Fix #4674: Improve error message

Author: nicolasstucki

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

@@ -2097,7 +2097,12 @@ object messages {
 
   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 msg = {
+      val addendum =
+        if (foundCls != testCls) s" is a subtype of $testCls"
+        else " is the same as the tested type"
+      s"The highlighted type test will always succeed since the scrutinee type ($foundCls)" + addendum
+    }
     val explanation = ""
   }
 }

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

@@ -91,8 +91,7 @@ object TypeTestsCasts {
 
           if (expr.tpe <:< testType)
             if (expr.tpe.isNotNull) {
-              if (foundCls != testCls)
-                ctx.warning(TypeTestAlwaysSucceeds(foundCls, testCls), tree.pos)
+              ctx.warning(TypeTestAlwaysSucceeds(foundCls, testCls), tree.pos)
               constant(expr, Literal(Constant(true)))
             }
             else expr.testNotNull