Make aliases of MatchAliases normal TypeAliases

Make `isMatch` false for applied `MatchAlias`es,
i.e. 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.
The logic for this distinction has also been extracted to the common `AliasingBounds` supertype.

`tryNormalize` on `AppliedType`s 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.

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/Types.scala

@@ -458,7 +458,10 @@ object Types extends TypeUtils {
 
     /** Is this a match type or a higher-kinded abstraction of one?
      */
-    def isMatch(using Context): Boolean = underlyingMatchType.exists
+    def isMatch(using Context): Boolean = stripped match
+      case tp: MatchType => true
+      case tp: HKTypeLambda => tp.resType.isMatch
+      case _ => false
 
     def underlyingMatchType(using Context): Type = stripped match {
       case tp: MatchType => tp
@@ -4587,16 +4590,22 @@ object Types extends TypeUtils {
 
     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
@@ -4606,7 +4615,12 @@ object Types extends TypeUtils {
     def isMatchAlias(using Context): Boolean = tycon.stripTypeVar match
       case tycon: TypeRef =>
         tycon.info match
-          case _: MatchAlias => true
+          case AliasingBounds(alias) =>
+            alias.underlyingMatchType.exists
+            /* This is the only true case since anything other than
+             * a TypeRef of an alias with an underlying match type
+             * should have been already reduced by `appliedTo` in the TypeAssigner.
+             */
           case _ => false
       case _ => false
 
@@ -5636,6 +5650,14 @@ 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/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/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

tests/pos-macros/i11795.scala

@@ -1,7 +1,17 @@
 import scala.quoted._
 import scala.deriving._
 
-def blah2[P <: Product, MEL <: Tuple: Type, MET <: Tuple: Type](m: Mirror.ProductOf[P] { type MirroredElemLabels = MEL; type MirroredElemTypes = MET})(using Quotes) = {
+def blah[P <: Product]
+    (m: Mirror.ProductOf[P])
+    (using Quotes, Type[m.MirroredElemLabels], Type[m.MirroredElemTypes]) = {
+  type z = Tuple.Zip[m.MirroredElemLabels, m.MirroredElemTypes]
+  Type.of[z] // error
+  ()
+}
+
+def blah2[P <: Product, MEL <: Tuple: Type, MET <: Tuple: Type]
+    (m: Mirror.ProductOf[P] { type MirroredElemLabels = MEL; type MirroredElemTypes = MET})
+    (using Quotes) = {
   Type.of[Tuple.Zip[MEL, MET]]
   ()
 }

tests/pos/i19821.scala

@@ -0,0 +1,26 @@
+
+object Test:
+
+  trait T:
+    type S
+    type F = T.F[S]
+
+    def foo: F
+    def bar: T.F[S]
+
+  object T:
+    type F[X] = X match
+      case String => Option[Int]
+
+    type G[X] = X match
+      case Option[x] => Int
+
+  val t: T {type S = String} = ???
+
+  val b = t.bar
+  val m1: T.G[b.type] = ???
+  val _: Int = m1 // Ok
+
+  val f = t.foo
+  val m: T.G[f.type] = ???
+  val _: Int = m // Error before changes