Propagate Range through MatchType cases

Author: odersky

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

@@ -5953,7 +5953,13 @@ object Types {
         case _ =>
           scrutinee match
             case Range(lo, hi) => range(bound.bounds.lo, bound.bounds.hi)
-            case _ => tp.derivedMatchType(bound, scrutinee, cases)
+            case _ =>
+              if cases.exists(isRange) then
+                Range(
+                  tp.derivedMatchType(bound, scrutinee, cases.map(lower)),
+                  tp.derivedMatchType(bound, scrutinee, cases.map(upper)))
+              else
+                tp.derivedMatchType(bound, scrutinee, cases)
 
     override protected def derivedSkolemType(tp: SkolemType, info: Type): Type =
       if info eq tp.info then tp

tests/neg/i15352.scala

@@ -0,0 +1,17 @@
+import scala.compiletime.*
+import scala.compiletime.ops.int.*
+
+abstract sealed class HList :
+  def ::[H](head: H): HNonEmpty[H, this.type] = HNonEmpty(head, this)
+
+case object HNil extends HList
+case class HNonEmpty[H, T <: HList](head: H, tail: T) extends HList :
+  type Elem[N <: Int] =
+    N match
+      case 0 => H
+      case S[n1] => Elem[n1]
+
+  inline def apply[N <: Int](n: N): Elem[N] =
+    n match
+      case _: 0 => head
+      case _: S[n1] => tail.asInstanceOf[HNonEmpty[?, ?]].apply(constValue[n1]) // error