error on custom definition of enum getters

Author: bishabosha

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -167,7 +167,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     ModifierNotAllowedForDefinitionID,
     CannotExtendJavaEnumID,
     InvalidReferenceInImplicitNotFoundAnnotationID,
-    TraitMayNotDefineNativeMethodID
+    TraitMayNotDefineNativeMethodID,
+    EnumGettersRedefinitionID
 
   def errorNumber = ordinal - 2
 }

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

@@ -2031,6 +2031,10 @@ import ast.tpd
     def explain = ""
   }
 
+  class EnumGettersRedefinition(decl: Symbol)(using Context) extends NamingMsg(EnumGettersRedefinitionID):
+    def msg = em"redefinition of $decl: ${decl.info} in an ${hl("enum")}"
+    def explain = em"users may not may not supply their own definition for $decl when inside an ${hl("enum")}"
+
   class DoubleDefinition(decl: Symbol, previousDecl: Symbol, base: Symbol)(using Context) extends NamingMsg(DoubleDefinitionID) {
     def msg = {
       def nameAnd = if (decl.name != previousDecl.name) " name and" else ""

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

@@ -889,7 +889,12 @@ trait Checking {
         if (decl.matches(other) && !javaFieldMethodPair) {
           def doubleDefError(decl: Symbol, other: Symbol): Unit =
             if (!decl.info.isErroneous && !other.info.isErroneous)
-              report.error(DoubleDefinition(decl, other, cls), decl.srcPos)
+              if decl.owner.is(Enum, butNot=JavaDefined|Case) && decl.span.isSynthetic && (
+                decl.name == nme.ordinal || decl.name == nme.enumLabel)
+              then
+                report.error(EnumGettersRedefinition(decl), other.srcPos)
+              else
+                report.error(DoubleDefinition(decl, other, cls), decl.srcPos)
           if (decl is Synthetic) doubleDefError(other, decl)
           else doubleDefError(decl, other)
         }

tests/neg/enumsLabel-overrides.scala

@@ -0,0 +1,15 @@
+trait Mixin { def enumLabel: String = "mixin" }
+
+enum Mixed extends Mixin {
+  case B // error: overriding method enumLabel in trait Mixin of type => String;
+}
+
+enum MixedAlso {
+  case C extends MixedAlso with Mixin // error: overriding method enumLabel in trait Mixin of type => String;
+}
+
+trait HasEnumLabel { def enumLabel: String }
+
+enum MyEnum extends HasEnumLabel {
+  case D // ok
+}

tests/neg/enumsLabel-singleimpl.scala

@@ -0,0 +1,13 @@
+enum Labelled {
+
+  case A
+
+  def enumLabel: String = "nolabel" // error: redefinition of method enumLabel: => String in an enum
+}
+
+enum Ordinalled {
+
+  case A
+
+  def ordinal: Int = -1 // error: redefinition of method ordinal: => Int in an enum
+}