Fix #13495: break completion cycles

Author: OlivierBlanvillain

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

@@ -1611,7 +1611,7 @@ object SymDenotations {
                   case _ => false
 
         if owner.isClass then
-          for c <- owner.info.decls.toList if maybeChild(c) do
+          for c <- owner.info.decls.toList if maybeChild(c) && (c.isClass || c.is(EnumVal)) do
             c.ensureCompleted()
       end completeChildrenIn
 

tests/pos/13495.scala

@@ -0,0 +1,20 @@
+import scala.annotation.showAsInfix
+
+object Test {
+  trait Component
+  sealed trait Deleted extends Component
+
+  type Deletable[L <: CList] <: CList = L match {
+    case h &: t => (h | Deleted) &: Deletable[t]
+    case CNil => CNil
+  }
+
+  sealed trait CList
+  sealed trait CNil extends CList
+  @showAsInfix case class &:[+C <: Component, +L <: CList](h: C, t: L) extends CList
+
+  case class A(x: Int, y: Int) extends Component
+  case class B(x: Int, y: Int) extends Component
+
+  val x: Deletable[A &: B &: CNil] = ???
+}