fix #9324: forbid no-arg java.lang.Enum ctor

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

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

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

@@ -1528,6 +1528,12 @@ import transform.SymUtils._
       def explain = ""
     }
 
+  class JavaEnumParentArgs(parent: Type)(using Context)
+    extends TypeMsg(JavaEnumParentArgsID) {
+      def msg = em"""not enough arguments for constructor Enum: ${hl("(name: String, ordinal: Int)")}: ${hl(parent.show)}"""
+      def explain = ""
+    }
+
   class CannotHaveSameNameAs(sym: Symbol, cls: Symbol, reason: CannotHaveSameNameAs.Reason)(using Context)
     extends SyntaxMsg(CannotHaveSameNameAsID) {
     import CannotHaveSameNameAs._

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

@@ -95,7 +95,7 @@ object RefChecks {
    *  and required classes. Also check that only `enum` constructs extend
    *  `java.lang.Enum`.
    */
-  private def checkParents(cls: Symbol)(using Context): Unit = cls.info match {
+  private def checkParents(cls: Symbol, parentTrees: List[Tree])(using Context): Unit = cls.info match {
     case cinfo: ClassInfo =>
       def checkSelfConforms(other: ClassSymbol, category: String, relation: String) = {
         val otherSelf = other.declaredSelfTypeAsSeenFrom(cls.thisType)
@@ -109,12 +109,26 @@ object RefChecks {
       for (reqd <- cinfo.cls.givenSelfType.classSymbols)
         checkSelfConforms(reqd, "missing requirement", "required")
 
+      def isClassExtendingJavaEnum =
+        !cls.isOneOf(Enum | Trait) && parents.exists(_.classSymbol == defn.JavaEnumClass)
+
       // Prevent wrong `extends` of java.lang.Enum
-      if !migrateTo3 &&
-         !cls.isOneOf(Enum | Trait) &&
-         parents.exists(_.classSymbol == defn.JavaEnumClass)
-      then
-        report.error(CannotExtendJavaEnum(cls), cls.sourcePos)
+      if isClassExtendingJavaEnum then
+        if !migrateTo3 then // always error, only traits or enum-syntax is possible under scala 3.x
+          report.error(CannotExtendJavaEnum(cls), cls.sourcePos)
+        else
+          // conditionally error, we allow classes to extend java.lang.Enum in scala 2 migration mode,
+          // however the no-arg constructor is forbidden, we must look at the parent trees to see
+          // which overload is called.
+          val javaEnumCtor = defn.JavaEnumClass.primaryConstructor
+          parentTrees.exists {
+            case parent @ tpd.Apply(tpd.TypeApply(fn, _), _) if fn.tpe.termSymbol eq javaEnumCtor =>
+              // here we are simulating the error for missing arguments to a constructor.
+              report.error(JavaEnumParentArgs(parent.tpe), cls.sourcePos)
+              true
+            case _ =>
+              false
+          }
 
     case _ =>
   }
@@ -1089,7 +1103,7 @@ class RefChecks extends MiniPhase { thisPhase =>
   override def transformTemplate(tree: Template)(using Context): Tree = try {
     val cls = ctx.owner.asClass
     checkOverloadedRestrictions(cls)
-    checkParents(cls)
+    checkParents(cls, tree.parents)
     if (cls.is(Trait)) tree.parents.foreach(checkParentPrefix(cls, _))
     checkCompanionNameClashes(cls)
     checkAllOverrides(cls)

tests/neg/extend-java-enum-migration.check

@@ -0,0 +1,16 @@
+-- [E160] Type Error: tests/neg/extend-java-enum-migration.scala:9:12 --------------------------------------------------
+9 |final class C extends jl.Enum[C] // error
+  |            ^
+  |            not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[C]
+-- [E160] Type Error: tests/neg/extend-java-enum-migration.scala:11:7 --------------------------------------------------
+11 |object O extends jl.Enum[O.type] // error
+   |       ^
+   |       not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[O.type]
+-- [E160] Type Error: tests/neg/extend-java-enum-migration.scala:14:6 --------------------------------------------------
+14 |class Sub extends T // error
+   |      ^
+   |      not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[T]
+-- [E160] Type Error: tests/neg/extend-java-enum-migration.scala:17:10 -------------------------------------------------
+17 |val foo = new java.lang.Enum[Color] {} // error
+   |          ^
+   |          not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[Color]

tests/neg/extend-java-enum-migration.scala

@@ -0,0 +1,17 @@
+import java.{lang => jl}
+
+import language.`3.0-migration`
+
+// This file is different from `tests/neg/extend-java-enum.scala` as we
+// are testing that it is illegal to *not* pass arguments to jl.Enum
+// in 3.0-migration
+
+final class C extends jl.Enum[C] // error
+
+object O extends jl.Enum[O.type] // error
+
+trait T extends jl.Enum[T] // ok
+class Sub extends T // error
+
+abstract class Color(name: String, ordinal: Int) extends java.lang.Enum[Color](name, ordinal) // ok
+val foo = new java.lang.Enum[Color] {} // error