Make abstract pattern error into an error message

Author: felixmulder

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -901,4 +901,12 @@ object messages {
     val msg = hl"trying to define package with same name as `$existing`"
     val explanation = ""
   }
+
+  case class ErasedType()(implicit ctx: Context)
+  extends Message(34) {
+    val kind = "Erased Type"
+    val msg =
+      i"abstract type pattern is unchecked since it is eliminated by erasure"
+    val explanation = ""
+  }
 }

compiler/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala

@@ -6,6 +6,7 @@ import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
 import core._
 import Contexts.Context, Types._, Constants._, Decorators._, Symbols._
 import TypeUtils._, TypeErasure._, Flags._
+import reporting.diagnostic.messages._
 
 
 /** Implements partial evaluation of `sc.isInstanceOf[Sel]` according to:
@@ -148,8 +149,8 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
           val inMatch = s.qualifier.symbol is Case
 
           if (valueClassesOrAny) {
-            if (selector eq defn.ObjectType)
-              ctx.warning(i"abstract type pattern is unchecked since it is eliminated by erasure", tree.pos)
+            if ((selector eq defn.ObjectType))
+              ctx.uncheckedWarning(ErasedType(), tree.pos)
             tree
           } else if (knownStatically)
             handleStaticallyKnown(s, scrutinee, selector, inMatch, tree.pos)