fix #9579: prevent non static enums that extend jl.Enum

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

@@ -1080,20 +1080,23 @@ trait Checking {
       report.error(em"$what can only be used in an inline method", pos)
 
   /** 1. Check that all case classes that extend `scala.Enum` are `enum` cases
-   *  2. Check that case class `enum` cases do not extend java.lang.Enum.
+   *  2. Check that parameterised `enum` cases do not extend java.lang.Enum.
+   *  3. Check that only a static `enum` base class can extend java.lang.Enum.
    */
   def checkEnum(cdef: untpd.TypeDef, cls: Symbol, firstParent: Symbol)(using Context): Unit = {
     def isEnumAnonCls =
       cls.isAnonymousClass
       && cls.owner.isTerm
       && (cls.owner.flagsUNSAFE.isAllOf(EnumCase)
         || ((cls.owner.name eq nme.DOLLAR_NEW) && cls.owner.flagsUNSAFE.isAllOf(Private | Synthetic)))
-    if (!isEnumAnonCls)
-      if (cdef.mods.isEnumCase) {
-        if (cls.derivesFrom(defn.JavaEnumClass))
+    val isJavaEnum = cls.derivesFrom(defn.JavaEnumClass)
+    if isJavaEnum && cdef.mods.isEnumClass && !cls.isStatic then
+      report.error(em"An enum extending java.lang.Enum must be declared in a static scope", cdef.srcPos)
+    if !isEnumAnonCls then
+      if cdef.mods.isEnumCase then
+        if isJavaEnum then
           report.error(em"paramerized case is not allowed in an enum that extends java.lang.Enum", cdef.srcPos)
-      }
-      else if (cls.is(Case) || firstParent.is(Enum))
+      else if cls.is(Case) || firstParent.is(Enum) then
         // Since enums are classes and Namer checks that classes don't extend multiple classes, we only check the class
         // parent.
         //

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

@@ -0,0 +1,22 @@
+import java.{lang => jl}
+
+object TestSuite:
+  def test(op: => Unit): Unit = op
+  test {
+    enum E extends jl.Enum[E] { case A } // error: enum extending java.lang.Enum must be declared in a static scope
+  }
+
+class Container:
+  enum E extends jl.Enum[E] { case A } // error: enum extending java.lang.Enum must be declared in a static scope
+
+object Wrap:
+  def force =
+    enum E extends jl.Enum[E] { case A } // error: enum extending java.lang.Enum must be declared in a static scope
+
+trait Universe:
+  enum E extends jl.Enum[E] { case A } // error: enum extending java.lang.Enum must be declared in a static scope
+
+enum E extends jl.Enum[E] { case A } // ok, a declaration at package level is static.
+
+object Static:
+  enum E extends jl.Enum[E] { case A } // ok, a declaration within a static object is static.