Fix compiler crash for repeated Enums

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

@@ -64,7 +64,8 @@ object DesugarEnums {
   }
 
   /** A type tree referring to `enumClass` */
-  def enumClassRef(implicit ctx: Context) = TypeTree(enumClass.typeRef)
+  def enumClassRef(implicit ctx: Context) =
+    if (enumClass.exists) TypeTree(enumClass.typeRef) else TypeTree()
 
   /** Add implied flags to an enum class or an enum case */
   def addEnumFlags(cdef: TypeDef)(implicit ctx: Context) =
@@ -89,7 +90,7 @@ object DesugarEnums {
       DefDef(name.toTermName, Nil, Nil, TypeTree(), valuesDot(select))
     val privateValuesDef =
       ValDef(nme.DOLLAR_VALUES, TypeTree(),
-             New(TypeTree(defn.EnumValuesType.appliedTo(enumClass.typeRef :: Nil)), ListOfNil))
+        New(TypeTree(defn.EnumValuesType.appliedTo(enumClass.typeRef :: Nil)), ListOfNil))
         .withFlags(Private)
     val valueOfDef = enumDefDef("enumValue", "fromInt")
     val withNameDef = enumDefDef("enumValueNamed", "fromName")
@@ -195,7 +196,8 @@ object DesugarEnums {
   /** Expand a module definition representing a parameterless enum case */
   def expandEnumModule(name: TermName, impl: Template, mods: Modifiers, pos: Position)(implicit ctx: Context): Tree = {
     assert(impl.body.isEmpty)
-    if (impl.parents.isEmpty)
+    if (!enumClass.exists) EmptyTree
+    else if (impl.parents.isEmpty)
       expandSimpleEnumCase(name, mods, pos)
     else {
       def toStringMeth =
@@ -210,7 +212,8 @@ object DesugarEnums {
 
   /** Expand a simple enum case */
   def expandSimpleEnumCase(name: TermName, mods: Modifiers, pos: Position)(implicit ctx: Context): Tree =
-    if (enumClass.typeParams.nonEmpty) {
+    if (!enumClass.exists) EmptyTree
+    else if (enumClass.typeParams.nonEmpty) {
       val parent = interpolatedEnumParent(pos)
       val impl = Template(emptyConstructor, parent :: Nil, EmptyValDef, Nil)
       expandEnumModule(name, impl, mods, pos)

tests/neg/i4470a.scala

@@ -0,0 +1,10 @@
+object RepeatedEnum {
+
+  enum Maybe { // error // error
+    case Foo
+  }
+
+  enum Maybe { // error
+    case Foo
+  }
+}

tests/neg/i4470b.scala

@@ -0,0 +1,10 @@
+object RepeatedExtendEnum {
+
+  enum Maybe[T] { // error // error
+    case Foo extends Maybe[Int]
+  }
+
+  enum Maybe[T] { // error
+    case Foo extends Maybe[Int]
+  }
+}

tests/neg/i4470c.scala

@@ -0,0 +1,9 @@
+object DuplicatedEnum {
+  enum Maybe[+T] { // error // error
+    case Some(x: T)
+  }
+
+  enum Maybe[+T] { // error
+    case Some(x: T)
+  }
+}