Keep the unsound behavior under a -Y flag

Author: OlivierBlanvillain

community-build/community-projects/shapeless

@@ -200,6 +200,7 @@ class ScalaSettings extends Settings.SettingGroup with CommonScalaSettings {
   val YerasedTerms: Setting[Boolean] = BooleanSetting("-Yerased-terms", "Allows the use of erased terms.")
   val YcheckInit: Setting[Boolean] = BooleanSetting("-Ycheck-init", "Check initialization of objects")
   val YrequireTargetName: Setting[Boolean] = BooleanSetting("-Yrequire-targetName", "Warn if an operator is defined without a @targetName annotation")
+  val YunsoundMatchTypes: Setting[Boolean] = BooleanSetting("-Yunsound-match-types", "Use unsound match type reduction algorithm.")
 
   /** Area-specific debug output */
   val YexplainLowlevel: Setting[Boolean] = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -2511,16 +2511,20 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case (_, tp2: NamedType) if gadtBounds(tp2.symbol) != null =>
         provablyDisjoint(tp1, gadtBounds(tp2.symbol).hi) || provablyDisjoint(tp1, tp2.superType)
       case (tp1: TypeProxy, tp2: TypeProxy) =>
-        provablyDisjoint(tp1.superType, tp2) || provablyDisjoint(tp1, tp2.superType)
+        provablyDisjoint(matchTypeSuperType(tp1), tp2) || provablyDisjoint(tp1, matchTypeSuperType(tp2))
       case (tp1: TypeProxy, _) =>
-        provablyDisjoint(tp1.superType, tp2)
+        provablyDisjoint(matchTypeSuperType(tp1), tp2)
       case (_, tp2: TypeProxy) =>
-        provablyDisjoint(tp1, tp2.superType)
+        provablyDisjoint(tp1, matchTypeSuperType(tp2))
       case _ =>
         false
     }
   }
 
+  /** Restores the buggy match type reduction under -Yunsound-match-types. */
+  private def matchTypeSuperType(tp: TypeProxy): Type =
+    if ctx.settings.YunsoundMatchTypes.value then tp.underlying else tp.superType
+
   protected def explainingTypeComparer = ExplainingTypeComparer(comparerContext)
   protected def trackingTypeComparer = TrackingTypeComparer(comparerContext)