Make aliases of MatchAliases normal TypeAliases (#19871)

Proposes to make `isMatch` true only for `MatchType`s and higher-kinded
abstraction of them.

As a result, code using `isMatch` to choose between a `TypeAlias` and
`MatchAlias` will now use a `TypeAlias` when aliasing a `MatchAlias`.
Which in turn allows for better de-aliasing, since `dealias` only
de-aliases standard type aliases.

`tryNormalize` on `AppliedType` should only attempt reduction if there
is an underlying match type. This could previously be identified by a
`MatchAlias` tycon. We now need a recursive check.

Fixes #19821

Author: odersky

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

@@ -461,7 +461,7 @@ class TypeApplications(val self: Type) extends AnyVal {
    */
   final def toBounds(using Context): TypeBounds = self match {
     case self: TypeBounds => self // this can happen for wildcard args
-    case _ => if (self.isMatch) MatchAlias(self) else TypeAlias(self)
+    case _ => AliasingBounds(self)
   }
 
   /** Translate a type of the form From[T] to either To[T] or To[? <: T] (if `wildcardArg` is set). Keep other types as they are.

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

@@ -1375,7 +1375,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
        *    tp1 <:< app2   using isSubType (this might instantiate params in tp2)
        */
       def compareLower(tycon2bounds: TypeBounds, tyconIsTypeRef: Boolean): Boolean =
-        if ((tycon2bounds.lo `eq` tycon2bounds.hi) && !tycon2bounds.isInstanceOf[MatchAlias])
+        if ((tycon2bounds.lo `eq` tycon2bounds.hi) && !tycon2bounds.isMatchAlias)
           if (tyconIsTypeRef) recur(tp1, tp2.superTypeNormalized) && recordGadtUsageIf(MatchType.thatReducesUsingGadt(tp2))
           else isSubApproxHi(tp1, tycon2bounds.lo.applyIfParameterized(args2))
         else

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

@@ -143,7 +143,7 @@ object TypeOps:
         defn.MatchCase(simplify(pat, theMap), body)
       case tp: AppliedType =>
         tp.tycon match
-          case tycon: TypeRef if tycon.info.isInstanceOf[MatchAlias] =>
+          case tycon: TypeRef if tp.isMatchAlias =>
             isFullyDefined(tp, ForceDegree.all)
           case _ =>
         val normed = tp.tryNormalize

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

@@ -456,14 +456,19 @@ object Types extends TypeUtils {
     /** Is this a MethodType for which the parameters will not be used? */
     def hasErasedParams(using Context): Boolean = false
 
-    /** Is this a match type or a higher-kinded abstraction of one?
-     */
-    def isMatch(using Context): Boolean = underlyingMatchType.exists
+    /** Is this a match type or a higher-kinded abstraction of one? */
+    def isMatch(using Context): Boolean = stripped match
+      case tp: MatchType => true
+      case tp: HKTypeLambda => tp.resType.isMatch
+      case _ => false
+
+    /** Does this application expand to a match type? */
+    def isMatchAlias(using Context): Boolean = underlyingMatchType.exists
 
     def underlyingMatchType(using Context): Type = stripped match {
       case tp: MatchType => tp
       case tp: HKTypeLambda => tp.resType.underlyingMatchType
-      case tp: AppliedType if tp.isMatchAlias => tp.superType.underlyingMatchType
+      case tp: AppliedType => tp.underlyingMatchType
       case _ => NoType
     }
 
@@ -4529,6 +4534,9 @@ object Types extends TypeUtils {
     private var myEvalRunId: RunId = NoRunId
     private var myEvalued: Type = uninitialized
 
+    private var validUnderlyingMatch: Period = Nowhere
+    private var cachedUnderlyingMatch: Type = uninitialized
+
     def isGround(acc: TypeAccumulator[Boolean])(using Context): Boolean =
       if myGround == 0 then myGround = if acc.foldOver(true, this) then 1 else -1
       myGround > 0
@@ -4585,31 +4593,38 @@ object Types extends TypeUtils {
         case nil => x
       foldArgs(op(x, tycon), args)
 
+    /** Exists if the tycon is a TypeRef of an alias with an underlying match type.
+     *  Anything else should have already been reduced in `appliedTo` by the TypeAssigner.
+     */
+    override def underlyingMatchType(using Context): Type =
+      if ctx.period != validUnderlyingMatch then
+        validUnderlyingMatch = if tycon.isProvisional then Nowhere else ctx.period
+        cachedUnderlyingMatch = superType.underlyingMatchType
+      cachedUnderlyingMatch
+
     override def tryNormalize(using Context): Type = tycon.stripTypeVar match {
       case tycon: TypeRef =>
-        def tryMatchAlias = tycon.info match {
-          case MatchAlias(alias) =>
+        def tryMatchAlias = tycon.info match
+          case AliasingBounds(alias) if isMatchAlias =>
             trace(i"normalize $this", typr, show = true) {
               MatchTypeTrace.recurseWith(this) {
                 alias.applyIfParameterized(args.map(_.normalized)).tryNormalize
+                /* `applyIfParameterized` may reduce several HKTypeLambda applications
+                 * before the underlying MatchType is reached.
+                 * Even if they do not involve any match type normalizations yet,
+                 * we still want to record these reductions in the MatchTypeTrace.
+                 * They should however only be attempted if they eventually expand
+                 * to a match type, which is ensured by the `isMatchAlias` guard.
+                 */
               }
             }
           case _ =>
             NoType
-        }
         tryCompiletimeConstantFold.orElse(tryMatchAlias)
       case _ =>
         NoType
     }
 
-    /** Does this application expand to a match type? */
-    def isMatchAlias(using Context): Boolean = tycon.stripTypeVar match
-      case tycon: TypeRef =>
-        tycon.info match
-          case _: MatchAlias => true
-          case _ => false
-      case _ => false
-
     /** Is this an unreducible application to wildcard arguments?
      *  This is the case if tycon is higher-kinded. This means
      *  it is a subtype of a hk-lambda, but not a match alias.
@@ -5137,20 +5152,9 @@ object Types extends TypeUtils {
     def apply(bound: Type, scrutinee: Type, cases: List[Type])(using Context): MatchType =
       unique(new CachedMatchType(bound, scrutinee, cases))
 
-    def thatReducesUsingGadt(tp: Type)(using Context): Boolean = tp match
-      case MatchType.InDisguise(mt) => mt.reducesUsingGadt
-      case mt: MatchType            => mt.reducesUsingGadt
-      case _                        => false
-
-    /** Extractor for match types hidden behind an AppliedType/MatchAlias. */
-    object InDisguise:
-      def unapply(tp: AppliedType)(using Context): Option[MatchType] = tp match
-        case AppliedType(tycon: TypeRef, args) => tycon.info match
-          case MatchAlias(alias) => alias.applyIfParameterized(args) match
-            case mt: MatchType => Some(mt)
-            case _ => None
-          case _ => None
-        case _ => None
+    def thatReducesUsingGadt(tp: Type)(using Context): Boolean = tp.underlyingMatchType match
+      case mt: MatchType => mt.reducesUsingGadt
+      case _ => false
   }
 
   enum MatchTypeCasePattern:
@@ -5636,6 +5640,14 @@ object Types extends TypeUtils {
     def lower(lo: Type)(using Context): TypeBounds = apply(lo, defn.AnyType)
   }
 
+  object AliasingBounds:
+    /** A MatchAlias if alias is a match type and a TypeAlias o.w.
+     *  Note that aliasing a MatchAlias returns a normal TypeAlias.
+     */
+    def apply(alias: Type)(using Context): AliasingBounds =
+      if alias.isMatch then MatchAlias(alias) else TypeAlias(alias)
+    def unapply(tp: AliasingBounds): Option[Type] = Some(tp.alias)
+
   object TypeAlias {
     def apply(alias: Type)(using Context): TypeAlias = unique(new TypeAlias(alias))
     def unapply(tp: TypeAlias): Option[Type] = Some(tp.alias)

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -412,9 +412,7 @@ class TreeUnpickler(reader: TastyReader,
               readType().appliedTo(until(end)(readType()))
             case TYPEBOUNDS =>
               val lo = readType()
-              if nothingButMods(end) then
-                if lo.isMatch then MatchAlias(readVariances(lo))
-                else TypeAlias(readVariances(lo))
+              if nothingButMods(end) then AliasingBounds(readVariances(lo))
               else
                 val hi = readVariances(readType())
                 createNullableTypeBounds(lo, hi)

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -452,9 +452,8 @@ object Inlines:
             unrollTupleTypes(tail).map(head :: _)
           case tpe: TermRef if tpe.symbol == defn.EmptyTupleModule =>
             Some(Nil)
-          case tpRef: TypeRef => tpRef.info match
-            case MatchAlias(alias) => unrollTupleTypes(alias.tryNormalize)
-            case _ => None
+          case tpe: AppliedType if tpe.isMatchAlias =>
+            unrollTupleTypes(tpe.tryNormalize)
           case _ =>
             None
 

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

@@ -622,7 +622,7 @@ trait ImplicitRunInfo:
     sym.isClass && !isExcluded(sym)
     || sym.isOpaqueAlias
     || sym.is(Deferred, butNot = Param)
-    || sym.info.isInstanceOf[MatchAlias]
+    || sym.info.isMatchAlias
 
   private def computeIScope(rootTp: Type): OfTypeImplicits =
 
@@ -636,7 +636,7 @@ trait ImplicitRunInfo:
         else if implicitScopeCache.contains(t) then parts += t
         else
           partSeen += t
-          t.dealias match
+          t.dealias.normalized match
             case t: TypeRef =>
               if isAnchor(t.symbol) then
                 parts += t
@@ -663,7 +663,6 @@ trait ImplicitRunInfo:
               traverseChildren(t)
             case t =>
               traverseChildren(t)
-              traverse(t.normalized)
       catch case ex: Throwable => handleRecursive("collectParts of", t.show, ex)
 
       def apply(tp: Type): collection.Set[Type] =
@@ -775,6 +774,7 @@ trait ImplicitRunInfo:
    *     if `T` is of the form `(P#x).type`, the anchors of `P`.
    *   - If `T` is the this-type of a static object, the anchors of a term reference to that object.
    *   - If `T` is some other this-type `P.this.type`, the anchors of `P`.
+   *   - If `T` is match type or an applied match alias, the anchors of the normalization of `T`.
    *   - If `T` is some other type, the union of the anchors of each constituent type of `T`.
    *
    *  The _implicit scope_ of a type `tp` is the smallest set S of term references (i.e. TermRefs)
@@ -787,7 +787,7 @@ trait ImplicitRunInfo:
    *   - If `T` is a reference to an opaque type alias named `A`, S includes
    *     a reference to an object `A` defined in the same scope as the type, if it exists,
    *     as well as the implicit scope of `T`'s underlying type or bounds.
-   *   - If `T` is a reference to an an abstract type or match type alias named `A`,
+   *   - If `T` is a reference to an an abstract type or unreducible match type alias named `A`,
    *     S includes a reference to an object `A` defined in the same scope as the type,
    *     if it exists, as well as the implicit scopes of `T`'s lower and upper bound,
    *     if present.

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

@@ -519,9 +519,7 @@ trait TypeAssigner {
   def assignType(tree: untpd.TypeBoundsTree, lo: Tree, hi: Tree, alias: Tree)(using Context): TypeBoundsTree =
     tree.withType(
       if !alias.isEmpty then alias.tpe
-      else if lo eq hi then
-        if lo.tpe.isMatch then MatchAlias(lo.tpe)
-        else TypeAlias(lo.tpe)
+      else if lo eq hi then AliasingBounds(lo.tpe)
       else TypeBounds(lo.tpe, hi.tpe))
 
   def assignType(tree: untpd.Bind, sym: Symbol)(using Context): Bind =

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

@@ -1841,11 +1841,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               case _ => false
             }
 
-        val result = pt match {
+        val result = pt.underlyingMatchType match {
           case mt: MatchType if isMatchTypeShaped(mt) =>
             typedDependentMatchFinish(tree, sel1, selType, tree.cases, mt)
-          case MatchType.InDisguise(mt) if isMatchTypeShaped(mt) =>
-            typedDependentMatchFinish(tree, sel1, selType, tree.cases, mt)
           case _ =>
             typedMatchFinish(tree, sel1, selType, tree.cases, pt)
         }

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

@@ -64,6 +64,7 @@ i17149.scala
 tuple-fold.scala
 mt-redux-norm.perspective.scala
 i18211.scala
+10867.scala
 
 # Opaque type
 i5720.scala