Merge pull request #7756 from dotty-staging/fix-#6533

Avoid nested IgnoredProtos

Author: odersky

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

@@ -113,10 +113,14 @@ object ProtoTypes {
   }
 
   /** A class marking ignored prototypes that can be revealed by `deepenProto` */
-  case class IgnoredProto(ignored: Type) extends UncachedGroundType with MatchAlways {
-    override def revealIgnored = ignored.revealIgnored
+  case class IgnoredProto(ignored: Type) extends UncachedGroundType with MatchAlways with
+    override def revealIgnored = ignored
     override def deepenProto(implicit ctx: Context): Type = ignored
-  }
+
+  object IgnoredProto with
+    def apply(ignored: Type): IgnoredProto = ignored match
+      case ignored: IgnoredProto => ignored
+      case _ => new IgnoredProto(ignored)
 
   /** A prototype for expressions [] that are part of a selection operation:
    *