add and use error message and id

Author: esarbe

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -102,6 +102,7 @@ public enum ErrorMessageID {
     UncheckedTypePatternID,
     ExtendFinalClassID,
     EnumCaseDefinitionInNonEnumOwnerID,
+    TailrecNotApplicableNeitherPrivateNorFinalID,
     ;
 
     public int errorNumber() {

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

@@ -1777,6 +1777,7 @@ object messages {
     val explanation =
       hl"""A class marked with the ${"final"} keyword cannot be extended"""
   }
+
   case class EnumCaseDefinitionInNonEnumOwner(owner: Symbol)(implicit ctx: Context)
     extends Message(EnumCaseDefinitionInNonEnumOwnerID) {
       val kind = "Syntax"
@@ -1786,4 +1787,15 @@ object messages {
             |If you want to create an ${"enum"} case, make sure the corresponding ${"enum class"} exists
             |and has the ${"enum"} keyword."""
   }
+
+
+  case class TailrecNotApplicableNeitherPrivateNorFinal(owner: Symbol)(implicit ctx: Context)
+    extends Message(TailrecNotApplicableNeitherPrivateNorFinalID) {
+      val kind = "Syntax"
+      val msg = hl"TailRec optimisation not applicable, ${owner} is neither `private` nor `final` so can be overridden"
+      val explanation =
+        hl"""
+            |The `@tailrec` annotation can only be used on methods that are either `private` or `final`.
+          """
+  }
 }

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

@@ -12,6 +12,7 @@ import Symbols._
 import Types._
 import NameKinds.TailLabelName
 import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+import reporting.diagnostic.messages.TailrecNotApplicableNeitherPrivateNorFinal
 
 /**
  * A Tail Rec Transformer
@@ -162,7 +163,7 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
           })
         }
       case d: DefDef if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
-        ctx.error("TailRec optimisation not applicable, method is neither private nor final so can be overridden", sym.pos)
+        ctx.error(TailrecNotApplicableNeitherPrivateNorFinal(sym), sym.pos)
         d
       case d if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
         ctx.error("TailRec optimisation not applicable, not a method", sym.pos)