Fix #6601: Check that an enum case extends its enum class

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -253,6 +253,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           cpy.Inlined(tree)(callTrace, transformSub(bindings), transform(expansion)(inlineContext(call)))
         case templ: Template =>
           withNoCheckNews(templ.parents.flatMap(newPart)) {
+            Checking.checkEnumParentOK(templ.symbol.owner)
             forwardParamAccessors(templ)
             synthMbr.addSyntheticMembers(
                 superAcc.wrapTemplate(templ)(

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

@@ -592,6 +592,17 @@ object Checking {
       stats.foreach(checkValueClassMember)
     }
   }
+
+  /** Check that an enum case extends its enum class */
+  def checkEnumParentOK(cls: Symbol)(using ctx: Context): Unit =
+    val enumCase =
+      if cls.isAllOf(EnumCase) then cls
+      else if cls.isAnonymousClass && cls.owner.isAllOf(EnumCase) then cls.owner
+      else NoSymbol
+    if enumCase.exists then
+      val enumCls = enumCase.owner.linkedClass
+      if !cls.info.parents.exists(_.typeSymbol == enumCls) then
+        ctx.error(i"enum case does not extend its enum $enumCls", enumCase.sourcePos)
 }
 
 trait Checking {

docs/docs/internals/syntax.md

@@ -105,7 +105,7 @@ yield
 
 ```
 as        derives   extension inline    on        opaque    open      using
-~         *         |         &         +         -
+*         +         -
 ```
 
 ## Context-free Syntax

docs/docs/reference/enums/desugarEnums.md

@@ -196,6 +196,9 @@ Cases such as `case C` expand to a `@static val` as opposed to a `val`. This all
 
 ### Other Rules
 
-A normal case class which is not produced from an enum case is not allowed to extend
+ - A normal case class which is not produced from an enum case is not allowed to extend
 `scala.Enum`. This ensures that the only cases of an enum are the ones that are
 explicitly declared in it.
+
+ - If an enum case has an extends clause, the enum class must be one of the
+   classes that's extended.

tests/neg/i6601.scala

@@ -0,0 +1,10 @@
+object GADTs2 {
+  enum Var[G, A] {
+    case Z[A, G]() extends Expr[(A, G), A] // error
+    case X extends AnyRef                  // error
+  }
+  enum Expr[G, A] {
+    case Lit[G](n: Int) extends Expr[G, Int]
+        // case S[A, G](x:
+  }
+}