Replace usages of MatchType.InDisguise by underlyingMatchType

Author: EugeneFlesselle

Diff for: 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

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -5151,20 +5151,9 @@ 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:

Diff for: 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)
         }