Fix #6047: Implement variance rules for match types

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -759,6 +759,9 @@ class Definitions {
 
     lazy val TypeBox_CAP: TypeSymbol = TypeBoxType.symbol.requiredType(tpnme.CAP)
 
+  lazy val MatchCaseType: TypeRef = ctx.requiredClassRef("scala.internal.MatchCase")
+  def MatchCaseClass(implicit ctx: Context): ClassSymbol = MatchCaseType.symbol.asClass
+
   lazy val NotType: TypeRef = ctx.requiredClassRef("scala.implicits.Not")
   def NotClass(implicit ctx: Context): ClassSymbol = NotType.symbol.asClass
   def NotModule(implicit ctx: Context): Symbol = NotClass.companionModule
@@ -931,6 +934,23 @@ class Definitions {
     }
   }
 
+  object MatchCase {
+    def apply(pat: Type, body: Type)(implicit ctx: Context): Type =
+      MatchCaseType.appliedTo(pat, body)
+    def unapply(tp: Type)(implicit ctx: Context): Option[(Type, Type)] = tp match {
+      case AppliedType(tycon, pat :: body :: Nil) if tycon.isRef(MatchCaseClass) =>
+        Some((pat, body))
+      case _ =>
+        None
+    }
+    def isInstance(tp: Type)(implicit ctx: Context): Boolean = tp match {
+      case AppliedType(tycon: TypeRef, _) =>
+        tycon.name == tpnme.MatchCase && // necessary pre-filter to avoid forcing symbols
+        tycon.isRef(MatchCaseClass)
+      case _ => false
+    }
+  }
+
   /** An extractor for multi-dimensional arrays.
    *  Note that this will also extract the high bound if an
    *  element type is a wildcard. E.g.

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -337,6 +337,7 @@ object StdNames {
     val Literal: N              = "Literal"
     val LiteralAnnotArg: N      = "LiteralAnnotArg"
     val longHash: N             = "longHash"
+    val MatchCase: N            = "MatchCase"
     val Modifiers: N            = "Modifiers"
     val NestedAnnotArg: N       = "NestedAnnotArg"
     val NoFlags: N              = "NoFlags"

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2134,45 +2134,46 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       }
     }
 
-    var result: Type = NoType
-    var remainingCases = cases
-    while (!remainingCases.isEmpty) {
-      val (cas :: cass) = remainingCases
-      remainingCases = cass
-      val saved = constraint
-      try {
-        inFrozenConstraint {
-          val cas1 = cas match {
-            case cas: HKTypeLambda =>
-              caseLambda = constrained(cas)
-              caseLambda.resultType
+    /** Match a single case.
+     *  @return  Some(tp)     if the match succeeds with type `tp`
+     *           Some(NoType) if the match fails, and there is an overlap between pattern and scrutinee
+     *           None         if the match fails and we should consider the following cases
+     *                        because scrutinee and pattern do not overlap
+     */
+    def matchCase(cas: Type): Option[Type] = {
+      val cas1 = cas match {
+        case cas: HKTypeLambda =>
+          caseLambda = constrained(cas)
+          caseLambda.resultType
+        case _ =>
+          cas
+      }
+      val defn.MatchCase(pat, body) = cas1
+      if (isSubType(scrut, pat))
+        // `scrut` is a subtype of `pat`: *It's a Match!*
+        Some {
+          caseLambda match {
+            case caseLambda: HKTypeLambda =>
+              val instances = paramInstances(new Array(caseLambda.paramNames.length), pat)
+              instantiateParams(instances)(body)
             case _ =>
-              cas
-          }
-          val defn.FunctionOf(pat :: Nil, body, _, _) = cas1
-          if (isSubType(scrut, pat)) {
-            // `scrut` is a subtype of `pat`: *It's a Match!*
-            result = caseLambda match {
-              case caseLambda: HKTypeLambda =>
-                val instances = paramInstances(new Array(caseLambda.paramNames.length), pat)
-                instantiateParams(instances)(body)
-              case _ =>
-                body
-            }
-            remainingCases = Nil
-          } else if (!intersecting(scrut, pat)) {
-            // We found a proof that `scrut` and  `pat` are incompatible.
-            // The search continues.
-          } else {
-            // We are stuck: this match type instanciation is irreducible.
-            result = NoType
-            remainingCases = Nil
+              body
           }
         }
-      }
-      finally constraint = saved
+      else if (intersecting(scrut, pat))
+        Some(NoType)
+      else
+        // We found a proof that `scrut` and  `pat` are incompatible.
+        // The search continues.
+        None
     }
-    result
+
+    def recur(cases: List[Type]): Type = cases match {
+      case cas :: cases1 => matchCase(cas).getOrElse(recur(cases1))
+      case Nil => NoType
+    }
+
+    inFrozenConstraint(recur(cases))
   }
 }
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3756,7 +3756,7 @@ object Types {
 
     def caseType(tp: Type)(implicit ctx: Context): Type = tp match {
       case tp: HKTypeLambda => caseType(tp.resType)
-      case defn.FunctionOf(_, restpe, _, _) => restpe
+      case defn.MatchCase(_, body) => body
     }
 
     def alternatives(implicit ctx: Context): List[Type] = cases.map(caseType)
@@ -4417,10 +4417,12 @@ object Types {
 
         case tp: LambdaType =>
           def mapOverLambda = {
-            variance = -variance
+            val restpe = tp.resultType
+            val saved = variance
+            variance = if (defn.MatchCase.isInstance(restpe)) 0 else -variance
             val ptypes1 = tp.paramInfos.mapConserve(this).asInstanceOf[List[tp.PInfo]]
-            variance = -variance
-            derivedLambdaType(tp)(ptypes1, this(tp.resultType))
+            variance = saved
+            derivedLambdaType(tp)(ptypes1, this(restpe))
           }
           mapOverLambda
 
@@ -4440,7 +4442,9 @@ object Types {
           derivedOrType(tp, this(tp.tp1), this(tp.tp2))
 
         case tp: MatchType =>
-          derivedMatchType(tp, this(tp.bound), this(tp.scrutinee), tp.cases.mapConserve(this))
+          val bound1 = this(tp.bound)
+          val scrut1 = atVariance(0)(this(tp.scrutinee))
+          derivedMatchType(tp, bound1, scrut1, tp.cases.mapConserve(this))
 
         case tp: SkolemType =>
           derivedSkolemType(tp, this(tp.info))
@@ -4804,10 +4808,12 @@ object Types {
       case _: BoundType | _: ThisType => x
 
       case tp: LambdaType =>
-        variance = -variance
+        val restpe = tp.resultType
+        val saved = variance
+        variance = if (defn.MatchCase.isInstance(restpe)) 0 else -variance
         val y = foldOver(x, tp.paramInfos)
-        variance = -variance
-        this(y, tp.resultType)
+        variance = saved
+        this(y, restpe)
 
       case tp: TermRef =>
         if (stopAtStatic && tp.currentSymbol.isStatic || (tp.prefix `eq` NoPrefix)) x
@@ -4835,7 +4841,9 @@ object Types {
         this(this(x, tp.tp1), tp.tp2)
 
       case tp: MatchType =>
-        foldOver(this(this(x, tp.bound), tp.scrutinee), tp.cases)
+        val x1 = this(x, tp.bound)
+        val x2 = atVariance(0)(this(x1, tp.scrutinee))
+        foldOver(x2, tp.cases)
 
       case AnnotatedType(underlying, annot) =>
         this(applyToAnnot(x, annot), underlying)

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -28,6 +28,7 @@ object Formatting {
       case arg: Showable =>
         try arg.show
         catch {
+          case ex: CyclicReference => "... (caught cyclic reference) ..."
           case NonFatal(ex)
           if !ctx.mode.is(Mode.PrintShowExceptions) &&
              !ctx.settings.YshowPrintErrors.value =>

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -166,7 +166,10 @@ class PlainPrinter(_ctx: Context) extends Printer {
         changePrec(OrTypePrec) { toText(tp1) ~ " | " ~ atPrec(OrTypePrec + 1) { toText(tp2) } }
       case MatchType(bound, scrutinee, cases) =>
         changePrec(GlobalPrec) {
-          def caseText(tp: Type): Text = "case " ~ toText(tp)
+          def caseText(tp: Type): Text = tp match {
+            case defn.MatchCase(pat, body) => "case " ~ toText(pat) ~ " => " ~ toText(body)
+            case _ => "case " ~ toText(tp)
+          }
           def casesText = Text(cases.map(caseText), "\n")
             atPrec(InfixPrec) { toText(scrutinee) } ~
             keywordStr(" match ") ~ "{" ~ casesText ~ "}" ~

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -13,6 +13,7 @@ import core.Mode
 import dotty.tools.dotc.core.Symbols.{Symbol, NoSymbol}
 import diagnostic.messages._
 import diagnostic._
+import ast.{tpd, Trees}
 import Message._
 
 object Reporter {
@@ -89,21 +90,25 @@ trait Reporting { this: Context =>
   }
 
   def warning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =
-    reportWarning(new Warning(msg, pos))
+    reportWarning(new Warning(msg, addInlineds(pos)))
 
-  def strictWarning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =
-    if (this.settings.strict.value) error(msg, pos)
-    else reportWarning(new ExtendMessage(() => msg)(_ + "\n(This would be an error under strict mode)").warning(pos))
+  def strictWarning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit = {
+    val fullPos = addInlineds(pos)
+    if (this.settings.strict.value) error(msg, fullPos)
+    else reportWarning(new ExtendMessage(() => msg)(_ + "\n(This would be an error under strict mode)").warning(fullPos))
+  }
 
   def error(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =
-    reporter.report(new Error(msg, pos))
+    reporter.report(new Error(msg, addInlineds(pos)))
 
-  def errorOrMigrationWarning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =
-    if (ctx.scala2Mode) migrationWarning(msg, pos) else error(msg, pos)
+  def errorOrMigrationWarning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit = {
+    val fullPos = addInlineds(pos)
+    if (ctx.scala2Mode) migrationWarning(msg, fullPos) else error(msg, fullPos)
+  }
 
   def restrictionError(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report {
-      new ExtendMessage(() => msg)(m => s"Implementation restriction: $m").error(pos)
+      new ExtendMessage(() => msg)(m => s"Implementation restriction: $m").error(addInlineds(pos))
     }
 
   def incompleteInputError(msg: => Message, pos: SourcePosition = NoSourcePosition)(implicit ctx: Context): Unit =
@@ -135,6 +140,14 @@ trait Reporting { this: Context =>
 
   def debugwarn(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     if (this.settings.Ydebug.value) warning(msg, pos)
+
+  private def addInlineds(pos: SourcePosition)(implicit ctx: Context) = {
+    def recur(pos: SourcePosition, inlineds: List[Trees.Tree[_]]): SourcePosition = inlineds match {
+      case inlined :: inlineds1 => pos.withOuter(recur(inlined.sourcePos, inlineds1))
+      case Nil => pos
+    }
+    recur(pos, tpd.enclosingInlineds)
+  }
 }
 
 /**

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -487,7 +487,7 @@ trait TypeAssigner {
         }
         HKTypeLambda.fromParams(
           params(new mutable.ListBuffer[TypeSymbol](), pat).toList,
-          defn.FunctionOf(pat.tpe :: Nil, body.tpe))
+          defn.MatchCase(pat.tpe, body.tpe))
       }
       else body.tpe
     tree.withType(ownType)

compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala

@@ -34,7 +34,7 @@ object VarianceChecker {
           val paramVarianceStr = if (v == 0) "contra" else "co"
           val occursStr = variance match {
             case -1 => "contra"
-            case 0 => "non"
+            case 0 => "in"
             case 1 => "co"
           }
           val pos = tree.tparams
@@ -123,18 +123,19 @@ class VarianceChecker()(implicit ctx: Context) {
     def apply(status: Option[VarianceError], tp: Type): Option[VarianceError] = trace(s"variance checking $tp of $base at $variance", variances) {
       try
         if (status.isDefined) status
-        else tp match {
+        else tp.normalized match {
           case tp: TypeRef =>
             val sym = tp.symbol
             if (sym.variance != 0 && base.isContainedIn(sym.owner)) checkVarianceOfSymbol(sym)
-            else if (sym.isAliasType) this(status, sym.info.bounds.hi)
-            else foldOver(status, tp)
+            else sym.info match {
+              case MatchAlias(_) => foldOver(status, tp)
+              case TypeAlias(alias) => this(status, alias)
+              case _ => foldOver(status, tp)
+            }
           case tp: MethodOrPoly =>
             this(status, tp.resultType) // params will be checked in their TypeDef or ValDef nodes.
           case AnnotatedType(_, annot) if annot.symbol == defn.UncheckedVarianceAnnot =>
             status
-          case tp: MatchType =>
-            apply(status, tp.bound)
           case tp: ClassInfo =>
             foldOver(status, tp.classParents)
           case _ =>
@@ -179,7 +180,7 @@ class VarianceChecker()(implicit ctx: Context) {
         sym.is(PrivateLocal) ||
         sym.name.is(InlineAccessorName) || // TODO: should we exclude all synthetic members?
         sym.is(TypeParam) && sym.owner.isClass // already taken care of in primary constructor of class
-      tree match {
+      try tree match {
         case defn: MemberDef if skip =>
           ctx.debuglog(s"Skipping variance check of ${sym.showDcl}")
         case tree: TypeDef =>
@@ -196,6 +197,9 @@ class VarianceChecker()(implicit ctx: Context) {
           vparamss foreach (_ foreach traverse)
         case _ =>
       }
+      catch {
+        case ex: TypeError => ctx.error(ex.toMessage, tree.sourcePos.focus)
+      }
     }
   }
 }

compiler/test-resources/repl/i5218

@@ -1,7 +1,7 @@
 scala> val tuple = (1, "2", 3L)
 val tuple: (Int, String, Long) = (1,2,3)
 scala> 0.0 *: tuple
-val res0: Double *: (Int, String, Long)(tuple) = (0.0,1,2,3)
+val res0: (Double, Int, String, Long) = (0.0,1,2,3)
 scala> tuple ++ tuple
 val res1: Int *: String *: Long *:
   scala.Tuple.Concat[Unit, (Int, String, Long)(tuple)] = (1,2,3,1,2,3)

compiler/test/dotc/run-test-pickling.blacklist

@@ -5,6 +5,7 @@ t3452g
 t7374
 tuples1.scala
 tuples1a.scala
+typeclass-derivation1.scala
 typeclass-derivation2.scala
 typeclass-derivation2a.scala
 typeclass-derivation3.scala