Error message for enum case in non-enum class companion object

compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala

@@ -6,6 +6,7 @@ import core._
 import util.Positions._, Types._, Contexts._, Constants._, Names._, NameOps._, Flags._
 import SymDenotations._, Symbols._, StdNames._, Annotations._, Trees._
 import Decorators._
+import reporting.diagnostic.messages.EnumCaseDefinitionInNonEnumOwner
 import collection.mutable.ListBuffer
 import util.Property
 import typer.ErrorReporting._
@@ -32,7 +33,7 @@ object DesugarEnums {
   /** Is enum case `tree` situated in a companion object of an enum class? */
   def enumCaseIsLegal(tree: Tree)(implicit ctx: Context): Boolean = (
     ctx.owner.is(ModuleClass) && enumClass.derivesFrom(defn.EnumClass)
-    || { ctx.error(em"case not allowed here, since owner ${ctx.owner} is not an `enum' object", tree.pos)
+    || { ctx.error(EnumCaseDefinitionInNonEnumOwner(ctx.owner), tree.pos)
          false
        }
     )

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

@@ -101,6 +101,7 @@ public enum ErrorMessageID {
     ReturnOutsideMethodDefinitionID,
     UncheckedTypePatternID,
     ExtendFinalClassID,
+    EnumCaseDefinitionInNonEnumOwnerID,
     ;
 
     public int errorNumber() {

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

@@ -1777,5 +1777,15 @@ object messages {
     val explanation =
       hl"""A class marked with the ${"final"} keyword cannot be extended"""
   }
-
+  case class EnumCaseDefinitionInNonEnumOwner(owner: Symbol)(implicit ctx: Context)
+    extends Message(EnumCaseDefinitionInNonEnumOwnerID) {
+      val kind = "Syntax"
+      val msg = em"case not allowed here, since owner `${owner}` is not an `${"enum"}` object"
+      val explanation =
+        hl"""
+            | `${"case"}` is only allowed at this place if the surrounding object is the companion object of an `${"enum"} ${"class"}`.
+            | If you wanted to create an enumeration make sure that the corresponding class has the `${"enum"}` keyword.
+            | Otherwise you might have forgotten `${"class"}` or `${"object"}` after this `${"case"}` here.
+          """
+    }
 }

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

@@ -998,6 +998,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val ReturnOutsideMethodDefinition(owner) :: Nil = messages
       assertEquals("object A", owner.show)
     }
+
   @Test def extendFinalClass = checkMessagesAfter("refchecks") {
     """final class A
       |
@@ -1010,4 +1011,17 @@ class ErrorMessagesTests extends ErrorMessagesTest {
     assertEquals(extender.show, "class B")
     assertEquals(parent.show, "class A")
   }
+
+  @Test def enumCaseDefinitionInNonEnumOwner =
+    checkMessagesAfter("frontend") {
+      """object Qux {
+        |  case Foo
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val EnumCaseDefinitionInNonEnumOwner(owner) :: Nil = messages
+      assertEquals("object Qux", owner.show)
+    }
 }