Stack of MT Reduction

Author: Lluc24

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

@@ -28,6 +28,8 @@ import NameKinds.WildcardParamName
 import MatchTypes.isConcrete
 import reporting.Message.Note
 import scala.util.boundary, boundary.break
+import dotty.tools.dotc.util.Property
+import scala.collection.mutable.Stack
 
 /** Provides methods to compare types.
  */
@@ -3594,6 +3596,8 @@ object MatchReducer:
       case Stuck              => "Stuck"
       case NoInstance(fails)  => "NoInstance(" ~ Text(fails.map(p.toText(_) ~ p.toText(_)), ", ") ~ ")"
 
+  val Reduction = new Property.Key[Stack[Type]]
+
 /** A type comparer for reducing match types.
  *  TODO: Not sure this needs to be a type comparer. Can we make it a
  *  separate class?
@@ -3940,7 +3944,23 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
             MatchTypeTrace.noInstance(scrut, cas, fails)
             NoType
           case MatchResult.Reduced(tp) =>
-            tp.simplified
+            val reductionStatus = ctx.property(Reduction) match
+              case Some(stack) => stack
+              case None     => Stack[Type]()
+
+            println(s"Scrutinee ${scrut.show}")
+            println(s"Reduction stack: ${reductionStatus.map(_.show)}")
+
+            if reductionStatus.contains(tp) then
+              ErrorType(em"Cyclic match type reduction detected")
+            else
+              val bodyType = {
+                reductionStatus.push(tp)
+                given Context = ctx.fresh.setProperty(Reduction, reductionStatus)
+                tp.simplified
+              }
+              bodyType
+
           case MatchResult.ReducedAndDisjoint =>
             // Empty types break the basic assumption that if a scrutinee and a
             // pattern are disjoint it's OK to reduce passed that pattern. Indeed,