diff --git a/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java b/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
index b9941ee11151..87136eda5ebe 100644
--- a/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
+++ b/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
@@ -130,6 +130,7 @@ public enum ErrorMessageID {
     DoubleDeclarationID,
     MatchCaseOnlyNullWarningID,
     ImportRenamedTwiceID,
+    TypeTestAlwaysSucceedsID,
     ;
 
     public int errorNumber() {
diff --git a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
index ef685b0dc0f2..93ac0c1832b2 100644
--- a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
@@ -2117,4 +2117,9 @@ object messages {
     val explanation: String = ""
   }
 
+  case class TypeTestAlwaysSucceeds(foundCls: Symbol, testCls: Symbol)(implicit ctx: Context) extends Message(TypeTestAlwaysSucceedsID) {
+    val kind = "Syntax"
+    val msg = s"The highlighted type test will always succeed since the scrutinee type ($foundCls) is a subtype of ${testCls}"
+    val explanation = ""
+  }
 }
diff --git a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
index c40cc7b213f2..39ea428c5ac9 100644
--- a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
@@ -10,6 +10,7 @@ import ValueClasses._
 import SymUtils._
 import core.Flags._
 import util.Positions._
+import reporting.diagnostic.messages.TypeTestAlwaysSucceeds
 import reporting.trace
 
 
@@ -90,9 +91,7 @@ object TypeTestsCasts {
 
           if (expr.tpe <:< testType)
             if (expr.tpe.isNotNull) {
-              ctx.warning(
-                em"this will always yield true, since `$foundCls` is a subclass of `$testCls`",
-                expr.pos)
+              ctx.warning(TypeTestAlwaysSucceeds(foundCls, testCls), tree.pos)
               constant(expr, Literal(Constant(true)))
             }
             else expr.testNotNull