fix: better error messages when an enum derives from AnyVal (#22236)

Closes #21944

Author: hamzaremmal

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

@@ -219,6 +219,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case DeprecatedAssignmentSyntaxID // errorNumber: 203
   case DeprecatedInfixNamedArgumentSyntaxID // errorNumber: 204
   case GivenSearchPriorityID // errorNumber: 205
+  case EnumMayNotBeValueClassesID // errorNumber: 206
 
   def errorNumber = ordinal - 1
 

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

@@ -3399,3 +3399,9 @@ class GivenSearchPriorityWarning(
        |$migrationHints"""
 
   def explain(using Context) = ""
+
+final class EnumMayNotBeValueClasses(sym: Symbol)(using Context) extends SyntaxMsg(EnumMayNotBeValueClassesID):
+    def msg(using Context): String = i"$sym may not be a value class"
+
+    def explain(using Context) = ""
+end EnumMayNotBeValueClasses

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

@@ -734,11 +734,16 @@ object Checking {
       case _ =>
         report.error(ValueClassesMayNotContainInitalization(clazz), stat.srcPos)
     }
-    if (clazz.isDerivedValueClass) {
+    // We don't check synthesised enum anonymous classes that are generated from
+    // enum extending a value class type (AnyVal or an alias of it)
+    // The error message 'EnumMayNotBeValueClassesID' will take care of generating the error message (See #22236)
+    if (clazz.isDerivedValueClass && !clazz.isEnumAnonymClass) {
       if (clazz.is(Trait))
         report.error(CannotExtendAnyVal(clazz), clazz.srcPos)
       if clazz.is(Module) then
         report.error(CannotExtendAnyVal(clazz), clazz.srcPos)
+      if (clazz.is(Enum))
+        report.error(EnumMayNotBeValueClasses(clazz), clazz.srcPos)
       if (clazz.is(Abstract))
         report.error(ValueClassesMayNotBeAbstract(clazz), clazz.srcPos)
       if (!clazz.isStatic)

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

@@ -1626,7 +1626,8 @@ class Namer { typer: Typer =>
           }
           else {
             val pclazz = pt.typeSymbol
-            if pclazz.is(Final) then
+            // The second condition avoids generating a useless message (See #22236 for more details)
+            if pclazz.is(Final) && !(pclazz.is(Enum) && pclazz.isDerivedValueClass) then
               report.error(ExtendFinalClass(cls, pclazz), cls.srcPos)
             else if pclazz.isEffectivelySealed && pclazz.associatedFile != cls.associatedFile then
               if pclazz.is(Sealed) && !pclazz.is(JavaDefined) then

tests/neg/i21944.check

@@ -0,0 +1,4 @@
+-- [E206] Syntax Error: tests/neg/i21944.scala:1:5 ---------------------------------------------------------------------
+1 |enum Orientation extends AnyVal: // error
+  |     ^
+  |     class Orientation may not be a value class

tests/neg/i21944.scala

@@ -0,0 +1,2 @@
+enum Orientation extends AnyVal: // error
+    case North, South, East, West