Lub MatchTypeTree case body types and store as the inferred bound

Author: dwijnand

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

@@ -1030,17 +1030,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         if tp1 ne tp1norm then recur(tp1norm, tp2)
         else either(recur(tp11, tp2), recur(tp12, tp2))
       case tp1: MatchType =>
-        def compareUpper =
-          val lub1 = tp1.cases.foldLeft(defn.NothingType: Type): (acc, case1) =>
-            if acc.exists then
-              val rhs = case1.resultType match { case defn.MatchCase(_, body) => body }
-              val isRecursive = rhs.existsPart(_.isInstanceOf[LazyRef])
-              if isRecursive then NoType else lub(acc, rhs)
-            else acc
-          if lub1.exists then
-            recur(lub1, tp2)
-          else
-            recur(tp1.underlying, tp2)
         def compareMatch = tp2 match {
           case tp2: MatchType =>
             // we allow a small number of scrutinee types to be widened:
@@ -1058,7 +1047,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             tp1.cases.corresponds(tp2.cases)(isSubType)
           case _ => false
         }
-        (!caseLambda.exists || canWidenAbstract) && compareUpper || compareMatch
+        (!caseLambda.exists || canWidenAbstract) && recur(tp1.underlying, tp2) || compareMatch
       case tp1: AnnotatedType if tp1.isRefining =>
         isNewSubType(tp1.parent)
       case JavaArrayType(elem1) =>

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

@@ -2373,7 +2373,15 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       report.error(MatchTypeScrutineeCannotBeHigherKinded(sel1Tpe), sel1.srcPos)
     val pt1 = if (bound1.isEmpty) pt else bound1.tpe
     val cases1 = tree.cases.mapconserve(typedTypeCase(_, sel1Tpe, pt1))
-    assignType(cpy.MatchTypeTree(tree)(bound1, sel1, cases1), bound1, sel1, cases1)
+    val bound2 = if tree.bound.isEmpty then
+      val lub = cases1.foldLeft(defn.NothingType: Type): (acc, case1) =>
+        if !acc.exists then NoType
+        else if case1.body.tpe.existsPart(_.isInstanceOf[LazyRef]) then NoType
+        else acc | TypeOps.avoid(case1.body.tpe, patVars(case1))
+      if lub.exists then TypeTree(lub, inferred = true)
+      else bound1
+    else bound1
+    assignType(cpy.MatchTypeTree(tree)(bound2, sel1, cases1), bound2, sel1, cases1)
   }
 
   def typedByNameTypeTree(tree: untpd.ByNameTypeTree)(using Context): ByNameTypeTree = tree.result match