path: root/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala

/* NSC -- new Scala compiler
 *
 * Copyright 2011-2013 LAMP/EPFL
 * @author Adriaan Moors
 */

package scala.tools.nsc.transform.patmat

import scala.collection.mutable
import scala.reflect.internal.util.Statistics

trait TreeAndTypeAnalysis extends Debugging {
  import global._
  import definitions._
  import analyzer.Typer

  /** Compute the type T implied for a value `v` matched by a pattern `pat` (with expected type `pt`).
   *
   * Usually, this is the pattern's type because pattern matching implies instance-of checks.
   *
   * However, Stable Identifier and Literal patterns are matched using `==`,
   * which does not imply a type for the binder that binds the matched value.
   *
   * See SI-1503, SI-5024: don't cast binders to types we're not sure they have
   *
   * TODO: update spec as follows (deviation between `**`):
   *
   *   A pattern binder x@p consists of a pattern variable x and a pattern p.
   *   The type of the variable x is the static type T **IMPLIED BY** the pattern p.
   *   This pattern matches any value v matched by the pattern p
   *     **Deleted: , provided the run-time type of v is also an instance of T, **
   *   and it binds the variable name to that value.
   *
   *   Addition:
   *     A pattern `p` _implies_ a type `T` if the pattern matches only values of the type `T`.
   */
  def binderTypeImpliedByPattern(pat: Tree, pt: Type, binder: Symbol): Type =
    pat match {
      // because `==` decides whether these patterns match, stable identifier patterns (ident or selection)
      // do not contribute any type information (beyond the pattern's expected type)
      // e.g., in case x@Nil => x --> all we know about `x` is that it satisfies Nil == x, which could be anything
      case Ident(_) | Select(_, _) =>
        if (settings.future) pt
        else {
          // TODO: don't warn unless this unsound assumption is actually used in a cast
          // I tried annotating the type returned here with an internal annotation (`pat.tpe withAnnotation UnsoundAssumptionAnnotation`),
          // and catching it in the patmat backend when used in a cast (because that would signal the unsound assumption was used),
          // but the annotation didn't bubble up...
          // This is a pretty poor approximation.
          def unsoundAssumptionUsed = binder.name != nme.WILDCARD && !(pt <:< pat.tpe)
          if (settings.warnUnsoundMatch && unsoundAssumptionUsed)
            reporter.warning(pat.pos,
              sm"""The value matched by $pat is bound to ${binder.name}, which may be used under the
                  |unsound assumption that it has type ${pat.tpe}, whereas we can only safely
                  |count on it having type $pt, as the pattern is matched using `==` (see SI-1503).""")

          pat.tpe
        }


      // the other patterns imply type tests, so we can safely assume the binder has the pattern's type when the pattern matches
      // concretely, a literal, type pattern, a case class (the constructor's result type) or extractor (the unapply's argument type) all imply type tests
      // (and, inductively, an alternative)
      case _ => pat.tpe
    }

  // we use subtyping as a model for implication between instanceof tests
  // i.e., when S <:< T we assume x.isInstanceOf[S] implies x.isInstanceOf[T]
  // unfortunately this is not true in general:
  // SI-6022 expects instanceOfTpImplies(ProductClass.tpe, AnyRefTpe)
  def instanceOfTpImplies(tp: Type, tpImplied: Type) = {
    val tpValue = isPrimitiveValueType(tp)

    // pretend we're comparing to Any when we're actually comparing to AnyVal or AnyRef
    // (and the subtype is respectively a value type or not a value type)
    // this allows us to reuse subtyping as a model for implication between instanceOf tests
    // the latter don't see a difference between AnyRef, Object or Any when comparing non-value types -- SI-6022
    val tpImpliedNormalizedToAny =
      if (tpImplied =:= (if (tpValue) AnyValTpe else AnyRefTpe)) AnyTpe
      else tpImplied

    tp <:< tpImpliedNormalizedToAny
  }

  def equivalentTree(a: Tree, b: Tree): Boolean = (a, b) match {
    case (Select(qual1, _), Select(qual2, _)) => equivalentTree(qual1, qual2) && a.symbol == b.symbol
    case (Ident(_), Ident(_)) => a.symbol == b.symbol
    case (Literal(c1), Literal(c2)) => c1 == c2
    case (This(_), This(_)) => a.symbol == b.symbol
    case (Apply(fun1, args1), Apply(fun2, args2)) => equivalentTree(fun1, fun2) && args1.corresponds(args2)(equivalentTree)
    // Those are the only cases we need to handle in the pattern matcher
    case _ => false
  }

  trait CheckableTreeAndTypeAnalysis {
    val typer: Typer

    // TODO: domain of other feasibly enumerable built-in types (char?)
    def enumerateSubtypes(tp: Type, grouped: Boolean): List[List[Type]] =
      tp.typeSymbol match {
        // TODO case _ if tp.isTupleType => // recurse into component types?
        case UnitClass if !grouped =>
          List(List(UnitTpe))
        case BooleanClass if !grouped =>
          List(ConstantTrue :: ConstantFalse :: Nil)
        // TODO case _ if tp.isTupleType => // recurse into component types
        case modSym: ModuleClassSymbol if !grouped =>
          List(List(tp))
        case sym: RefinementClassSymbol =>
          val parentSubtypes = tp.parents.flatMap(parent => enumerateSubtypes(parent, grouped))
          if (parentSubtypes exists (_.nonEmpty)) {
            // If any of the parents is enumerable, then the refinement type is enumerable.
            // We must only include subtypes of the parents that conform to `tp`.
            // See neg/virtpatmat_exhaust_compound.scala for an example.
            parentSubtypes map (_.filter(_ <:< tp))
          }
          else Nil
        // make sure it's not a primitive, else (5: Byte) match { case 5 => ... } sees no Byte
        case sym if sym.isSealed =>

          val tpApprox = typer.infer.approximateAbstracts(tp)
          val pre = tpApprox.prefix

          def filterChildren(children: List[Symbol]): List[Type] = {
            children flatMap { sym =>
              // have to filter out children which cannot match: see ticket #3683 for an example
              // compare to the fully known type `tp` (modulo abstract types),
              // so that we can rule out stuff like: sealed trait X[T]; class XInt extends X[Int] --> XInt not valid when enumerating X[String]
              // however, must approximate abstract types in

              val memberType = nestedMemberType(sym, pre, tpApprox.typeSymbol.owner)
              val subTp = appliedType(memberType, sym.typeParams.map(_ => WildcardType))
              val subTpApprox = typer.infer.approximateAbstracts(subTp) // TODO: needed?
              // debug.patmat("subtp"+(subTpApprox <:< tpApprox, subTpApprox, tpApprox))
              if (subTpApprox <:< tpApprox) Some(checkableType(subTp))
              else None
            }
          }

          if(grouped) {
            def enumerateChildren(sym: Symbol) = {
              sym.sealedChildren.toList
                .sortBy(_.sealedSortName)
                .filterNot(x => x.isSealed && x.isAbstractClass && !isPrimitiveValueClass(x))
            }

            // enumerate only direct subclasses,
            // subclasses of subclasses are enumerated in the next iteration
            // and added to a new group
            def groupChildren(wl: List[Symbol],
                              acc: List[List[Type]]): List[List[Type]] = wl match {
              case hd :: tl =>
                val children = enumerateChildren(hd)
                // put each trait in a new group, since traits could belong to the same
                // group as a derived class
                val (traits, nonTraits) = children.partition(_.isTrait)
                val filtered = (traits.map(List(_)) ++ List(nonTraits)).map(filterChildren)
                groupChildren(tl ++ children, acc ++ filtered)
              case Nil      => acc
            }

            groupChildren(sym :: Nil, Nil)
          } else {
            val subclasses = debug.patmatResult(s"enum $sym sealed, subclasses")(
              // symbols which are both sealed and abstract need not be covered themselves, because
              // all of their children must be and they cannot otherwise be created.
              sym.sealedDescendants.toList
                sortBy (_.sealedSortName)
                filterNot (x => x.isSealed && x.isAbstractClass && !isPrimitiveValueClass(x))
            )

            List(debug.patmatResult(s"enum sealed tp=$tp, tpApprox=$tpApprox as") {
              // valid subtypes are turned into checkable types, as we are entering the realm of the dynamic
              filterChildren(subclasses)
            })
          }
        case sym if sym.isCase =>
          List(List(tp))

        case sym =>
          debug.patmat("enum unsealed "+ ((tp, sym, sym.isSealed, isPrimitiveValueClass(sym))))
          Nil
      }

    // approximate a type to the static type that is fully checkable at run time,
    // hiding statically known but dynamically uncheckable information using existential quantification
    // TODO: this is subject to the availability of TypeTags (since an abstract type with a type tag is checkable at run time)
    def checkableType(tp: Type): Type = {
      // TODO: this is extremely rough...
      // replace type args by wildcards, since they can't be checked (don't use existentials: overkill)
      // TODO: when type tags are available, we will check -- when this is implemented, can we take that into account here?
      // similar to typer.infer.approximateAbstracts
      object typeArgsToWildcardsExceptArray extends TypeMap {
        // SI-6771 dealias would be enough today, but future proofing with the dealiasWiden.
        // See neg/t6771b.scala for elaboration
        def apply(tp: Type): Type = tp.dealias match {
          case TypeRef(pre, sym, args) if args.nonEmpty && (sym ne ArrayClass) =>
            TypeRef(pre, sym, args map (_ => WildcardType))
          case _ =>
            mapOver(tp)
        }
      }
      val result = typeArgsToWildcardsExceptArray(tp)
      debug.patmatResult(s"checkableType($tp)")(result)
    }

    // a type is "uncheckable" (for exhaustivity) if we don't statically know its subtypes (i.e., it's unsealed)
    // we consider tuple types with at least one component of a checkable type as a checkable type
    def uncheckableType(tp: Type): Boolean = {
      val checkable = (
           (isTupleType(tp) && tupleComponents(tp).exists(tp => !uncheckableType(tp)))
        || enumerateSubtypes(tp, grouped = false).nonEmpty)
      // if (!checkable) debug.patmat("deemed uncheckable: "+ tp)
      !checkable
    }
  }
}

trait MatchApproximation extends TreeAndTypeAnalysis with ScalaLogic with MatchTreeMaking {
  import global._
  import global.definitions._

  /**
   * Represent a match as a formula in propositional logic that encodes whether the match matches (abstractly: we only consider types)
   *
   */
  trait MatchApproximator extends TreeMakers with TreesAndTypesDomain {
    object Test {
      var currId = 0
    }
    case class Test(prop: Prop, treeMaker: TreeMaker) {
      // private val reusedBy = new mutable.HashSet[Test]
      var reuses: Option[Test] = None
      def registerReuseBy(later: Test): Unit = {
        assert(later.reuses.isEmpty, later.reuses)
        // reusedBy += later
        later.reuses = Some(this)
      }
      val id = { Test.currId += 1; Test.currId}
      override def toString = s"T${id}C($prop)"
    }

    class TreeMakersToPropsIgnoreNullChecks(root: Symbol) extends TreeMakersToProps(root) {
      override def uniqueNonNullProp(p: Tree): Prop = True
    }

    // returns (tree, tests), where `tree` will be used to refer to `root` in `tests`
    class TreeMakersToProps(val root: Symbol) {
      prepareNewAnalysis() // reset hash consing for Var and Const

      private[this] val uniqueEqualityProps = new mutable.HashMap[(Tree, Tree), Eq]
      private[this] val uniqueNonNullProps  = new mutable.HashMap[Tree, Not]
      private[this] val uniqueTypeProps     = new mutable.HashMap[(Tree, Type), Eq]

      def uniqueEqualityProp(testedPath: Tree, rhs: Tree): Prop =
        uniqueEqualityProps getOrElseUpdate((testedPath, rhs), Eq(Var(testedPath), ValueConst(rhs)))

      // overridden in TreeMakersToPropsIgnoreNullChecks
      def uniqueNonNullProp (testedPath: Tree): Prop =
        uniqueNonNullProps getOrElseUpdate(testedPath, Not(Eq(Var(testedPath), NullConst)))

      def uniqueTypeProp(testedPath: Tree, pt: Type): Prop =
        uniqueTypeProps getOrElseUpdate((testedPath, pt), Eq(Var(testedPath), TypeConst(checkableType(pt))))

      // a variable in this set should never be replaced by a tree that "does not consist of a selection on a variable in this set" (intuitively)
      private val pointsToBound = mutable.HashSet(root)
      private val trees         = mutable.HashSet.empty[Tree]

      // the substitution that renames variables to variables in pointsToBound
      private var normalize: Substitution  = EmptySubstitution
      private var substitutionComputed = false

      // replaces a variable (in pointsToBound) by a selection on another variable in pointsToBound
      // in the end, instead of having x1, x1.hd, x2, x2.hd, ... flying around,
      // we want something like x1, x1.hd, x1.hd.tl, x1.hd.tl.hd, so that we can easily recognize when
      // we're testing the same variable
      // TODO check:
      //   pointsToBound -- accumSubst.from == Set(root) && (accumSubst.from.toSet -- pointsToBound) isEmpty
      private var accumSubst: Substitution = EmptySubstitution

      // hashconsing trees (modulo value-equality)
      def unique(t: Tree, tpOverride: Type = NoType): Tree =
        trees find (a => equivalentTree(a, t)) match {
          case Some(orig) =>
            // debug.patmat("unique: "+ (t eq orig, orig))
            orig
          case _ =>
            trees += t
            if (tpOverride != NoType) t setType tpOverride
            else t
        }

      def uniqueTp(tp: Type): Type = tp match {
        // typerefs etc are already hashconsed
        case _ : UniqueType                      => tp
        case tp@RefinedType(parents, EmptyScope) => tp.memo(tp: Type)(identity) // TODO: does this help?
        case _                                   => tp
      }

      // produce the unique tree used to refer to this binder
      // the type of the binder passed to the first invocation
      // determines the type of the tree that'll be returned for that binder as of then
      final def binderToUniqueTree(b: Symbol) =
        unique(accumSubst(normalize(gen.mkAttributedStableRef(b))), b.tpe)

      // note that the sequencing of operations is important: must visit in same order as match execution
      // binderToUniqueTree uses the type of the first symbol that was encountered as the type for all future binders
      abstract class TreeMakerToProp extends (TreeMaker => Prop) {
        // requires(if (!substitutionComputed))
        def updateSubstitution(subst: Substitution): Unit = {
          // find part of substitution that replaces bound symbols by new symbols, and reverse that part
          // so that we don't introduce new aliases for existing symbols, thus keeping the set of bound symbols minimal
          val (boundSubst, unboundSubst) = (subst.from zip subst.to) partition {
            case (f, t) =>
              t.isInstanceOf[Ident] && t.symbol.exists && pointsToBound(f)
          }
          val (boundFrom, boundTo) = boundSubst.unzip
          val (unboundFrom, unboundTo) = unboundSubst.unzip

          // reverse substitution that would otherwise replace a variable we already encountered by a new variable
          // NOTE: this forgets the more precise type we have for these later variables, but that's probably okay
          normalize >>= Substitution(boundTo map (_.symbol), boundFrom map (CODE.REF(_)))
          // debug.patmat ("normalize subst: "+ normalize)

          val okSubst = Substitution(unboundFrom, unboundTo map (normalize(_))) // it's important substitution does not duplicate trees here -- it helps to keep hash consing simple, anyway
          pointsToBound ++= ((okSubst.from, okSubst.to).zipped filter { (f, t) => pointsToBound exists (sym => t.exists(_.symbol == sym)) })._1
          // debug.patmat("pointsToBound: "+ pointsToBound)

          accumSubst >>= okSubst
          // debug.patmat("accumSubst: "+ accumSubst)
        }

        def handleUnknown(tm: TreeMaker): Prop

        /** apply itself must render a faithful representation of the TreeMaker
         *
         * Concretely, True must only be used to represent a TreeMaker that is sure to match and that does not do any computation at all
         * e.g., doCSE relies on apply itself being sound in this sense (since it drops TreeMakers that are approximated to True -- SI-6077)
         *
         * handleUnknown may be customized by the caller to approximate further
         *
         * TODO: don't ignore outer-checks
         */
        def apply(tm: TreeMaker): Prop = {
          if (!substitutionComputed) updateSubstitution(tm.subPatternsAsSubstitution)

          tm match {
            case ttm@TypeTestTreeMaker(prevBinder, testedBinder, pt, _)   =>
              object condStrategy extends TypeTestTreeMaker.TypeTestCondStrategy {
                type Result                                           = Prop
                def and(a: Result, b: Result)                         = And(a, b)
                def withOuterTest(testedBinder: Symbol, expectedTp: Type) = True // TODO OuterEqProp(testedBinder, expectedType)
                def typeTest(b: Symbol, pt: Type) = { // a type test implies the tested path is non-null (null.isInstanceOf[T] is false for all T)
                  val p = binderToUniqueTree(b);                        And(uniqueNonNullProp(p), uniqueTypeProp(p, uniqueTp(pt)))
                }
                def nonNullTest(testedBinder: Symbol)                 = uniqueNonNullProp(binderToUniqueTree(testedBinder))
                def equalsTest(pat: Tree, testedBinder: Symbol)       = uniqueEqualityProp(binderToUniqueTree(testedBinder), unique(pat))
                // rewrite eq test to type test against the singleton type `pat.tpe`; unrelated to == (uniqueEqualityProp), could be null
                def eqTest(pat: Tree, testedBinder: Symbol)           = uniqueTypeProp(binderToUniqueTree(testedBinder), uniqueTp(pat.tpe))
                def tru                                               = True
              }
              ttm.renderCondition(condStrategy)
            case EqualityTestTreeMaker(prevBinder, patTree, _)        => uniqueEqualityProp(binderToUniqueTree(prevBinder), unique(patTree))
            case AlternativesTreeMaker(_, altss, _)                   => \/(altss map (alts => /\(alts map this)))
            case ProductExtractorTreeMaker(testedBinder, None)        => uniqueNonNullProp(binderToUniqueTree(testedBinder))
            case SubstOnlyTreeMaker(_, _)                             => True
            case GuardTreeMaker(guard) =>
              guard.tpe match {
                case ConstantTrue  => True
                case ConstantFalse => False
                case _             => handleUnknown(tm)
              }
            case ExtractorTreeMaker(_, _, _) |
                 ProductExtractorTreeMaker(_, _) |
                 BodyTreeMaker(_, _)               => handleUnknown(tm)
          }
        }
      }


      private val irrefutableExtractor: PartialFunction[TreeMaker, Prop] = {
        // the extra condition is None, the extractor's result indicates it always succeeds,
        // (the potential type-test for the argument is represented by a separate TypeTestTreeMaker)
        case IrrefutableExtractorTreeMaker(_, _) => True
      }

      // special-case: interpret pattern `List()` as `Nil`
      // TODO: make it more general List(1, 2) => 1 :: 2 :: Nil  -- not sure this is a good idea...
      private val rewriteListPattern: PartialFunction[TreeMaker, Prop] = {
        case p @ ExtractorTreeMaker(_, _, testedBinder)
          if testedBinder.tpe.typeSymbol == ListClass && p.checkedLength == Some(0) =>
            uniqueEqualityProp(binderToUniqueTree(p.prevBinder), unique(Ident(NilModule) setType NilModule.tpe))
      }
      val fullRewrite      = (irrefutableExtractor orElse rewriteListPattern)
      val refutableRewrite = irrefutableExtractor

      @inline def onUnknown(handler: TreeMaker => Prop) = new TreeMakerToProp {
        def handleUnknown(tm: TreeMaker) = handler(tm)
      }

      // used for CSE -- rewrite all unknowns to False (the most conservative option)
      object conservative extends TreeMakerToProp {
        def handleUnknown(tm: TreeMaker) = False
      }

      final def approximateMatch(cases: List[List[TreeMaker]], treeMakerToProp: TreeMakerToProp = conservative) ={
        val testss = cases.map { _ map (tm => Test(treeMakerToProp(tm), tm)) }
        substitutionComputed = true // a second call to approximateMatch should not re-compute the substitution (would be wrong)
        testss
      }
    }

    def approximateMatchConservative(root: Symbol, cases: List[List[TreeMaker]]): List[List[Test]] =
      (new TreeMakersToProps(root)).approximateMatch(cases)

    // turns a case (represented as a list of abstract tests)
    // into a proposition that is satisfiable if the case may match
    protected final def caseWithoutBodyToProp(tests: List[Test]): Prop =
      /\(tests.takeWhile(t => !t.treeMaker.isInstanceOf[BodyTreeMaker]).map(t => t.prop))

    def showTreeMakers(cases: List[List[TreeMaker]]) = {
      debug.patmat("treeMakers:")
      debug.patmat(alignAcrossRows(cases, ">>"))
    }
  }
}

trait MatchAnalysis extends MatchApproximation {
  import PatternMatchingStats._
  import global._
  import global.definitions._

  trait MatchAnalyzer extends MatchApproximator  {
    def uncheckedWarning(pos: Position, msg: String) = currentRun.reporting.uncheckedWarning(pos, msg)
    def warn(pos: Position, ex: AnalysisBudget.Exception, kind: String) = uncheckedWarning(pos, s"Cannot check match for $kind.\n${ex.advice}")
    def reportWarning(message: String) = global.reporter.warning(typer.context.tree.pos, message)

  // TODO: model dependencies between variables: if V1 corresponds to (x: List[_]) and V2 is (x.hd), V2 cannot be assigned when V1 = null or V1 = Nil
    // right now hackily implement this by pruning counter-examples
    // unreachability would also benefit from a more faithful representation


    // reachability (dead code)

    // computes the first 0-based case index that is unreachable (if any)
    // a case is unreachable if it implies its preceding cases
    // call C the formula that is satisfiable if the considered case matches
    // call P the formula that is satisfiable if the cases preceding it match
    // the case is reachable if there is a model for -P /\ C,
    // thus, the case is unreachable if there is no model for -(-P /\ C),
    // or, equivalently, P \/ -C, or C => P
    def unreachableCase(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type): Option[Int] = {
      val start = if (Statistics.canEnable) Statistics.startTimer(patmatAnaReach) else null

      // use the same approximator so we share variables,
      // but need different conditions depending on whether we're conservatively looking for failure or success
      // don't rewrite List-like patterns, as List() and Nil need to distinguished for unreachability
      val approx = new TreeMakersToProps(prevBinder)
      def approximate(default: Prop) = approx.approximateMatch(cases, approx.onUnknown { tm =>
        approx.refutableRewrite.applyOrElse(tm, (_: TreeMaker) => default )
      })

      val propsCasesOk   = approximate(True)  map caseWithoutBodyToProp
      val propsCasesFail = approximate(False) map (t => Not(caseWithoutBodyToProp(t)))

      try {
        val (eqAxiomsFail, symbolicCasesFail) = removeVarEq(propsCasesFail, modelNull = true)
        val (eqAxiomsOk, symbolicCasesOk) = removeVarEq(propsCasesOk, modelNull = true)
        val eqAxioms = simplify(And(eqAxiomsOk, eqAxiomsFail)) // I'm pretty sure eqAxiomsOk == eqAxiomsFail, but not 100% sure.

        val prefix = mutable.ArrayBuffer[Prop]()
        prefix += eqAxioms

        var prefixRest = symbolicCasesFail
        var current = symbolicCasesOk
        var reachable = true
        var caseIndex = 0

        debug.patmat("reachability, vars:\n" + ((propsCasesFail flatMap gatherVariables).distinct map (_.describe) mkString ("\n")))
        debug.patmat(s"equality axioms:\n$eqAxiomsOk")

        // invariant (prefixRest.length == current.length) && (prefix.reverse ++ prefixRest == symbolicCasesFail)
        // termination: prefixRest.length decreases by 1
        while (prefixRest.nonEmpty && reachable) {
          val prefHead = prefixRest.head
          caseIndex += 1
          prefixRest = prefixRest.tail
          if (prefixRest.isEmpty) reachable = true
          else {
            prefix += prefHead
            current = current.tail
          val and = And((current.head +: prefix): _*)
          val model = findModelFor(eqFreePropToSolvable(and))

            // debug.patmat("trying to reach:\n"+ cnfString(current.head) +"\nunder prefix:\n"+ cnfString(prefix))
            // if (NoModel ne model) debug.patmat("reached: "+ modelString(model))

            reachable = NoModel ne model
          }
        }

        if (Statistics.canEnable) Statistics.stopTimer(patmatAnaReach, start)

        if (reachable) None else Some(caseIndex)
      } catch {
        case ex: AnalysisBudget.Exception =>
          warn(prevBinder.pos, ex, "unreachability")
          None // CNF budget exceeded
      }
    }

    // exhaustivity

    def exhaustive(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type): List[String] = if (uncheckableType(prevBinder.info)) Nil else {
      // customize TreeMakersToProps (which turns a tree of tree makers into a more abstract DAG of tests)
      // - approximate the pattern `List()` (unapplySeq on List with empty length) as `Nil`,
      //   otherwise the common (xs: List[Any]) match { case List() => case x :: xs => } is deemed unexhaustive
      // - back off (to avoid crying exhaustive too often) when:
      //    - there are guards -->
      //    - there are extractor calls (that we can't secretly/soundly) rewrite
      val start = if (Statistics.canEnable) Statistics.startTimer(patmatAnaExhaust) else null
      var backoff = false

      val approx = new TreeMakersToPropsIgnoreNullChecks(prevBinder)
      val symbolicCases = approx.approximateMatch(cases, approx.onUnknown { tm =>
        approx.fullRewrite.applyOrElse[TreeMaker, Prop](tm, {
          case BodyTreeMaker(_, _) => True // irrelevant -- will be discarded by symbolCase later
          case _ => // debug.patmat("backing off due to "+ tm)
            backoff = true
            False
        })
      }) map caseWithoutBodyToProp

      if (backoff) Nil else {
        val prevBinderTree = approx.binderToUniqueTree(prevBinder)

        // TODO: null tests generate too much noise, so disabled them -- is there any way to bring them back?
        // assuming we're matching on a non-null scrutinee (prevBinder), when does the match fail?
        // val nonNullScrutineeCond =
        //   assume non-null for all the components of the tuple we're matching on (if we're matching on a tuple)
        //   if (isTupleType(prevBinder.tpe))
        //     prevBinder.tpe.typeArgs.mapWithIndex{case (_, i) => NonNullProp(codegen.tupleSel(prevBinderTree)(i))}.reduceLeft(And)
        //   else
        //     NonNullProp(prevBinderTree)
        // val matchFails = And(symbolic(nonNullScrutineeCond), Not(symbolicCases reduceLeft (Or(_, _))))

        // when does the match fail?
        val matchFails = Not(\/(symbolicCases))

        // debug output:
        debug.patmat("analysing:")
        showTreeMakers(cases)

        // debug.patmat("\nvars:\n"+ (vars map (_.describe) mkString ("\n")))
        // debug.patmat("\nmatchFails as CNF:\n"+ cnfString(propToSolvable(matchFails)))

        try {
          // find the models (under which the match fails)
          val matchFailModels = findAllModelsFor(propToSolvable(matchFails), prevBinder.pos)

          val scrutVar = Var(prevBinderTree)
          val counterExamples = {
            matchFailModels.flatMap {
              model =>
                val varAssignments = expandModel(model)
                varAssignments.flatMap(modelToCounterExample(scrutVar) _)
            }
          }

          // sorting before pruning is important here in order to
          // keep neg/t7020.scala stable
          // since e.g. List(_, _) would cover List(1, _)
          val pruned = CounterExample.prune(counterExamples.sortBy(_.toString)).map(_.toString)

          if (Statistics.canEnable) Statistics.stopTimer(patmatAnaExhaust, start)
          pruned
        } catch {
          case ex: AnalysisBudget.Exception =>
            warn(prevBinder.pos, ex, "exhaustivity")
            Nil // CNF budget exceeded
        }
      }
    }

    object CounterExample {
      def prune(examples: List[CounterExample]): List[CounterExample] = {
        // SI-7669 Warning: we don't used examples.distinct here any more as
        //         we can have A != B && A.coveredBy(B) && B.coveredBy(A)
        //         with Nil and List().
        val result = mutable.Buffer[CounterExample]()
        for (example <- examples if (!result.exists(example coveredBy _)))
          result += example
        result.toList
      }
    }

    // a way to construct a value that will make the match fail: a constructor invocation, a constant, an object of some type)
    class CounterExample {
      protected[MatchAnalyzer] def flattenConsArgs: List[CounterExample] = Nil
      def coveredBy(other: CounterExample): Boolean = this == other || other == WildcardExample
    }
    case class ValueExample(c: ValueConst) extends CounterExample { override def toString = c.toString }
    case class TypeExample(c: Const)  extends CounterExample { override def toString = "(_ : "+ c +")" }
    case class NegativeExample(eqTo: Const, nonTrivialNonEqualTo: List[Const]) extends CounterExample {
      // require(nonTrivialNonEqualTo.nonEmpty, nonTrivialNonEqualTo)
      override def toString = {
        val negation =
          if (nonTrivialNonEqualTo.tail.isEmpty) nonTrivialNonEqualTo.head.toString
          else nonTrivialNonEqualTo.map(_.toString).sorted.mkString("(", ", ", ")")
        "(x: "+ eqTo +" forSome x not in "+ negation +")"
      }
    }
    case class ListExample(ctorArgs: List[CounterExample]) extends CounterExample {
      protected[MatchAnalyzer] override def flattenConsArgs: List[CounterExample] = ctorArgs match {
        case hd :: tl :: Nil => hd :: tl.flattenConsArgs
        case _ => Nil
      }
      protected[MatchAnalyzer] lazy val elems = flattenConsArgs

      override def coveredBy(other: CounterExample): Boolean =
        other match {
          case other@ListExample(_) =>
            this == other || ((elems.length == other.elems.length) && (elems zip other.elems).forall{case (a, b) => a coveredBy b})
          case _ => super.coveredBy(other)
        }

      override def toString = elems.mkString("List(", ", ", ")")
    }
    case class TupleExample(ctorArgs: List[CounterExample]) extends CounterExample {
      override def toString = ctorArgs.mkString("(", ", ", ")")

      override def coveredBy(other: CounterExample): Boolean =
        other match {
          case TupleExample(otherArgs) =>
            this == other || ((ctorArgs.length == otherArgs.length) && (ctorArgs zip otherArgs).forall{case (a, b) => a coveredBy b})
          case _ => super.coveredBy(other)
        }
    }
    case class ConstructorExample(cls: Symbol, ctorArgs: List[CounterExample]) extends CounterExample {
      override def toString = cls.decodedName + (if (cls.isModuleClass) "" else ctorArgs.mkString("(", ", ", ")"))
    }

    case object WildcardExample extends CounterExample { override def toString = "_" }
    case object NoExample extends CounterExample { override def toString = "??" }

    // returns a mapping from variable to
    // equal and notEqual symbols
    def modelToVarAssignment(model: Model): Map[Var, (Seq[Const], Seq[Const])] =
      model.toSeq.groupBy{f => f match {case (sym, value) => sym.variable} }.mapValues{ xs =>
        val (trues, falses) = xs.partition(_._2)
        (trues map (_._1.const), falses map (_._1.const))
        // should never be more than one value in trues...
      }

    def varAssignmentString(varAssignment: Map[Var, (Seq[Const], Seq[Const])]) =
      varAssignment.toSeq.sortBy(_._1.toString).map { case (v, (trues, falses)) =>
         val assignment = "== "+ (trues mkString("(", ", ", ")")) +"  != ("+ (falses mkString(", ")) +")"
         v +"(="+ v.path +": "+ v.staticTpCheckable +") "+ assignment
       }.mkString("\n")

    /**
     * The models we get from the DPLL solver need to be mapped back to counter examples.
     * However there's no precalculated mapping model -> counter example. Even worse,
     * not every valid model corresponds to a valid counter example.
     * The reason is that restricting the valid models further would for example require
     * a quadratic number of additional clauses. So to keep the optimistic case fast
     * (i.e., all cases are covered in a pattern match), the infeasible counter examples
     * are filtered later.
     *
     * The DPLL procedure keeps the literals that do not contribute to the solution
     * unassigned, e.g., for  `(a \/ b)`
     * only {a = true} or {b = true} is required and the other variable can have any value.
     *
     * This function does a smart expansion of the model and avoids models that
     * have conflicting mappings.
     *
     * For example for in case of the given set of symbols (taken from `t7020.scala`):
     *  "V2=2#16"
     *  "V2=6#19"
     *  "V2=5#18"
     *  "V2=4#17"
     *  "V2=7#20"
     *
     * One possibility would be to group the symbols by domain but
     * this would only work for equality tests and would not be compatible
     * with type tests.
     * Another observation leads to a much simpler algorithm:
     * Only one of these symbols can be set to true,
     * since `V2` can at most be equal to one of {2,6,5,4,7}.
     */
    def expandModel(solution: Solution): List[Map[Var, (Seq[Const], Seq[Const])]] = {

      val model = solution.model

      // x1 = ...
      // x1.hd = ...
      // x1.tl = ...
      // x1.hd.hd = ...
      // ...
      val varAssignment = modelToVarAssignment(model)
      debug.patmat("var assignment for model " + model + ":\n" + varAssignmentString(varAssignment))

      // group symbols that assign values to the same variables (i.e., symbols are mutually exclusive)
      // (thus the groups are sets of disjoint assignments to variables)
      val groupedByVar: Map[Var, List[Sym]] = solution.unassigned.groupBy(_.variable)

      val expanded = for {
        (variable, syms) <- groupedByVar.toList
      } yield {

        val (equal, notEqual) = varAssignment.getOrElse(variable, Nil -> Nil)

        def addVarAssignment(equalTo: List[Const], notEqualTo: List[Const]) =
          Map(variable ->((equal ++ equalTo, notEqual ++ notEqualTo)))

        // this assignment is needed in case that
        // there exists already an assign
        val allNotEqual = addVarAssignment(Nil, syms.map(_.const))

        // this assignment is conflicting on purpose:
        // a list counter example could contain wildcards: e.g. `List(_,_)`
        val allEqual = addVarAssignment(syms.map(_.const), Nil)

        if(equal.isEmpty) {
          val oneHot = for {
            s <- syms
          } yield {
            addVarAssignment(List(s.const), syms.filterNot(_ == s).map(_.const))
          }
          allEqual :: allNotEqual :: oneHot
        } else {
          allEqual :: allNotEqual :: Nil
        }
      }

      if (expanded.isEmpty) {
        List(varAssignment)
      } else {
        // we need the Cartesian product here,
        // since we want to report all missing cases
        // (i.e., combinations)
        val cartesianProd = expanded.reduceLeft((xs, ys) =>
          for {map1 <- xs
               map2 <- ys} yield {
            map1 ++ map2
          })

        // add expanded variables
        // note that we can just use `++`
        // since the Maps have disjoint keySets
        for {
          m <- cartesianProd
        } yield {
          varAssignment ++ m
        }
      }
    }

    // return constructor call when the model is a true counter example
    // (the variables don't take into account type information derived from other variables,
    //  so, naively, you might try to construct a counter example like _ :: Nil(_ :: _, _ :: _),
    //  since we didn't realize the tail of the outer cons was a Nil)
    def modelToCounterExample(scrutVar: Var)(varAssignment: Map[Var, (Seq[Const], Seq[Const])]): Option[CounterExample] = {
      // chop a path into a list of symbols
      def chop(path: Tree): List[Symbol] = path match {
        case Ident(_) => List(path.symbol)
        case Select(pre, name) => chop(pre) :+ path.symbol
        case _ =>
          // debug.patmat("don't know how to chop "+ path)
          Nil
      }

      // turn the variable assignments into a tree
      // the root is the scrutinee (x1), edges are labelled by the fields that are assigned
      // a node is a variable example (which is later turned into a counter example)
      object VariableAssignment {
        private def findVar(path: List[Symbol]) = path match {
          case List(root) if root == scrutVar.path.symbol => Some(scrutVar)
          case _ => varAssignment.find{case (v, a) => chop(v.path) == path}.map(_._1)
        }

        private val uniques = new mutable.HashMap[Var, VariableAssignment]
        private def unique(variable: Var): VariableAssignment =
          uniques.getOrElseUpdate(variable, {
            val (eqTo, neqTo) = varAssignment.getOrElse(variable, (Nil, Nil)) // TODO
            VariableAssignment(variable, eqTo.toList, neqTo.toList)
          })

        def apply(variable: Var): VariableAssignment = {
          val path  = chop(variable.path)
          val pre   = path.init
          val field = path.last

          val newCtor = unique(variable)

          if (pre.isEmpty) newCtor
          else {
            findVar(pre) foreach { preVar =>
              val outerCtor = this(preVar)
              outerCtor.addField(field, newCtor)
            }
            newCtor
          }
        }
      }

      // node in the tree that describes how to construct a counter-example
      case class VariableAssignment(variable: Var, equalTo: List[Const], notEqualTo: List[Const]) {
        private val fields: mutable.Map[Symbol, VariableAssignment] = mutable.HashMap.empty
        // need to prune since the model now incorporates all super types of a constant (needed for reachability)
        private lazy val uniqueEqualTo = equalTo filterNot (subsumed => equalTo.exists(better => (better ne subsumed) && instanceOfTpImplies(better.tp, subsumed.tp)))
        private lazy val inSameDomain = uniqueEqualTo forall (const => variable.domainSyms.exists(_.exists(_.const.tp =:= const.tp)))
        private lazy val prunedEqualTo = uniqueEqualTo filterNot (subsumed => variable.staticTpCheckable <:< subsumed.tp)
        private lazy val ctor       = (prunedEqualTo match { case List(TypeConst(tp)) => tp case _ => variable.staticTpCheckable }).typeSymbol.primaryConstructor
        private lazy val ctorParams = if (ctor.paramss.isEmpty) Nil else ctor.paramss.head
        private lazy val cls        = ctor.safeOwner
        private lazy val caseFieldAccs = cls.caseFieldAccessors

        def addField(symbol: Symbol, assign: VariableAssignment) {
          // SI-7669 Only register this field if if this class contains it.
          val shouldConstrainField = !symbol.isCaseAccessor || caseFieldAccs.contains(symbol)
          if (shouldConstrainField) fields(symbol) = assign
        }

        def allFieldAssignmentsLegal: Boolean =
          (fields.keySet subsetOf caseFieldAccs.toSet) && fields.values.forall(_.allFieldAssignmentsLegal)

        private lazy val nonTrivialNonEqualTo = notEqualTo.filterNot{c => c.isAny }

        // NoExample if the constructor call is ill-typed
        // (thus statically impossible -- can we incorporate this into the formula?)
        // beBrief is used to suppress negative information nested in tuples -- it tends to get too noisy
        def toCounterExample(beBrief: Boolean = false): Option[CounterExample] =
          if (!allFieldAssignmentsLegal) Some(NoExample)
          else {
            debug.patmat("describing "+ ((variable, equalTo, notEqualTo, fields, cls, allFieldAssignmentsLegal)))
            val res = prunedEqualTo match {
              // a definite assignment to a value
              case List(eq: ValueConst) if fields.isEmpty => Some(ValueExample(eq))

              // constructor call
              // or we did not gather any information about equality but we have information about the fields
              //  --> typical example is when the scrutinee is a tuple and all the cases first unwrap that tuple and only then test something interesting
              case _ if cls != NoSymbol && !isPrimitiveValueClass(cls) &&
                        (  uniqueEqualTo.nonEmpty
                        || (fields.nonEmpty && prunedEqualTo.isEmpty && notEqualTo.isEmpty)) =>

                def args(brevity: Boolean = beBrief) = {
                  // figure out the constructor arguments from the field assignment
                  val argLen = (caseFieldAccs.length min ctorParams.length)

                  val examples = (0 until argLen).map(i => fields.get(caseFieldAccs(i)).map(_.toCounterExample(brevity)) getOrElse Some(WildcardExample)).toList
                  sequence(examples)
                }

                cls match {
                  case ConsClass                                =>
                    args().map {
                      case List(NoExample, l: ListExample) =>
                        // special case for neg/t7020.scala:
                        // if we find a counter example `??::*` we report `*::*` instead
                        // since the `??` originates from uniqueEqualTo containing several instanced of the same type
                        List(WildcardExample, l)
                      case args                            => args
                    }.map(ListExample)
                  case _ if isTupleSymbol(cls)                  => args(brevity = true).map(TupleExample)
                  case _ if cls.isSealed && cls.isAbstractClass =>
                    // don't report sealed abstract classes, since
                    // 1) they can't be instantiated
                    // 2) we are already reporting any missing subclass (since we know the full domain)
                    // (see patmatexhaust.scala)
                    None
                  case _                                        => args().map(ConstructorExample(cls, _))
                }

              // a definite assignment to a type
              case List(eq) if fields.isEmpty => Some(TypeExample(eq))

              // negative information
              case Nil if nonTrivialNonEqualTo.nonEmpty =>
                // negation tends to get pretty verbose
                if (beBrief) Some(WildcardExample)
                else {
                  val eqTo = equalTo.headOption getOrElse TypeConst(variable.staticTpCheckable)
                  Some(NegativeExample(eqTo, nonTrivialNonEqualTo))
                }

              // if uniqueEqualTo contains more than one symbol of the same domain
              // then we can safely ignore these counter examples since we will eventually encounter
              // both counter examples separately
              case _ if inSameDomain => None

              // not a valid counter-example, possibly since we have a definite type but there was a field mismatch
              // TODO: improve reasoning -- in the mean time, a false negative is better than an annoying false positive
              case _ => Some(NoExample)
            }
            debug.patmatResult("described as")(res)
          }

        override def toString = toCounterExample().toString
      }

      // slurp in information from other variables
      varAssignment.keys.foreach{ v => if (v != scrutVar) VariableAssignment(v) }

      // this is the variable we want a counter example for
      VariableAssignment(scrutVar).toCounterExample()
    }

    def analyzeCases(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type, suppression: Suppression): Unit = {
      if (!suppression.suppressUnreachable) {
        unreachableCase(prevBinder, cases, pt) foreach { caseIndex =>
          reportUnreachable(cases(caseIndex).last.pos)
        }
      }
      if (!suppression.suppressExhaustive) {
        val counterExamples = exhaustive(prevBinder, cases, pt)
        if (counterExamples.nonEmpty)
          reportMissingCases(prevBinder.pos, counterExamples)
      }
    }
  }
}