Update the error message for 'CaseClassCannotExtendEnum' to include enum name

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

@@ -2121,9 +2121,9 @@ object messages {
            |""".stripMargin
   }
 
-  case class CaseClassCannotExtendEnum(cls: Symbol)(implicit ctx: Context) extends Message(CaseClassCannotExtendEnumID) {
+  case class CaseClassCannotExtendEnum(cls: Symbol, parent: Symbol)(implicit ctx: Context) extends Message(CaseClassCannotExtendEnumID) {
     override def kind: String = "Syntax"
-    override def msg: String = hl"""normal case $cls in ${cls.owner} cannot extend an enum"""
+    override def msg: String = hl"""normal case class cannot extend an enum. case $cls in ${cls.owner} is extending enum ${parent.name}."""
     override def explanation: String = ""
   }
 }

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

@@ -853,14 +853,14 @@ trait Checking {
       ctx.error(em"$what can only be used in a rewrite method", pos)
 
   /** Check that all case classes that extend `scala.Enum` are `enum` cases */
-  def checkEnum(cdef: untpd.TypeDef, cls: Symbol)(implicit ctx: Context): Unit = {
+  def checkEnum(cdef: untpd.TypeDef, cls: Symbol, parent: Symbol)(implicit ctx: Context): Unit = {
     import untpd.modsDeco
     def isEnumAnonCls =
       cls.isAnonymousClass &&
       cls.owner.isTerm &&
       (cls.owner.flagsUNSAFE.is(Case) || cls.owner.name == nme.DOLLAR_NEW)
     if (!cdef.mods.isEnumCase && !isEnumAnonCls)
-      ctx.error(CaseClassCannotExtendEnum(cls), cdef.pos)
+      ctx.error(CaseClassCannotExtendEnum(cls, parent), cdef.pos)
   }
 
   /** Check that all references coming from enum cases in an enum companion object
@@ -938,7 +938,7 @@ trait Checking {
 
 trait ReChecking extends Checking {
   import tpd._
-  override def checkEnum(cdef: untpd.TypeDef, cls: Symbol)(implicit ctx: Context): Unit = ()
+  override def checkEnum(cdef: untpd.TypeDef, cls: Symbol, parent: Symbol)(implicit ctx: Context): Unit = ()
   override def checkRefsLegal(tree: tpd.Tree, badOwner: Symbol, allowed: (Name, Symbol) => Boolean, where: String)(implicit ctx: Context): Unit = ()
   override def checkEnumCompanions(stats: List[Tree], enumContexts: collection.Map[Symbol, Context])(implicit ctx: Context): List[Tree] = stats
 }

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

@@ -1578,7 +1578,10 @@ class Typer extends Namer
         .withType(dummy.termRef)
       if (!cls.is(AbstractOrTrait) && !ctx.isAfterTyper)
         checkRealizableBounds(cls, cdef.namePos)
-      if (cls.is(Case) && cls.derivesFrom(defn.EnumClass)) checkEnum(cdef, cls)
+      if (cls.is(Case) && cls.derivesFrom(defn.EnumClass)) {
+        val firstParent = parents1.head.tpe.dealias.typeSymbol
+        checkEnum(cdef, cls, firstParent)
+      }
       val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1), cls)
       checkVariance(cdef1)
       if (ctx.phase.isTyper && cdef1.tpe.derivesFrom(defn.DynamicClass) && !ctx.dynamicsEnabled) {

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -30,8 +30,9 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         implicit val ctx: Context = ictx
         assertMessageCount(1, messages)
         val errorMsg = messages.head
-        val CaseClassCannotExtendEnum(cls) :: Nil = messages
+        val CaseClassCannotExtendEnum(cls, parent) :: Nil = messages
         assertEquals("Bar", cls.name.show)
+        assertEquals("Foo", parent.name.show)
         assertEquals("<empty>", cls.owner.name.show)
       }