Fix #5008: forbid any class from extending Enums

Author: Blaisorblade

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

@@ -2138,7 +2138,7 @@ object messages {
 
   case class ClassCannotExtendEnum(cls: Symbol, parent: Symbol)(implicit ctx: Context) extends Message(ClassCannotExtendEnumID) {
     override def kind: String = "Syntax"
-    override def msg: String = hl"""Normal case class cannot extend an enum. case $cls in ${cls.owner} is extending enum ${parent.name}."""
+    override def msg: String = hl"""$cls in ${cls.owner} extends enum ${parent.name}, but extending enums is prohibited."""
     override def explanation: String = ""
   }
 

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

@@ -908,7 +908,12 @@ trait Checking {
       cls.owner.isTerm &&
       (cls.owner.flagsUNSAFE.is(Case) || cls.owner.name == nme.DOLLAR_NEW)
     if (!cdef.mods.isEnumCase && !isEnumAnonCls) {
-      if (cls.is(Case))
+      // Since enums are classes and Namer checks that classes don't extend multiple classes, we only check the class
+      // parent.
+      //
+      // Unlike firstParent.derivesFrom(defn.EnumClass), this test allows inheriting from `Enum` by hand;
+      // see enum-List-control.scala.
+      if (cls.is(Case) || firstParent.is(Enum))
         ctx.error(ClassCannotExtendEnum(cls, firstParent), cdef.pos)
     }
   }

tests/neg/i5008.scala

@@ -0,0 +1,8 @@
+enum Foo { case A }
+enum Bar { case A }
+enum Baz extends Foo { case Z } // error
+
+enum Quux extends Foo with Bar { case Z } // error
+
+class Quuw extends Foo // error
+class Quuz extends Foo { val enumTag = 1 } // error