback
    }

    def selectWithFallback(fallBack: Context => Tree) =
      tryAlternatively(typeSelectOnTerm(_))(fallBack)

    if (tree.qualifier.isType) {
      val qual1 = typedType(tree.qualifier, selectionProto(tree.name, pt, this))
      assignType(cpy.Select(tree)(qual1, tree.name), qual1)
    }
    else if (ctx.compilationUnit.isJava && tree.name.isTypeName)
      // SI-3120 Java uses the same syntax, A.B, to express selection from the
      // value A and from the type A. We have to try both.
      selectWithFallback(tryJavaSelectOnType(_)) // !!! possibly exponential bcs of qualifier retyping
    else if (tree.name == nme.withFilter && tree.getAttachment(desugar.MaybeFilter).isDefined)
      selectWithFallback {
        implicit ctx =>
          typedSelect(untpd.cpy.Select(tree)(tree.qualifier, nme.filter), pt) // !!! possibly exponential bcs of qualifier retyping
      }
    else
      typeSelectOnTerm(ctx)
  }

  def typedThis(tree: untpd.This)(implicit ctx: Context): Tree = track("typedThis") {
    assignType(tree)
  }

  def typedSuper(tree: untpd.Super, pt: Type)(implicit ctx: Context): Tree = track("typedSuper") {
    val qual1 = typed(tree.qual)
    val inConstrCall = pt match {
      case pt: SelectionProto if pt.name == nme.CONSTRUCTOR => true
      case _ => false
    }
    pt match {
      case pt: SelectionProto if pt.name.isTypeName =>
        qual1 // don't do super references for types; they are meaningless anyway
      case _ =>
        assignType(cpy.Super(tree)(qual1, tree.mix), qual1, inConstrCall)
    }
  }

  def typedLiteral(tree: untpd.Literal)(implicit ctx: Context) = track("typedLiteral") {
    assignType(tree)
  }

  def typedNew(tree: untpd.New, pt: Type)(implicit ctx: Context) = track("typedNew") {
    tree.tpt match {
      case templ: untpd.Template =>
        import untpd._
        templ.parents foreach {
          case parent: RefTree =>
            typedAheadImpl(parent, tree => inferTypeParams(typedType(tree), pt))
          case _ =>
        }
        val x = tpnme.ANON_CLASS
        val clsDef = TypeDef(x, templ).withFlags(Final)
        typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(x), Nil)), pt)
      case _ =>
        var tpt1 = typedType(tree.tpt)
        tpt1 = tpt1.withType(ensureAccessible(tpt1.tpe, superAccess = false, tpt1.pos))
        tpt1.tpe.dealias match {
          case TypeApplications.EtaExpansion(tycon) => tpt1 = tpt1.withType(tycon)
          case _ =>
        }
        checkClassType(tpt1.tpe, tpt1.pos, traitReq = false, stablePrefixReq = true)

        tpt1 match {
          case AppliedTypeTree(_, targs) =>
            for (targ @ TypeBoundsTree(_, _) <- targs)
              ctx.error("type argument must be fully defined", targ.pos)
          case _ =>
        }

        assignType(cpy.New(tree)(tpt1), tpt1)
        // todo in a later phase: checkInstantiatable(cls, tpt1.pos)
    }
  }

  def typedTyped(tree: untpd.Typed, pt: Type)(implicit ctx: Context): Tree = track("typedTyped") {
    /*  Handles three cases:
     *  @param  ifPat    how to handle a pattern (_: T)
     *  @param  ifExpr   how to handle an expression (e: T)
     *  @param  wildName what name `w` to use in the rewriting of
     *                   (x: T) to (x @ (w: T)). This is either `_` or `_*`.
     */
    def cases(ifPat: => Tree, ifExpr: => Tree, wildName: TermName) = tree.expr match {
      case id: untpd.Ident if (ctx.mode is Mode.Pattern) && isVarPattern(id) =>
        if (id.name == nme.WILDCARD || id.name == nme.WILDCARD_STAR) ifPat
        else {
          import untpd._
          typed(Bind(id.name, Typed(Ident(wildName), tree.tpt)).withPos(id.pos), pt)
        }
      case _ => ifExpr
    }
    def ascription(tpt: Tree, isWildcard: Boolean) = {
      val underlyingTreeTpe =
        if (isRepeatedParamType(tpt)) TypeTree(defn.SeqType.appliedTo(pt :: Nil))
        else tpt

      val expr1 =
        if (isRepeatedParamType(tpt)) tree.expr.withType(defn.SeqType.appliedTo(pt :: Nil))
        else if (isWildcard) tree.expr.withType(tpt.tpe)
        else typed(tree.expr, tpt.tpe.widenSkolem)
      assignType(cpy.Typed(tree)(expr1, tpt), underlyingTreeTpe)
    }
    if (untpd.isWildcardStarArg(tree))
      cases(
        ifPat = ascription(TypeTree(defn.RepeatedParamType.appliedTo(pt)), isWildcard = true),
        ifExpr = seqToRepeated(typedExpr(tree.expr, defn.SeqType)),
        wildName = nme.WILDCARD_STAR)
    else {
      def typedTpt = checkSimpleKinded(typedType(tree.tpt))
      def handlePattern: Tree = {
        val tpt1 = typedTpt
        // special case for an abstract type that comes with a class tag
        tpt1.tpe.dealias match {
          case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper =>
            inferImplicit(defn.ClassTagType.appliedTo(tref),
               EmptyTree, tpt1.pos)(ctx.retractMode(Mode.Pattern)) match {
              case SearchSuccess(arg, _, _, _) =>
                return typed(untpd.Apply(untpd.TypedSplice(arg), tree.expr), pt)
              case _ =>
            }
          case _ =>
            if (!ctx.isAfterTyper) tpt1.tpe.<:<(pt)(ctx.addMode(Mode.GADTflexible))
        }
        ascription(tpt1, isWildcard = true)
      }
      cases(
        ifPat = handlePattern,
        ifExpr = ascription(typedTpt, isWildcard = false),
        wildName = nme.WILDCARD)
    }
  }

  def typedNamedArg(tree: untpd.NamedArg, pt: Type)(implicit ctx: Context) = track("typedNamedArg") {
    val arg1 = typed(tree.arg, pt)
    assignType(cpy.NamedArg(tree)(tree.name, arg1), arg1)
  }

  def typedAssign(tree: untpd.Assign, pt: Type)(implicit ctx: Context) = track("typedAssign") {
    tree.lhs match {
      case lhs @ Apply(fn, args) =>
        typed(cpy.Apply(lhs)(untpd.Select(fn, nme.update), args :+ tree.rhs), pt)
      case untpd.TypedSplice(Apply(MaybePoly(Select(fn, app), targs), args)) if app == nme.apply =>
        val rawUpdate: untpd.Tree = untpd.Select(untpd.TypedSplice(fn), nme.update)
        val wrappedUpdate =
          if (targs.isEmpty) rawUpdate
          else untpd.TypeApply(rawUpdate, targs map (untpd.TypedSplice(_)))
        val appliedUpdate = cpy.Apply(fn)(wrappedUpdate, (args map (untpd.TypedSplice(_))) :+ tree.rhs)
        typed(appliedUpdate, pt)
      case lhs =>
        val lhsCore = typedUnadapted(lhs, AssignProto)
        def lhs1 = typed(untpd.TypedSplice(lhsCore))
        def canAssign(sym: Symbol) = // allow assignments from the primary constructor to class fields
          sym.is(Mutable, butNot = Accessor) ||
          ctx.owner.isPrimaryConstructor && !sym.is(Method) && sym.owner == ctx.owner.owner ||
          ctx.owner.name.isTraitSetterName || ctx.owner.isStaticConstructor
        lhsCore.tpe match {
          case ref: TermRef if canAssign(ref.symbol) =>
            assignType(cpy.Assign(tree)(lhs1, typed(tree.rhs, ref.info)))
          case _ =>
            def reassignmentToVal =
              errorTree(cpy.Assign(tree)(lhsCore, typed(tree.rhs, lhs1.tpe.widen)),
                  "reassignment to val")
            lhsCore.tpe match {
              case ref: TermRef => // todo: further conditions to impose on getter?
                val pre = ref.prefix
                val setterName = ref.name.setterName
                val setter = pre.member(setterName)
                lhsCore match {
                  case lhsCore: RefTree if setter.exists =>
                    val setterTypeRaw = pre.select(setterName, setter)
                    val setterType = ensureAccessible(setterTypeRaw, isSuperSelection(lhsCore), tree.pos)
                    val lhs2 = healNonvariant(
                      untpd.rename(lhsCore, setterName).withType(setterType), WildcardType)
                    typedUnadapted(cpy.Apply(tree)(untpd.TypedSplice(lhs2), tree.rhs :: Nil))
                  case _ =>
                    reassignmentToVal
                }
              case TryDynamicCallType =>
                typedDynamicAssign(tree, pt)
              case tpe =>
                reassignmentToVal
            }
        }
    }
  }

  def typedBlockStats(stats: List[untpd.Tree])(implicit ctx: Context): (Context, List[tpd.Tree]) =
    (indexAndAnnotate(stats), typedStats(stats, ctx.owner))

  def typedBlock(tree: untpd.Block, pt: Type)(implicit ctx: Context) = track("typedBlock") {
    val (exprCtx, stats1) = typedBlockStats(tree.stats)
    val ept =
      if (tree.isInstanceOf[untpd.InfixOpBlock])
        // Right-binding infix operations are expanded to InfixBlocks, which may be followed by arguments.
        // Example: `(a /: bs)(op)` expands to `{ val x = a; bs./:(x) } (op)` where `{...}` is an InfixBlock.
        pt
      else pt.notApplied
    val expr1 = typedExpr(tree.expr, ept)(exprCtx)
    ensureNoLocalRefs(
        assignType(cpy.Block(tree)(stats1, expr1), stats1, expr1), pt, localSyms(stats1))
  }

  def escapingRefs(block: Tree, localSyms: => List[Symbol])(implicit ctx: Context): collection.Set[NamedType] = {
    lazy val locals = localSyms.toSet
    block.tpe namedPartsWith (tp => locals.contains(tp.symbol))
  }

  /** Check that expression's type can be expressed without references to locally defined
   *  symbols. The following two remedies are tried before giving up:
   *  1. If the expected type of the expression is fully defined, pick it as the
   *     type of the result expressed by adding a type ascription.
   *  2. If (1) fails, force all type variables so that the block's type is
   *     fully defined and try again.
   */
  protected def ensureNoLocalRefs(tree: Tree, pt: Type, localSyms: => List[Symbol], forcedDefined: Boolean = false)(implicit ctx: Context): Tree = {
    def ascribeType(tree: Tree, pt: Type): Tree = tree match {
      case block @ Block(stats, expr) =>
        val expr1 = ascribeType(expr, pt)
        cpy.Block(block)(stats, expr1) withType expr1.tpe // no assignType here because avoid is redundant
      case _ =>
        Typed(tree, TypeTree(pt.simplified))
    }
    val leaks = escapingRefs(tree, localSyms)
    if (leaks.isEmpty) tree
    else if (isFullyDefined(pt, ForceDegree.none)) ascribeType(tree, pt)
    else if (!forcedDefined) {
      fullyDefinedType(tree.tpe, "block", tree.pos)
      val tree1 = ascribeType(tree, avoid(tree.tpe, localSyms))
      ensureNoLocalRefs(tree1, pt, localSyms, forcedDefined = true)
    } else
      errorTree(tree,
          em"local definition of ${leaks.head.name} escapes as part of expression's type ${tree.tpe}"/*; full type: ${result.tpe.toString}"*/)
  }

  def typedIf(tree: untpd.If, pt: Type)(implicit ctx: Context): Tree = track("typedIf") {
    val cond1 = typed(tree.cond, defn.BooleanType)
    val thenp1 = typed(tree.thenp, pt.notApplied)
    val elsep1 = typed(tree.elsep orElse (untpd.unitLiteral withPos tree.pos), pt.notApplied)
    val thenp2 :: elsep2 :: Nil = harmonize(thenp1 :: elsep1 :: Nil)
    assignType(cpy.If(tree)(cond1, thenp2, elsep2), thenp2, elsep2)
  }

  private def decomposeProtoFunction(pt: Type, defaultArity: Int)(implicit ctx: Context): (List[Type], Type) = pt match {
    case _ if defn.isFunctionType(pt) =>
      // if expected parameter type(s) are wildcards, approximate from below.
      // if expected result type is a wildcard, approximate from above.
      // this can type the greatest set of admissible closures.
      (pt.dealias.argTypesLo.init, pt.dealias.argTypesHi.last)
    case SAMType(meth) =>
      val mt @ MethodType(_, paramTypes) = meth.info
      (paramTypes, mt.resultType)
    case _ =>
      (List.range(0, defaultArity) map alwaysWildcardType, WildcardType)
  }

  def typedFunction(tree: untpd.Function, pt: Type)(implicit ctx: Context) = track("typedFunction") {
    val untpd.Function(args, body) = tree
    if (ctx.mode is Mode.Type) {
      val funCls =
        if (tree.isInstanceOf[untpd.ImplicitFunction]) defn.ImplicitFunctionClass(args.length)
        else defn.FunctionClass(args.length)
      typed(cpy.AppliedTypeTree(tree)(
        untpd.TypeTree(funCls.typeRef), args :+ body), pt)
    }
    else {
      val params = args.asInstanceOf[List[untpd.ValDef]]

      pt match {
        case pt: TypeVar if untpd.isFunctionWithUnknownParamType(tree) =>
          // try to instantiate `pt` if this is possible. If it does not
          // work the error will be reported later in `inferredParam`,
          // when we try to infer the parameter type.
          isFullyDefined(pt, ForceDegree.noBottom)
        case _ =>
      }

      val (protoFormals, protoResult) = decomposeProtoFunction(pt, params.length)

      def refersTo(arg: untpd.Tree, param: untpd.ValDef): Boolean = arg match {
        case Ident(name) => name == param.name
        case _ => false
      }

      /** The function body to be returned in the closure. Can become a TypedSplice
       *  of a typed expression if this is necessary to infer a parameter type.
       */
      var fnBody = tree.body

      /** If function is of the form
       *      (x1, ..., xN) => f(x1, ..., XN)
       *  the type of `f`, otherwise NoType. (updates `fnBody` as a side effect).
       */
      def calleeType: Type = fnBody match {
        case Apply(expr, args) if (args corresponds params)(refersTo) =>
          expr match {
            case untpd.TypedSplice(expr1) =>
              expr1.tpe
            case _ =>
              val protoArgs = args map (_ withType WildcardType)
              val callProto = FunProto(protoArgs, WildcardType, this)
              val expr1 = typedExpr(expr, callProto)
              fnBody = cpy.Apply(fnBody)(untpd.TypedSplice(expr1), args)
              expr1.tpe
          }
        case _ =>
          NoType
      }

      /** Two attempts: First, if expected type is fully defined pick this one.
       *  Second, if function is of the form
       *      (x1, ..., xN) => f(x1, ..., XN)
       *  and f has a method type MT, pick the corresponding parameter type in MT,
       *  if this one is fully defined.
       *  If both attempts fail, issue a "missing parameter type" error.
       */
      def inferredParamType(param: untpd.ValDef, formal: Type): Type = {
        if (isFullyDefined(formal, ForceDegree.noBottom)) return formal
        calleeType.widen match {
          case mtpe: MethodType =>
            val pos = params indexWhere (_.name == param.name)
            if (pos < mtpe.paramTypes.length) {
              val ptype = mtpe.paramTypes(pos)
              if (isFullyDefined(ptype, ForceDegree.noBottom)) return ptype
            }
          case _ =>
        }
        val ofFun =
          if (nme.syntheticParamNames(args.length + 1) contains param.name)
            i" of expanded function $tree"
          else
            ""
        errorType(i"missing parameter type for parameter ${param.name}$ofFun, expected = $pt", param.pos)
      }

      def protoFormal(i: Int): Type =
        if (protoFormals.length == params.length) protoFormals(i)
        else errorType(i"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)

      /** Is `formal` a product type which is elementwise compatible with `params`? */
      def ptIsCorrectProduct(formal: Type) = {
        val pclass = defn.ProductNType(params.length).symbol
        isFullyDefined(formal, ForceDegree.noBottom) &&
        formal.derivesFrom(pclass) &&
        formal.baseArgTypes(pclass).corresponds(params) {
          (argType, param) =>
            param.tpt.isEmpty || argType <:< typedAheadType(param.tpt).tpe
        }
      }

      val desugared =
        if (protoFormals.length == 1 && params.length != 1 && ptIsCorrectProduct(protoFormals.head)) {
          desugar.makeTupledFunction(params, fnBody)
        }
        else {
          val inferredParams: List[untpd.ValDef] =
            for ((param, i) <- params.zipWithIndex) yield
              if (!param.tpt.isEmpty) param
              else cpy.ValDef(param)(
                tpt = untpd.TypeTree(
                  inferredParamType(param, protoFormal(i)).underlyingIfRepeated(isJava = false)))

          // Define result type of closure as the expected type, thereby pushing
          // down any implicit searches. We do this even if the expected type is not fully
          // defined, which is a bit of a hack. But it's needed to make the following work
          // (see typers.scala and printers/PlainPrinter.scala for examples).
          //
          //     def double(x: Char): String = s"$x$x"
          //     "abc" flatMap double
          //
          val resultTpt = protoResult match {
            case WildcardType(_) => untpd.TypeTree()
            case _ => untpd.TypeTree(protoResult)
          }
          val inlineable = pt.hasAnnotation(defn.InlineParamAnnot)
          desugar.makeClosure(inferredParams, fnBody, resultTpt, inlineable)
        }
      typed(desugared, pt)
    }
  }

  def typedClosure(tree: untpd.Closure, pt: Type)(implicit ctx: Context): Tree = track("typedClosure") {
    val env1 = tree.env mapconserve (typed(_))
    val meth1 = typedUnadapted(tree.meth)
    val target =
      if (tree.tpt.isEmpty)
        meth1.tpe.widen match {
          case mt: MethodType =>
            pt match {
              case SAMType(meth) if !defn.isFunctionType(pt) && mt <:< meth.info =>
                if (!isFullyDefined(pt, ForceDegree.all))
                  ctx.error(ex"result type of closure is an underspecified SAM type $pt", tree.pos)
                TypeTree(pt)
              case _ =>
                if (!mt.isDependent) EmptyTree
                else throw new java.lang.Error(i"internal error: cannot turn dependent method type $mt into closure, position = ${tree.pos}, raw type = ${mt.toString}") // !!! DEBUG. Eventually, convert to an error?
            }
          case tp =>
            throw new java.lang.Error(i"internal error: closing over non-method $tp, pos = ${tree.pos}")
        }
      else typed(tree.tpt)
    //println(i"typing closure $tree : ${meth1.tpe.widen}")
    assignType(cpy.Closure(tree)(env1, meth1, target), meth1, target)
  }

  def typedMatch(tree: untpd.Match, pt: Type)(implicit ctx: Context) = track("typedMatch") {
    tree.selector match {
      case EmptyTree =>
        val (protoFormals, _) = decomposeProtoFunction(pt, 1)
        val unchecked = pt <:< defn.PartialFunctionType
        typed(desugar.makeCaseLambda(tree.cases, protoFormals.length, unchecked) withPos tree.pos, pt)
      case _ =>
        val sel1 = typedExpr(tree.selector)
        val selType = widenForMatchSelector(
            fullyDefinedType(sel1.tpe, "pattern selector", tree.pos))

        val cases1 = typedCases(tree.cases, selType, pt.notApplied)
        val cases2 = harmonize(cases1).asInstanceOf[List[CaseDef]]
        assignType(cpy.Match(tree)(sel1, cases2), cases2)
    }
  }

  def typedCases(cases: List[untpd.CaseDef], selType: Type, pt: Type)(implicit ctx: Context) = {

    /** gadtSyms = "all type parameters of enclosing methods that appear
     *              non-variantly in the selector type" todo: should typevars
     *              which appear with variances +1 and -1 (in different
     *              places) be considered as well?
     */
    val gadtSyms: Set[Symbol] = ctx.traceIndented(i"GADT syms of $selType", gadts) {
      val accu = new TypeAccumulator[Set[Symbol]] {
        def apply(tsyms: Set[Symbol], t: Type): Set[Symbol] = {
          val tsyms1 = t match {
            case tr: TypeRef if (tr.symbol is TypeParam) && tr.symbol.owner.isTerm && variance == 0 =>
              tsyms + tr.symbol
            case _ =>
              tsyms
          }
          foldOver(tsyms1, t)
        }
      }
      accu(Set.empty, selType)
    }

    cases mapconserve (typedCase(_, pt, selType, gadtSyms))
  }

  /** Type a case. Overridden in ReTyper, that's why it's separate from
   *  typedCases.
   */
  def typedCase(tree: untpd.CaseDef, pt: Type, selType: Type, gadtSyms: Set[Symbol])(implicit ctx: Context): CaseDef = track("typedCase") {
    val originalCtx = ctx

    /** - replace all references to symbols associated with wildcards by their GADT bounds
     *  - enter all symbols introduced by a Bind in current scope
     */
    val indexPattern = new TreeMap {
      val elimWildcardSym = new TypeMap {
        def apply(t: Type) = t match {
          case ref @ TypeRef(_, tpnme.WILDCARD) if ctx.gadt.bounds.contains(ref.symbol) =>
            ctx.gadt.bounds(ref.symbol)
          case TypeAlias(ref @ TypeRef(_, tpnme.WILDCARD)) if ctx.gadt.bounds.contains(ref.symbol) =>
            ctx.gadt.bounds(ref.symbol)
          case _ =>
            mapOver(t)
        }
      }
      override def transform(trt: Tree)(implicit ctx: Context) =
        super.transform(trt.withType(elimWildcardSym(trt.tpe))) match {
          case b: Bind =>
            if (ctx.scope.lookup(b.name) == NoSymbol) ctx.enter(b.symbol)
            else ctx.error(new DuplicateBind(b, tree), b.pos)
            b.symbol.info = elimWildcardSym(b.symbol.info)
            b
          case t => t
        }
    }

    def caseRest(pat: Tree)(implicit ctx: Context) = {
      val pat1 = indexPattern.transform(pat)
      val guard1 = typedExpr(tree.guard, defn.BooleanType)
      val body1 = ensureNoLocalRefs(typedExpr(tree.body, pt), pt, ctx.scope.toList)
        .ensureConforms(pt)(originalCtx) // insert a cast if body does not conform to expected type if we disregard gadt bounds
      assignType(cpy.CaseDef(tree)(pat1, guard1, body1), body1)
    }

    val gadtCtx =
      if (gadtSyms.isEmpty) ctx
      else {
        val c = ctx.fresh.setFreshGADTBounds
        for (sym <- gadtSyms)
          if (!c.gadt.bounds.contains(sym))
            c.gadt.setBounds(sym, TypeBounds.empty)
        c
      }
    val pat1 = typedPattern(tree.pat, selType)(gadtCtx)
    caseRest(pat1)(gadtCtx.fresh.setNewScope)
  }

  def typedReturn(tree: untpd.Return)(implicit ctx: Context): Return = track("typedReturn") {
    def returnProto(owner: Symbol, locals: Scope): Type =
      if (owner.isConstructor) defn.UnitType
      else owner.info match {
        case info: PolyType =>
          val tparams = locals.toList.takeWhile(_ is TypeParam)
          assert(info.paramNames.length == tparams.length,
                 i"return mismatch from $owner, tparams = $tparams, locals = ${locals.toList}%, %")
          info.instantiate(tparams.map(_.typeRef)).finalResultType
        case info =>
          info.finalResultType
      }
    def enclMethInfo(cx: Context): (Tree, Type) = {
      val owner = cx.owner
      if (cx == NoContext || owner.isType) {
        ctx.error("return outside method definition", tree.pos)
        (EmptyTree, WildcardType)
      }
      else if (owner != cx.outer.owner && owner.isRealMethod) {
        if (owner.isInlineMethod)
          (EmptyTree, errorType(em"no explicit return allowed from inline $owner", tree.pos))
        else if (!owner.isCompleted)
          (EmptyTree, errorType(em"$owner has return statement; needs result type", tree.pos))
        else {
          val from = Ident(TermRef(NoPrefix, owner.asTerm))
          val proto = returnProto(owner, cx.scope)
          (from, proto)
        }
      }
      else enclMethInfo(cx.outer)
    }
    val (from, proto) =
      if (tree.from.isEmpty) enclMethInfo(ctx)
      else {
        val from = tree.from.asInstanceOf[tpd.Tree]
        val proto =
          if (ctx.erasedTypes) from.symbol.info.finalResultType
          else WildcardType // We cannot reliably detect the internal type view of polymorphic or dependent methods
                            // because we do not know the internal type params and method params.
                            // Hence no adaptation is possible, and we assume WildcardType as prototype.
        (from, proto)
      }
    val expr1 = typedExpr(tree.expr orElse untpd.unitLiteral.withPos(tree.pos), proto)
    assignType(cpy.Return(tree)(expr1, from))
  }

  def typedTry(tree: untpd.Try, pt: Type)(implicit ctx: Context): Try = track("typedTry") {
    val expr1 = typed(tree.expr, pt.notApplied)
    val cases1 = typedCases(tree.cases, defn.ThrowableType, pt.notApplied)
    val finalizer1 = typed(tree.finalizer, defn.UnitType)
    val expr2 :: cases2x = harmonize(expr1 :: cases1)
    val cases2 = cases2x.asInstanceOf[List[CaseDef]]
    assignType(cpy.Try(tree)(expr2, cases2, finalizer1), expr2, cases2)
  }

  def typedThrow(tree: untpd.Throw)(implicit ctx: Context): Tree = track("typedThrow") {
    val expr1 = typed(tree.expr, defn.ThrowableType)
    Throw(expr1).withPos(tree.pos)
  }

  def typedSeqLiteral(tree: untpd.SeqLiteral, pt: Type)(implicit ctx: Context): SeqLiteral = track("typedSeqLiteral") {
    val proto1 = pt.elemType match {
      case NoType => WildcardType
      case bounds: TypeBounds => WildcardType(bounds)
      case elemtp => elemtp
    }
    val elems1 = tree.elems mapconserve (typed(_, proto1))
    val proto2 = // the computed type of the `elemtpt` field
      if (!tree.elemtpt.isEmpty) WildcardType
      else if (isFullyDefined(proto1, ForceDegree.none)) proto1
      else if (tree.elems.isEmpty && tree.isInstanceOf[Trees.JavaSeqLiteral[_]])
        defn.ObjectType // generic empty Java varargs are of type Object[]
      else ctx.typeComparer.lub(elems1.tpes)
    val elemtpt1 = typed(tree.elemtpt, proto2)
    assignType(cpy.SeqLiteral(tree)(elems1, elemtpt1), elems1, elemtpt1)
  }

  def typedInlined(tree: untpd.Inlined, pt: Type)(implicit ctx: Context): Inlined = {
    val (exprCtx, bindings1) = typedBlockStats(tree.bindings)
    val expansion1 = typed(tree.expansion, pt)(inlineContext(tree.call)(exprCtx))
    assignType(cpy.Inlined(tree)(tree.call, bindings1.asInstanceOf[List[MemberDef]], expansion1),
        bindings1, expansion1)
  }

  def typedTypeTree(tree: untpd.TypeTree, pt: Type)(implicit ctx: Context): TypeTree = track("typedTypeTree") {
    tree match {
      case tree: untpd.DerivedTypeTree =>
        tree.ensureCompletions
        try
          TypeTree(tree.derivedType(tree.attachment(untpd.OriginalSymbol))) withPos tree.pos
        // btw, no need to remove the attachment. The typed
        // tree is different from the untyped one, so the
        // untyped tree is no longer accessed after all
        // accesses with typedTypeTree are done.
        catch {
          case ex: NoSuchElementException =>
            println(s"missing OriginalSymbol for ${ctx.owner.ownersIterator.toList}")
            throw ex
        }
      case _ =>
        assert(isFullyDefined(pt, ForceDegree.none))
        tree.withType(pt)
    }
  }

  def typedSingletonTypeTree(tree: untpd.SingletonTypeTree)(implicit ctx: Context): SingletonTypeTree = track("typedSingletonTypeTree") {
    val ref1 = typedExpr(tree.ref)
    checkStable(ref1.tpe, tree.pos)
    assignType(cpy.SingletonTypeTree(tree)(ref1), ref1)
  }

  def typedAndTypeTree(tree: untpd.AndTypeTree)(implicit ctx: Context): AndTypeTree = track("typedAndTypeTree") {
    val left1 = typed(tree.left)
    val right1 = typed(tree.right)
    assignType(cpy.AndTypeTree(tree)(left1, right1), left1, right1)
  }

  def typedOrTypeTree(tree: untpd.OrTypeTree)(implicit ctx: Context): OrTypeTree = track("typedOrTypeTree") {
    val where = "in a union type"
    val left1 = checkNotSingleton(typed(tree.left), where)
    val right1 = checkNotSingleton(typed(tree.right), where)
    assignType(cpy.OrTypeTree(tree)(left1, right1), left1, right1)
  }

  def typedRefinedTypeTree(tree: untpd.RefinedTypeTree)(implicit ctx: Context): RefinedTypeTree = track("typedRefinedTypeTree") {
    val tpt1 = if (tree.tpt.isEmpty) TypeTree(defn.ObjectType) else typedAheadType(tree.tpt)
    val refineClsDef = desugar.refinedTypeToClass(tpt1, tree.refinements)
    val refineCls = createSymbol(refineClsDef).asClass
    val TypeDef(_, impl: Template) = typed(refineClsDef)
    val refinements1 = impl.body
    assert(tree.refinements.length == refinements1.length, s"${tree.refinements} != $refinements1")
    val seen = mutable.Set[Symbol]()
    for (refinement <- refinements1) { // TODO: get clarity whether we want to enforce these conditions
      typr.println(s"adding refinement $refinement")
      checkRefinementNonCyclic(refinement, refineCls, seen)
      val rsym = refinement.symbol
      if (rsym.info.isInstanceOf[PolyType] && rsym.allOverriddenSymbols.isEmpty)
        ctx.error(i"polymorphic refinement $rsym without matching type in parent $tpt1 is no longer allowed", refinement.pos)    }
    assignType(cpy.RefinedTypeTree(tree)(tpt1, refinements1), tpt1, refinements1, refineCls)
  }

  def typedAppliedTypeTree(tree: untpd.AppliedTypeTree)(implicit ctx: Context): Tree = track("typedAppliedTypeTree") {
    val tpt1 = typed(tree.tpt, AnyTypeConstructorProto)(ctx.retractMode(Mode.Pattern))
    val tparams = tpt1.tpe.typeParams
    if (tparams.isEmpty) {
      ctx.error(ex"${tpt1.tpe} does not take type parameters", tree.pos)
      tpt1
    }
    else {
      var args = tree.args
      val args1 =
        if (hasNamedArg(args)) typedNamedArgs(args)
        else {
          if (args.length != tparams.length) {
            wrongNumberOfTypeArgs(tpt1.tpe, tparams, args, tree.pos)
            args = args.take(tparams.length)
          }
          def typedArg(arg: untpd.Tree, tparam: TypeParamInfo) = {
            val (desugaredArg, argPt) =
              if (ctx.mode is Mode.Pattern)
                (if (isVarPattern(arg)) desugar.patternVar(arg) else arg, tparam.paramBounds)
              else
                (arg, WildcardType)
            typed(desugaredArg, argPt)
          }
          args.zipWithConserve(tparams)(typedArg(_, _)).asInstanceOf[List[Tree]]
        }
      // check that arguments conform to bounds is done in phase PostTyper
      assignType(cpy.AppliedTypeTree(tree)(tpt1, args1), tpt1, args1)
    }
  }

  def typedPolyTypeTree(tree: untpd.PolyTypeTree)(implicit ctx: Context): Tree = track("typedPolyTypeTree") {
    val PolyTypeTree(tparams, body) = tree
    indexAndAnnotate(tparams)
    val tparams1 = tparams.mapconserve(typed(_).asInstanceOf[TypeDef])
    val body1 = typedType(tree.body)
    assignType(cpy.PolyTypeTree(tree)(tparams1, body1), tparams1, body1)
  }

  def typedByNameTypeTree(tree: untpd.ByNameTypeTree)(implicit ctx: Context): ByNameTypeTree = track("typedByNameTypeTree") {
    val result1 = typed(tree.result)
    assignType(cpy.ByNameTypeTree(tree)(result1), result1)
  }

  /** Define a new symbol associated with a Bind or pattern wildcard and
   *  make it gadt narrowable.
   */
  private def newPatternBoundSym(name: Name, info: Type, pos: Position)(implicit ctx: Context) = {
    val flags = if (name.isTypeName) BindDefinedType else EmptyFlags
    val sym = ctx.newSymbol(ctx.owner, name, flags | Case, info, coord = pos)
    if (name.isTypeName) ctx.gadt.setBounds(sym, info.bounds)
    sym
  }

  def typedTypeBoundsTree(tree: untpd.TypeBoundsTree)(implicit ctx: Context): TypeBoundsTree = track("typedTypeBoundsTree") {
    val TypeBoundsTree(lo, hi) = desugar.typeBoundsTree(tree)
    val lo1 = typed(lo)
    val hi1 = typed(hi)
    val tree1 = assignType(cpy.TypeBoundsTree(tree)(lo1, hi1), lo1, hi1)
    if (ctx.mode.is(Mode.Pattern)) {
      // Associate a pattern-bound type symbol with the wildcard.
      // The bounds of the type symbol can be constrained when comparing a pattern type
      // with an expected type in typedTyped. The type symbol is eliminated once
      // the enclosing pattern has been typechecked; see `indexPattern` in `typedCase`.
      val wildcardSym = newPatternBoundSym(tpnme.WILDCARD, tree1.tpe, tree.pos)
      tree1.withType(wildcardSym.typeRef)
    }
    else tree1
  }

  def typedBind(tree: untpd.Bind, pt: Type)(implicit ctx: Context): Tree = track("typedBind") {
    val pt1 = fullyDefinedType(pt, "pattern variable", tree.pos)
    val body1 = typed(tree.body, pt1)
    typr.println(i"typed bind $tree pt = $pt1 bodytpe = ${body1.tpe}")
    body1 match {
      case UnApply(fn, Nil, arg :: Nil) if tree.body.isInstanceOf[untpd.Typed] =>
        // A typed pattern `x @ (_: T)` with an implicit `ctag: ClassTag[T]`
        // was rewritten to `x @ ctag(_)`.
        // Rewrite further to `ctag(x @ _)`
        assert(fn.symbol.owner == defn.ClassTagClass)
        tpd.cpy.UnApply(body1)(fn, Nil,
            typed(untpd.Bind(tree.name, arg).withPos(tree.pos), arg.tpe) :: Nil)
      case _ =>
        val sym = newPatternBoundSym(tree.name, body1.tpe, tree.pos)
        assignType(cpy.Bind(tree)(tree.name, body1), sym)
    }
  }

  def typedAlternative(tree: untpd.Alternative, pt: Type)(implicit ctx: Context): Alternative = track("typedAlternative") {
    val trees1 = tree.trees mapconserve (typed(_, pt))
    assignType(cpy.Alternative(tree)(trees1), trees1)
  }

  def completeAnnotations(mdef: untpd.MemberDef, sym: Symbol)(implicit ctx: Context): Unit = {
    // necessary to force annotation trees to be computed.
    sym.annotations.foreach(_.ensureCompleted)
    lazy val annotCtx = {
      val c = ctx.outersIterator.dropWhile(_.owner == sym).next
      c.property(ExprOwner) match {
        case Some(exprOwner) if c.owner.isClass =>
          // We need to evaluate annotation arguments in an expression context, since
          // classes defined in a such arguments should not be entered into the
          // enclosing class.
          c.exprContext(mdef, exprOwner)
        case _ => c
      }
    }
    // necessary in order to mark the typed ahead annotations as definitely typed:
    untpd.modsDeco(mdef).mods.annotations.foreach(typedAnnotation(_)(annotCtx))
  }

  def typedAnnotation(annot: untpd.Tree)(implicit ctx: Context): Tree = track("typedAnnotation") {
    typed(annot, defn.AnnotationType)
  }

  def typedValDef(vdef: untpd.ValDef, sym: Symbol)(implicit ctx: Context) = track("typedValDef") {
    val ValDef(name, tpt, _) = vdef
    completeAnnotations(vdef, sym)
    val tpt1 = checkSimpleKinded(typedType(tpt))
    val rhs1 = vdef.rhs match {
      case rhs @ Ident(nme.WILDCARD) => rhs withType tpt1.tpe
      case rhs => typedExpr(rhs, tpt1.tpe)
    }
    val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
    if (sym.is(Inline, butNot = DeferredOrParamAccessor))
      checkInlineConformant(rhs1, em"right-hand side of inline $sym")
    patchIfLazy(vdef1)
    vdef1
  }

  /** Add a @volitile to lazy vals when rewriting from Scala2 */
  private def patchIfLazy(vdef: ValDef)(implicit ctx: Context): Unit = {
    val sym = vdef.symbol
    if (sym.is(Lazy, butNot = Deferred | Module | Synthetic) && !sym.isVolatile &&
        ctx.scala2Mode && ctx.settings.rewrite.value.isDefined &&
        !ctx.isAfterTyper)
      patch(Position(toUntyped(vdef).pos.start), "@volatile ")
  }

  def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(implicit ctx: Context) = track("typedDefDef") {
    val DefDef(name, tparams, vparamss, tpt, _) = ddef
    completeAnnotations(ddef, sym)
    val tparams1 = tparams mapconserve (typed(_).asInstanceOf[TypeDef])
    val vparamss1 = vparamss nestedMapconserve (typed(_).asInstanceOf[ValDef])
    if (sym is Implicit) checkImplicitParamsNotSingletons(vparamss1)
    var tpt1 = checkSimpleKinded(typedType(tpt))

    var rhsCtx = ctx
    if (sym.isConstructor && !sym.isPrimaryConstructor && tparams1.nonEmpty) {
      // for secondary constructors we need a context that "knows"
      // that their type parameters are aliases of the class type parameters.
      // See pos/i941.scala
      rhsCtx = ctx.fresh.setFreshGADTBounds
      (tparams1, sym.owner.typeParams).zipped.foreach ((tdef, tparam) =>
        rhsCtx.gadt.setBounds(tdef.symbol, TypeAlias(tparam.typeRef)))
    }
    val rhs1 = typedExpr(ddef.rhs, tpt1.tpe)(rhsCtx)

    // Overwrite inline body to make sure it is not evaluated twice
    if (sym.isInlineMethod) Inliner.registerInlineInfo(sym, _ => rhs1)

    if (sym.isAnonymousFunction) {
      // If we define an anonymous function, make sure the return type does not
      // refer to parameters. This is necessary because closure types are
      // function types so no dependencies on parameters are allowed.
      tpt1 = tpt1.withType(avoid(tpt1.tpe, vparamss1.flatMap(_.map(_.symbol))))
    }

    assignType(cpy.DefDef(ddef)(name, tparams1, vparamss1, tpt1, rhs1), sym)
    //todo: make sure dependent method types do not depend on implicits or by-name params
  }

  def typedTypeDef(tdef: untpd.TypeDef, sym: Symbol)(implicit ctx: Context): Tree = track("typedTypeDef") {
    val TypeDef(name, rhs) = tdef
    completeAnnotations(tdef, sym)
    val rhs1 = tdef.rhs match {
      case rhs @ PolyTypeTree(tparams, body) =>
        val tparams1 = tparams.map(typed(_)).asInstanceOf[List[TypeDef]]
        val body1 = typedType(body)
        assignType(cpy.PolyTypeTree(rhs)(tparams1, body1), tparams1, body1)
      case rhs =>
        typedType(rhs)
    }
    assignType(cpy.TypeDef(tdef)(name, rhs1), sym)
  }

  def typedClassDef(cdef: untpd.TypeDef, cls: ClassSymbol)(implicit ctx: Context) = track("typedClassDef") {
    val TypeDef(name, impl @ Template(constr, parents, self, _)) = cdef
    val superCtx = ctx.superCallContext

    /** If `ref` is an implicitly parameterized trait, pass an implicit argument list.
     *  Otherwise, if `ref` is a parameterized trait, error.
     *  Note: Traits and classes currently always have at least an empty parameter list ()
     *        before the implicit parameters (this is inserted if not given in source).
     *        We skip this parameter list when deciding whether a trait is parameterless or not.
     *  @param ref   The tree referring to the (parent) trait
     *  @param psym  Its type symbol
     *  @param cinfo The info of its constructor
     */
    def maybeCall(ref: Tree, psym: Symbol, cinfo: Type): Tree = cinfo match {
      case cinfo: PolyType =>
        maybeCall(ref, psym, cinfo.resultType)
      case cinfo @ MethodType(Nil, _) if cinfo.resultType.isInstanceOf[ImplicitMethodType] =>
        val icall = New(ref).select(nme.CONSTRUCTOR).appliedToNone
        typedExpr(untpd.TypedSplice(icall))(superCtx)
      case cinfo @ MethodType(Nil, _) if !cinfo.resultType.isInstanceOf[MethodType] =>
        ref
      case cinfo: MethodType =>
        if (!ctx.erasedTypes) { // after constructors arguments are passed in super call.
          typr.println(i"constr type: $cinfo")
          ctx.error(em"parameterized $psym lacks argument list", ref.pos)
        }
        ref
      case _ =>
        ref
    }

    def typedParent(tree: untpd.Tree): Tree =
      if (tree.isType) {
        val result = typedType(tree)(superCtx)
        val psym = result.tpe.typeSymbol
        if (psym.is(Trait) && !cls.is(Trait) && !cls.superClass.isSubClass(psym))
          maybeCall(result, psym, psym.primaryConstructor.info)
        else
          result
      }
      else {
        val result = typedExpr(tree)(superCtx)
        checkParentCall(result, cls)
        result
      }

    completeAnnotations(cdef, cls)
    val constr1 = typed(constr).asInstanceOf[DefDef]
    val parentsWithClass = ensureFirstIsClass(parents mapconserve typedParent, cdef.pos.toSynthetic)
    val parents1 = ensureConstrCall(cls, parentsWithClass)(superCtx)
    val self1 = typed(self)(ctx.outer).asInstanceOf[ValDef] // outer context where class members are not visible
    val dummy = localDummy(cls, impl)
    val body1 = typedStats(impl.body, dummy)(inClassContext(self1.symbol))

    // Expand comments and type usecases
    cookComments(body1.map(_.symbol), self1.symbol)(localContext(cdef, cls).setNewScope)

    checkNoDoubleDefs(cls)
    val impl1 = cpy.Template(impl)(constr1, parents1, self1, body1)
      .withType(dummy.nonMemberTermRef)
    checkVariance(impl1)
    if (!cls.is(AbstractOrTrait) && !ctx.isAfterTyper) checkRealizableBounds(cls.typeRef, cdef.namePos)
    val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1), cls)
    if (ctx.phase.isTyper && cdef1.tpe.derivesFrom(defn.DynamicClass) && !ctx.dynamicsEnabled) {
      val isRequired = parents1.exists(_.tpe.isRef(defn.DynamicClass))
      ctx.featureWarning(nme.dynamics.toString, "extension of type scala.Dynamic", isScala2Feature = true,
          cls, isRequired, cdef.pos)
    }

    // check value class constraints
    checkDerivedValueClass(cls, body1)

    cdef1

    // todo later: check that
    //  1. If class is non-abstract, it is instantiatable:
    //  - self type is s supertype of own type
    //  - all type members have consistent bounds
    // 2. all private type members have consistent bounds
    // 3. Types do not override classes.
    // 4. Polymorphic type defs override nothing.
  }

  /** Ensure that the first type in a list of parent types Ps points to a non-trait class.
   *  If that's not already the case, add one. The added class type CT is determined as follows.
   *  First, let C be the unique class such that
   *  - there is a parent P_i such that P_i derives from C, and
   *  - for every class D: If some parent P_j, j <= i derives from D, then C derives from D.
   *  Then, let CT be the smallest type which
   *  - has C as its class symbol, and
   *  - for all parents P_i: If P_i derives from C then P_i <:< CT.
   */
  def ensureFirstIsClass(parents: List[Type])(implicit ctx: Context): List[Type] = {
    def realClassParent(cls: Symbol): ClassSymbol =
      if (!cls.isClass) defn.ObjectClass
      else if (!(cls is Trait)) cls.asClass
      else cls.asClass.classParents match {
        case parentRef :: _ => realClassParent(parentRef.symbol)
        case nil => defn.ObjectClass
      }
    def improve(candidate: ClassSymbol, parent: Type): ClassSymbol = {
      val pcls = realClassParent(parent.classSymbol)
      if (pcls derivesFrom candidate) pcls else candidate
    }
    parents match {
      case p :: _ if p.classSymbol.isRealClass => parents
      case _ =>
        val pcls = (defn.ObjectClass /: parents)(improve)
        typr.println(i"ensure first is class $parents%, % --> ${parents map (_ baseTypeWithArgs pcls)}%, %")
        val ptype = ctx.typeComparer.glb(
            defn.ObjectType :: (parents map (_ baseTypeWithArgs pcls)))
        ptype :: parents
    }
  }

  /** Ensure that first parent tree refers to a real class. */
  def ensureFirstIsClass(parents: List[Tree], pos: Position)(implicit ctx: Context): List[Tree] = parents match {
    case p :: ps if p.tpe.classSymbol.isRealClass => parents
    case _ =>
      // add synthetic class type
      val first :: _ = ensureFirstIsClass(parents.tpes)
      TypeTree(checkFeasible(first, pos, em"\n in inferred parent $first")).withPos(pos) :: parents
  }

  /** If this is a real class, make sure its first parent is a
   *  constructor call. Cannot simply use a type. Overridden in ReTyper.
   */
  def ensureConstrCall(cls: ClassSymbol, parents: List[Tree])(implicit ctx: Context): List[Tree] = {
    val firstParent :: otherParents = parents
    if (firstParent.isType && !(cls is Trait) && !cls.is(JavaDefined))
      typed(untpd.New(untpd.TypedSplice(firstParent), Nil)) :: otherParents
    else parents
  }

  /** Overridden in retyper */
  def checkVariance(tree: Tree)(implicit ctx: Context) = VarianceChecker.check(tree)

  def localDummy(cls: ClassSymbol, impl: untpd.Template)(implicit ctx: Context): Symbol =
    ctx.newLocalDummy(cls, impl.pos)

  def typedImport(imp: untpd.Import, sym: Symbol)(implicit ctx: Context): Import = track("typedImport") {
    val expr1 = typedExpr(imp.expr, AnySelectionProto)
    checkStable(expr1.tpe, imp.expr.pos)
    if (!ctx.isAfterTyper) checkRealizable(expr1.tpe, imp.expr.pos)
    assignType(cpy.Import(imp)(expr1, imp.selectors), sym)
  }

  def typedPackageDef(tree: untpd.PackageDef)(implicit ctx: Context): Tree = track("typedPackageDef") {
    val pid1 = typedExpr(tree.pid, AnySelectionProto)(ctx.addMode(Mode.InPackageClauseName))
    val pkg = pid1.symbol

    // Package will not exist if a duplicate type has already been entered, see
    // `tests/neg/1708.scala`, else branch's error message should be supressed
    if (pkg.exists) {
      val packageContext =
        if (pkg is Package) ctx.fresh.setOwner(pkg.moduleClass).setTree(tree)
        else {
          ctx.error(em"$pkg is already defined, cannot be a package", tree.pos)
          ctx
        }
      val stats1 = typedStats(tree.stats, pkg.moduleClass)(packageContext)
      cpy.PackageDef(tree)(pid1.asInstanceOf[RefTree], stats1) withType pkg.valRef
    } else errorTree(tree, i"package ${tree.pid.name} does not exist")
  }

  def typedAnnotated(tree: untpd.Annotated, pt: Type)(implicit ctx: Context): Tree = track("typedAnnotated") {
    val annot1 = typedExpr(tree.annot, defn.AnnotationType)
    val arg1 = typed(tree.arg, pt)
    if (ctx.mode is Mode.Type)
      assignType(cpy.Annotated(tree)(arg1, annot1), arg1, annot1)
    else {
      val tpt = TypeTree(AnnotatedType(arg1.tpe.widen, Annotation(annot1)))
      assignType(cpy.Typed(tree)(arg1, tpt), tpt)
    }
  }

  def typedTypedSplice(tree: untpd.TypedSplice)(implicit ctx: Context): Tree =
    tree.tree match {
      case tree1: TypeTree => tree1  // no change owner necessary here ...
      case tree1: Ident => tree1     // ... or here, since these trees cannot contain bindings
      case tree1 =>
        if (ctx.owner ne tree.owner) tree1.changeOwner(tree.owner, ctx.owner)
        else tree1
    }


  def typedAsFunction(tree: untpd.PostfixOp, pt: Type)(implicit ctx: Context): Tree = {
    val untpd.PostfixOp(qual, nme.WILDCARD) = tree
    val pt1 = if (defn.isFunctionType(pt)) pt else AnyFunctionProto
    var res = typed(qual, pt1)
    if (pt1.eq(AnyFunctionProto) && !defn.isFunctionClass(res.tpe.classSymbol)) {
      def msg = i"not a function: ${res.tpe}; cannot be followed by `_'"
      if (ctx.scala2Mode) {
        // Under -rewrite, patch `x _` to `(() => x)`
        ctx.migrationWarning(msg, tree.pos)
        patch(Position(tree.pos.start), "(() => ")
        patch(Position(qual.pos.end, tree.pos.end), ")")
        res = typed(untpd.Function(Nil, untpd.TypedSplice(res)))
      }
      else ctx.error(msg, tree.pos)
    }
    res
  }

  /** Retrieve symbol attached to given tree */
  protected def retrieveSym(tree: untpd.Tree)(implicit ctx: Context) = tree.removeAttachment(SymOfTree) match {
    case Some(sym) =>
      sym.ensureCompleted()
      sym
    case none =>
      NoSymbol
  }

  /** A fresh local context with given tree and owner.
   *  Owner might not exist (can happen for self valdefs), in which case
   *  no owner is set in result context
   */
  protected def localContext(tree: untpd.Tree, owner: Symbol)(implicit ctx: Context): FreshContext = {
    val freshCtx = ctx.fresh.setTree(tree)
    if (owner.exists) freshCtx.setOwner(owner) else freshCtx
  }

  protected def localTyper(sym: Symbol): Typer = nestedTyper.remove(sym).get

  def typedUnadapted(initTree: untpd.Tree, pt: Type = WildcardType)(implicit ctx: Context): Tree = {
    record("typedUnadapted")
    val xtree = expanded(initTree)
    xtree.removeAttachment(TypedAhead) match {
      case Some(ttree) => ttree
      case none =>

        def typedNamed(tree: untpd.NameTree, pt: Type)(implicit ctx: Context): Tree = {
          val sym = retrieveSym(xtree)
          tree match {
            case tree: untpd.Ident => typedIdent(tree, pt)
            case tree: untpd.Select => typedSelect(tree, pt)
            case tree: untpd.Bind => typedBind(tree, pt)
            case tree: untpd.ValDef =>
              if (tree.isEmpty) tpd.EmptyValDef
              else typedValDef(tree, sym)(localContext(tree, sym).setNewScope)
            case tree: untpd.DefDef =>
              val typer1 = localTyper(sym)
              typer1.typedDefDef(tree, sym)(localContext(tree, sym).setTyper(typer1))
            case tree: untpd.TypeDef =>
              if (tree.isClassDef)
                typedClassDef(tree, sym.asClass)(localContext(tree, sym).setMode(ctx.mode &~ Mode.InSuperCall))
              else
                typedTypeDef(tree, sym)(localContext(tree, sym).setNewScope)
            case _ => typedUnadapted(desugar(tree), pt)
          }
        }

        def typedUnnamed(tree: untpd.Tree): Tree = tree match {
          case tree: untpd.Apply =>
            if (ctx.mode is Mode.Pattern) typedUnApply(tree, pt) else typedApply(tree, pt)
          case tree: untpd.This => typedThis(tree)
          case tree: untpd.Literal => typedLiteral(tree)
          case tree: untpd.New => typedNew(tree, pt)
          case tree: untpd.Typed => typedTyped(tree, pt)
          case tree: untpd.NamedArg => typedNamedArg(tree, pt)
          case tree: untpd.Assign => typedAssign(tree, pt)
          case tree: untpd.Block => typedBlock(desugar.block(tree), pt)(ctx.fresh.setNewScope)
          case tree: untpd.If => typedIf(tree, pt)
          case tree: untpd.Function => typedFunction(tree, pt)
          case tree: untpd.Closure => typedClosure(tree, pt)
          case tree: untpd.Import => typedImport(tree, retrieveSym(tree))
          case tree: untpd.Match => typedMatch(tree, pt)
          case tree: untpd.Return => typedReturn(tree)
          case tree: untpd.Try => typedTry(tree, pt)
          case tree: untpd.Throw => typedThrow(tree)
          case tree: untpd.TypeApply => typedTypeApply(tree, pt)
          case tree: untpd.Super => typedSuper(tree, pt)
          case tree: untpd.SeqLiteral => typedSeqLiteral(tree, pt)
          case tree: untpd.Inlined => typedInlined(tree, pt)
          case tree: untpd.TypeTree => typedTypeTree(tree, pt)
          case tree: untpd.SingletonTypeTree => typedSingletonTypeTree(tree)
          case tree: untpd.AndTypeTree => typedAndTypeTree(tree)
          case tree: untpd.OrTypeTree => typedOrTypeTree(tree)
          case tree: untpd.RefinedTypeTree => typedRefinedTypeTree(tree)
          case tree: untpd.AppliedTypeTree => typedAppliedTypeTree(tree)
          case tree: untpd.PolyTypeTree => typedPolyTypeTree(tree)(localContext(tree, NoSymbol).setNewScope)
          case tree: untpd.ByNameTypeTree => typedByNameTypeTree(tree)
          case tree: untpd.TypeBoundsTree => typedTypeBoundsTree(tree)
          case tree: untpd.Alternative => typedAlternative(tree, pt)
          case tree: untpd.PackageDef => typedPackageDef(tree)
          case tree: untpd.Annotated => typedAnnotated(tree, pt)
          case tree: untpd.TypedSplice => typedTypedSplice(tree)
          case tree:  untpd.UnApply => typedUnApply(tree, pt)
          case tree @ untpd.PostfixOp(qual, nme.WILDCARD) => typedAsFunction(tree, pt)
          case untpd.EmptyTree => tpd.EmptyTree
          case _ => typedUnadapted(desugar(tree), pt)
        }

        if (defn.isImplicitFunctionType(pt) &&
            xtree.isTerm &&
            !untpd.isImplicitClosure(xtree) &&
            !ctx.isAfterTyper)
          makeImplicitFunction(xtree, pt)
        else xtree match {
          case xtree: untpd.NameTree => typedNamed(encodeName(xtree), pt)
          case xtree => typedUnnamed(xtree)
        }
    }
  }

  protected def encodeName(tree: untpd.NameTree)(implicit ctx: Context): untpd.NameTree =
    untpd.rename(tree, tree.name.encode)

  protected def makeImplicitFunction(tree: untpd.Tree, pt: Type)(implicit ctx: Context): Tree = {
    val defn.FunctionOf(formals, resType, true) = pt.dealias
    val paramTypes = formals.map(fullyDefinedType(_, "implicit function parameter", tree.pos))
    val ifun = desugar.makeImplicitFunction(paramTypes, tree)
    typr.println(i"make implicit function $tree / $pt ---> $ifun")
    typedUnadapted(ifun)
  }

  def typed(tree: untpd.Tree, pt: Type = WildcardType)(implicit ctx: Context): Tree = /*>|>*/ ctx.traceIndented (i"typing $tree", typr, show = true) /*<|<*/ {
    assertPositioned(tree)
    try adapt(typedUnadapted(tree, pt), pt, tree)
    catch {
      case ex: CyclicReference => errorTree(tree, cyclicErrorMsg(ex))
      case ex: TypeError => errorTree(tree, ex.getMessage)
    }
  }

  def typedTrees(trees: List[untpd.Tree])(implicit ctx: Context): List[Tree] =
    trees mapconserve (typed(_))

  def typedStats(stats: List[untpd.Tree], exprOwner: Symbol)(implicit ctx: Context): List[tpd.Tree] = {
    val buf = new mutable.ListBuffer[Tree]
    @tailrec def traverse(stats: List[untpd.Tree])(implicit ctx: Context): List[Tree] = stats match {
      case (imp: untpd.Import) :: rest =>
        val imp1 = typed(imp)
        buf += imp1
        traverse(rest)(importContext(imp1.symbol, imp.selectors))
      case (mdef: untpd.DefTree) :: rest =>
        mdef.removeAttachment(ExpandedTree) match {
          case Some(xtree) =>
            traverse(xtree :: rest)
          case none =>
            typed(mdef) match {
              case mdef1: DefDef if Inliner.hasBodyToInline(mdef1.symbol) =>
                buf ++= inlineExpansion(mdef1)
              case mdef1 =>
                buf += mdef1
            }
            traverse(rest)
        }
      case Thicket(stats) :: rest =>
        traverse(stats ++ rest)
      case stat :: rest =>
        buf += typed(stat)(ctx.exprContext(stat, exprOwner))
        traverse(rest)
      case nil =>
        buf.toList
    }
    val localCtx = {
      val exprOwnerOpt = if (exprOwner == ctx.owner) None else Some(exprOwner)
      ctx.withProperty(ExprOwner, exprOwnerOpt)
    }
    traverse(stats)(localCtx)
  }

  /** Given an inline method `mdef`, the method rewritten so that its body
   *  uses accessors to access non-public members, followed by the accessor definitions.
   *  Overwritten in Retyper to return `mdef` unchanged.
   */
  protected def inlineExpansion(mdef: DefDef)(implicit ctx: Context): List[Tree] =
    tpd.cpy.DefDef(mdef)(rhs = Inliner.bodyToInline(mdef.symbol)) ::
        Inliner.removeInlineAccessors(mdef.symbol)

  def typedExpr(tree: untpd.Tree, pt: Type = WildcardType)(implicit ctx: Context): Tree =
    typed(tree, pt)(ctx retractMode Mode.PatternOrType)
  def typedType(tree: untpd.Tree, pt: Type = WildcardType)(implicit ctx: Context): Tree = // todo: retract mode between Type and Pattern?
    typed(tree, pt)(ctx addMode Mode.Type)
  def typedPattern(tree: untpd.Tree, selType: Type = WildcardType)(implicit ctx: Context): Tree =
    typed(tree, selType)(ctx addMode Mode.Pattern)

  def tryEither[T](op: Context => T)(fallBack: (T, TyperState) => T)(implicit ctx: Context) = {
    val nestedCtx = ctx.fresh.setNewTyperState
    val result = op(nestedCtx)
    if (nestedCtx.reporter.hasErrors)
      fallBack(result, nestedCtx.typerState)
    else {
      nestedCtx.typerState.commit()
      result
    }
  }

  /** Try `op1`, if there are errors, try `op2`, if `op2` also causes errors, fall back
   *  to errors and result of `op1`.
   */
  def tryAlternatively[T](op1: Context => T)(op2: Context => T)(implicit ctx: Context): T =
    tryEither(op1) { (failedVal, failedState) =>
      tryEither(op2) { (_, _) =>
        failedState.commit
        failedVal
      }
    }

  /** Is `pt` a prototype of an `apply` selection, or a parameterless function yielding one? */
  def isApplyProto(pt: Type)(implicit ctx: Context): Boolean = pt match {
    case pt: SelectionProto => pt.name == nme.apply
    case pt: FunProto       => pt.args.isEmpty && isApplyProto(pt.resultType)
    case pt: IgnoredProto   => isApplyProto(pt.ignored)
    case _                  => false
  }

  /** Potentially add apply node or implicit conversions. Before trying either,
   *  if the function is applied to an empty parameter list (), we try
   *
   *  0th strategy: If `tree` overrides a nullary method, mark the prototype
   *                so that the argument is dropped and return `tree` itself.
   *
   *  After that, two strategies are tried, and the first that is successful is picked.
   *  If neither of the strategies are successful, continues with`fallBack`.
   *
   *  1st strategy: Try to insert `.apply` so that the result conforms to prototype `pt`.
   *                This strategy is not tried if the prototype represents already
   *                another `.apply` or `.apply()` selection.
   *
   *  2nd strategy: If tree is a select `qual.name`, try to insert an implicit conversion
   *    around the qualifier part `qual` so that the result conforms to the expected type
   *    with wildcard result type.
   */
  def tryInsertApplyOrImplicit(tree: Tree, pt: ProtoType)(fallBack: => Tree)(implicit ctx: Context): Tree = {

    def tryApply(implicit ctx: Context) = {
      val sel = typedSelect(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt)
      if (sel.tpe.isError) sel else adapt(sel, pt)
    }

    def tryImplicit =
      tryInsertImplicitOnQualifier(tree, pt).getOrElse(fallBack)

    pt match {
      case pt @ FunProto(Nil, _, _)
      if tree.symbol.allOverriddenSymbols.exists(_.info.isNullaryMethod) =>
        pt.markAsDropped()
        tree
      case _ =>
        if (isApplyProto(pt)) tryImplicit
        else tryEither(tryApply(_))((_, _) => tryImplicit)
     }
  }

  /** If this tree is a select node `qual.name`, try to insert an implicit conversion
   *  `c` around `qual` so that `c(qual).name` conforms to `pt`.
   */
  def tryInsertImplicitOnQualifier(tree: Tree, pt: Type)(implicit ctx: Context): Option[Tree] = ctx.traceIndented(i"try insert impl on qualifier $tree $pt") {
    tree match {
      case Select(qual, name) =>
        val qualProto = SelectionProto(name, pt, NoViewsAllowed)
        tryEither { implicit ctx =>
          val qual1 = adaptInterpolated(qual, qualProto, EmptyTree)
          if ((qual eq qual1) || ctx.reporter.hasErrors) None
          else Some(typed(cpy.Select(tree)(untpd.TypedSplice(qual1), name), pt))
        } { (_, _) => None
        }
      case _ => None
    }
  }

  def adapt(tree: Tree, pt: Type, original: untpd.Tree = untpd.EmptyTree)(implicit ctx: Context): Tree = /*>|>*/ track("adapt") /*<|<*/ {
    /*>|>*/ ctx.traceIndented(i"adapting $tree of type ${tree.tpe} to $pt", typr, show = true) /*<|<*/ {
      if (tree.isDef) interpolateUndetVars(tree, tree.symbol)
      else if (!tree.tpe.widen.isInstanceOf[MethodOrPoly]) interpolateUndetVars(tree, NoSymbol)
      tree.overwriteType(tree.tpe.simplified)
      adaptInterpolated(tree, pt, original)
    }
  }

  /** (-1) For expressions with annotated types, let AnnotationCheckers decide what to do
   *  (0) Convert expressions with constant types to literals (unless in interactive/scaladoc mode)
   */

  /** Perform the following adaptations of expression, pattern or type `tree` wrt to
   *  given prototype `pt`:
   *  (1) Resolve overloading
   *  (2) Apply parameterless functions
   *  (3) Apply polymorphic types to fresh instances of their type parameters and
   *      store these instances in context.undetparams,
   *      unless followed by explicit type application.
   *  (4) Do the following to unapplied methods used as values:
   *  (4.1) If the method has only implicit parameters pass implicit arguments
   *  (4.2) otherwise, if `pt` is a function type and method is not a constructor,
   *        convert to function by eta-expansion,
   *  (4.3) otherwise, if the method is nullary with a result type compatible to `pt`
   *        and it is not a constructor, apply it to ()
   *  otherwise issue an error
   *  (5) Convert constructors in a pattern as follows:
   *  (5.1) If constructor refers to a case class factory, set tree's type to the unique
   *        instance of its primary constructor that is a subtype of the expected type.
   *  (5.2) If constructor refers to an extractor, convert to application of
   *        unapply or unapplySeq method.
   *
   *  (6) Convert all other types to TypeTree nodes.
   *  (7) When in TYPEmode but not FUNmode or HKmode, check that types are fully parameterized
   *      (7.1) In HKmode, higher-kinded types are allowed, but they must have the expected kind-arity
   *  (8) When in both EXPRmode and FUNmode, add apply method calls to values of object type.
   *  (9) If there are undetermined type variables and not POLYmode, infer expression instance
   *  Then, if tree's type is not a subtype of expected type, try the following adaptations:
   *  (10) If the expected type is Byte, Short or Char, and the expression
   *      is an integer fitting in the range of that type, convert it to that type.
   *  (11) Widen numeric literals to their expected type, if necessary
   *  (12) When in mode EXPRmode, convert E to { E; () } if expected type is scala.Unit.
   *  (13) When in mode EXPRmode, apply AnnotationChecker conversion if expected type is annotated.
   *  (14) When in mode EXPRmode, apply a view
   *  If all this fails, error
   */
  def adaptInterpolated(tree: Tree, pt: Type, original: untpd.Tree)(implicit ctx: Context): Tree = {

    assert(pt.exists)

    def methodStr = err.refStr(methPart(tree).tpe)

    def missingArgs = errorTree(tree,
      em"""missing arguments for $methodStr
          |follow this method with `_' if you want to treat it as a partially applied function""")

    def adaptOverloaded(ref: TermRef) = {
      val altDenots = ref.denot.alternatives
      typr.println(i"adapt overloaded $ref with alternatives ${altDenots map (_.info)}%, %")
      val alts = altDenots map (alt =>
        TermRef.withSigAndDenot(ref.prefix, ref.name, alt.info.signature, alt))
      def expectedStr = err.expectedTypeStr(pt)
      resolveOverloaded(alts, pt) match {
        case alt :: Nil =>
          adapt(tree.withType(alt), pt, original)
        case Nil =>
          def noMatches =
            errorTree(tree,
              em"""none of the ${err.overloadedAltsStr(altDenots)}
                  |match $expectedStr""")
          def hasEmptyParams(denot: SingleDenotation) = denot.info.paramTypess == ListOfNil
          pt match {
            case pt: FunProto =>
              tryInsertApplyOrImplicit(tree, pt)(noMatches)
            case _ =>
              if (altDenots exists (_.info.paramTypess == ListOfNil))
                typed(untpd.Apply(untpd.TypedSplice(tree), Nil), pt)
              else
                noMatches
          }
        case alts =>
          val remainingDenots = alts map (_.denot.asInstanceOf[SingleDenotation])
          def all = if (remainingDenots.length == 2) "both" else "all"
          errorTree(tree,
            em"""Ambiguous overload. The ${err.overloadedAltsStr(remainingDenots)}
                |$all match $expectedStr""")
      }
    }

    def isUnary(tp: Type): Boolean = tp match {
      case tp: MethodicType =>
        tp.firstParamTypes match {
          case ptype :: Nil => !ptype.isRepeatedParam
          case _ => false
        }
      case tp: TermRef =>
        tp.denot.alternatives.forall(alt => isUnary(alt.info))
      case _ =>
        false
    }

    def adaptToArgs(wtp: Type, pt: FunProto): Tree = wtp match {
      case _: MethodType | _: PolyType =>
        if (pt.args.lengthCompare(1) > 0 && isUnary(wtp) && ctx.canAutoTuple)
          adaptInterpolated(tree, pt.tupled, original)
        else
          tree
      case _ => tryInsertApplyOrImplicit(tree, pt) {
        val more = tree match {
          case Apply(_, _) => " more"
          case _ => ""
        }
        errorTree(tree, em"$methodStr does not take$more parameters")
      }
    }

    /** If `tp` is a TypeVar which is fully constrained (i.e. its upper bound `hi` conforms
     *  to its lower bound `lo`), replace `tp` by `hi`. This is necessary to
     *  keep the right constraints for some implicit search problems. The paradigmatic case
     *  is `implicitNums.scala`. Without the healing done in `followAlias`, we cannot infer
     *  implicitly[_3], where _2 is the typelevel number 3. The problem here is that if a
     *  prototype is, say, Succ[Succ[Zero]], we can infer that it's argument type is Succ[Zero].
     *  But if the prototype is N? >: Succ[Succ[Zero]] <: Succ[Succ[Zero]], the same
     *  decomposition does not work - we'd get a N?#M where M is the element type name of Succ
     *  instead.
     */
    def followAlias(tp: Type)(implicit ctx: Context): Type = {
      val constraint = ctx.typerState.constraint
      def inst(tp: Type): Type = tp match {
        case TypeBounds(lo, hi)
        if (lo eq hi) || (hi <:< lo)(ctx.fresh.setExploreTyperState) =>
          inst(lo)
        case tp: PolyParam =>
          constraint.typeVarOfParam(tp).orElse(tp)
        case _ => tp
      }
      tp match {
        case tp: TypeVar if constraint.contains(tp) => inst(constraint.entry(tp.origin))
        case _ => tp
      }
    }

    def adaptNoArgs(wtp: Type): Tree = wtp match {
      case wtp: ExprType =>
        adaptInterpolated(tree.withType(wtp.resultType), pt, original)
      case wtp: ImplicitMethodType if constrainResult(wtp, followAlias(pt)) =>
        val tvarsToInstantiate = tvarsInParams(tree)
        wtp.paramTypes.foreach(instantiateSelected(_, tvarsToInstantiate))
        val constr = ctx.typerState.constraint
        def addImplicitArgs(implicit ctx: Context) = {
          val errors = new mutable.ListBuffer[() => String]
          def implicitArgError(msg: => String) = {
            errors += (() => msg)
            EmptyTree
          }
          def issueErrors() = {
            for (err <- errors) ctx.error(err(), tree.pos.endPos)
            tree.withType(wtp.resultType)
          }
          val args = (wtp.paramNames, wtp.paramTypes).zipped map { (pname, formal) =>
            def implicitArgError(msg: String => String) =
              errors += (() => msg(em"parameter $pname of $methodStr"))
            inferImplicitArg(formal, implicitArgError, tree.pos.endPos)
          }
          if (errors.nonEmpty) {
            // If there are several arguments, some arguments might already
            // have influenced the context, binding variables, but later ones
            // might fail. In that case the constraint needs to be reset.
            ctx.typerState.constraint = constr

            // If method has default params, fall back to regular application
            // where all inferred implicits are passed as named args.
            if (tree.symbol.hasDefaultParams) {
              val namedArgs = (wtp.paramNames, args).zipped.flatMap { (pname, arg) =>
                arg match {
                  case EmptyTree => Nil
                  case _ => untpd.NamedArg(pname, untpd.TypedSplice(arg)) :: Nil
                }
              }
              tryEither { implicit ctx =>
                typed(untpd.Apply(untpd.TypedSplice(tree), namedArgs), pt)
              } { (_, _) =>
                issueErrors()
              }
            } else issueErrors()
          }
          else adapt(tpd.Apply(tree, args), pt)
        }
        if ((pt eq WildcardType) || original.isEmpty) addImplicitArgs(argCtx(tree))
        else
          ctx.typerState.tryWithFallback(addImplicitArgs(argCtx(tree))) {
            adapt(typed(original, WildcardType), pt, EmptyTree)
          }
      case wtp: MethodType if !pt.isInstanceOf[SingletonType] =>
        val arity =
          if (defn.isFunctionType(pt))
            if (!isFullyDefined(pt, ForceDegree.none) && isFullyDefined(wtp, ForceDegree.none))
              // if method type is fully defined, but expected type is not,
              // prioritize method parameter types as parameter types of the eta-expanded closure
              0
            else defn.functionArity(pt)
          else if (pt eq AnyFunctionProto) wtp.paramTypes.length
          else -1
        if (arity >= 0 && !tree.symbol.isConstructor)
          typed(etaExpand(tree, wtp, arity), pt)
        else if (wtp.paramTypes.isEmpty)
          adaptInterpolated(tpd.Apply(tree, Nil), pt, EmptyTree)
        else if (wtp.isImplicit)
          err.typeMismatch(tree, pt)
        else
          missingArgs
      case _ =>
        ctx.typeComparer.GADTused = false
        if (defn.isImplicitFunctionClass(wtp.underlyingClassRef(refinementOK = false).classSymbol) &&
            !untpd.isImplicitClosure(tree) &&
            !isApplyProto(pt) &&
            !ctx.isAfterTyper) {
          typr.println("insert apply on implicit $tree")
          typed(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt)
        }
        else if (ctx.mode is Mode.Pattern) {
          tree match {
            case _: RefTree | _: Literal
            if !isVarPattern(tree) &&
               !(tree.tpe <:< pt)(ctx.addMode(Mode.GADTflexible)) =>
              checkCanEqual(pt, wtp, tree.pos)(ctx.retractMode(Mode.Pattern))
            case _ =>
          }
          tree
        }
        else if (tree.tpe <:< pt) {
          if (pt.hasAnnotation(defn.InlineParamAnnot))
            checkInlineConformant(tree, "argument to inline parameter")
          if (Inliner.hasBodyToInline(tree.symbol) &&
              !ctx.owner.ownersIterator.exists(_.isInlineMethod) &&
              !ctx.settings.YnoInline.value &&
              !ctx.isAfterTyper)
            adapt(Inliner.inlineCall(tree, pt), pt)
          else if (ctx.typeComparer.GADTused && pt.isValueType)
            // Insert an explicit cast, so that -Ycheck in later phases succeeds.
            // I suspect, but am not 100% sure that this might affect inferred types,
            // if the expected type is a supertype of the GADT bound. It would be good to come
            // up with a test case for this.
            tree.asInstance(pt)
          else
            tree
        }
        else if (wtp.isInstanceOf[MethodType]) missingArgs
        else {
          typr.println(i"adapt to subtype ${tree.tpe} !<:< $pt")
          //typr.println(TypeComparer.explained(implicit ctx => tree.tpe <:< pt))
          adaptToSubType(wtp)
        }
    }
    /** Adapt an expression of constant type to a different constant type `tpe`. */
    def adaptConstant(tree: Tree, tpe: ConstantType): Tree = {
      def lit = Literal(tpe.value).withPos(tree.pos)
      tree match {
        case Literal(c) => lit
        case tree @ Block(stats, expr) => tpd.cpy.Block(tree)(stats, adaptConstant(expr, tpe))
        case tree =>
          if (isIdempotentExpr(tree)) lit // See discussion in phase Literalize why we demand isIdempotentExpr
          else Block(tree :: Nil, lit)
      }
    }

    def adaptToSubType(wtp: Type): Tree = {
      // try converting a constant to the target type
      val folded = ConstFold(tree, pt)
      if (folded ne tree) return adaptConstant(folded, folded.tpe.asInstanceOf[ConstantType])
      // drop type if prototype is Unit
      if (pt isRef defn.UnitClass)
        // local adaptation makes sure every adapted tree conforms to its pt
        // so will take the code path that decides on inlining
        return tpd.Block(adapt(tree, WildcardType) :: Nil, Literal(Constant(())))
      // convert function literal to SAM closure
      tree match {
        case Closure(Nil, id @ Ident(nme.ANON_FUN), _)
        if defn.isFunctionType(wtp) && !defn.isFunctionType(pt) =>
          pt match {
            case SAMType(meth)
            if wtp <:< meth.info.toFunctionType() =>
              // was ... && isFullyDefined(pt, ForceDegree.noBottom)
              // but this prevents case blocks from implementing polymorphic partial functions,
              // since we do not know the result parameter a priori. Have to wait until the
              // body is typechecked.
              return cpy.Closure(tree)(Nil, id, TypeTree(pt)).withType(pt)
            case _ =>
          }
        case _ =>
      }
      // try an implicit conversion
      inferView(tree, pt) match {
        case SearchSuccess(inferred, _, _, _) =>
          adapt(inferred, pt)
        case failure: SearchFailure =>
          if (pt.isInstanceOf[ProtoType] && !failure.isInstanceOf[AmbiguousImplicits]) tree
          else err.typeMismatch(tree, pt, failure)
      }
    }

    def adaptType(tp: Type): Tree = {
      val tree1 =
        if ((pt eq AnyTypeConstructorProto) || tp.typeParamSymbols.isEmpty) tree
        else tree.withType(tree.tpe.EtaExpand(tp.typeParamSymbols))
      if ((ctx.mode is Mode.Pattern) || tree1.tpe <:< pt) tree1
      else err.typeMismatch(tree1, pt)
    }

    /** If tree has an error type but no errors are reported yet, issue
     *  the error message stored in the type.
     *  One way this can happen is if implicit search causes symbols and types
     *  to be completed. The types are stored by `typedAhead` so that they can be
     *  retrieved later and thus avoid duplication of typechecking work.
     *  But if the implicit search causing the `typedAhead` fails locally but
     *  another alternative succeeds we can be left with an ErrorType in the
     *  tree that went unreported. A scenario where this happens is i1802.scala.
     */
    def ensureReported(tp: Type) = tp match {
      case err: ErrorType if !ctx.reporter.hasErrors => ctx.error(err.msg, tree.pos)
      case _ =>
    }

    tree match {
      case _: MemberDef | _: PackageDef | _: Import | _: WithoutTypeOrPos[_] => tree
      case _ => tree.tpe.widen match {
        case tp: FlexType =>
          ensureReported(tp)
          tree
        case ref: TermRef =>
          pt match {
            case pt: FunProto
            if pt.args.lengthCompare(1) > 0 && isUnary(ref) && ctx.canAutoTuple =>
              adaptInterpolated(tree, pt.tupled, original)
            case _ =>
              adaptOverloaded(ref)
          }
        case poly: PolyType if !(ctx.mode is Mode.Type) =>
          if (pt.isInstanceOf[PolyProto]) tree
          else {
            var typeArgs = tree match {
              case Select(qual, nme.CONSTRUCTOR) => qual.tpe.widenDealias.argTypesLo.map(TypeTree)
              case _ => Nil
            }
            if (typeArgs.isEmpty) typeArgs = constrained(poly, tree)._2
            convertNewGenericArray(
              adaptInterpolated(tree.appliedToTypeTrees(typeArgs), pt, original))
          }
        case wtp =>
          if (isStructuralTermSelect(tree)) adapt(handleStructural(tree), pt)
          else pt match {
            case pt: FunProto =>
              adaptToArgs(wtp, pt)
            case pt: PolyProto =>
              tryInsertApplyOrImplicit(tree, pt)(tree) // error will be reported in typedTypeApply
            case _ =>
              if (ctx.mode is Mode.Type) adaptType(tree.tpe)
              else adaptNoArgs(wtp)
          }
      }
    }
  }
}