fix scala#9260: copy variance in derived type parameters

Author: bishabosha

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

@@ -136,8 +136,9 @@ object desugar {
     )
 
   /** A derived type definition watching `sym` */
-  def derivedTypeParam(sym: TypeSymbol)(using Context): TypeDef =
-    TypeDef(sym.name, DerivedFromParamTree().watching(sym)).withFlags(TypeParam)
+  def derivedTypeParamWithVariance(sym: TypeSymbol)(using Context): TypeDef =
+    val variance = (Covariant | Contravariant) & sym.flags
+    TypeDef(sym.name, DerivedFromParamTree().watching(sym)).withFlags(TypeParam | variance)
 
   /** A value definition copied from `vdef` with a tpt typetree derived from it */
   def derivedTermParam(vdef: ValDef)(using Context): ValDef =
@@ -406,7 +407,7 @@ object desugar {
 
     val originalTparams = constr1.tparams
     val originalVparamss = constr1.vparamss
-    lazy val derivedEnumParams = enumClass.typeParams.map(derivedTypeParam)
+    lazy val derivedEnumParams = enumClass.typeParams.map(derivedTypeParamWithVariance)
     val impliedTparams =
       if (isEnumCase) {
         val tparamReferenced = typeParamIsReferenced(

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

@@ -166,7 +166,13 @@ class VarianceChecker(using Context) {
   private object Traverser extends TreeTraverser {
     def checkVariance(sym: Symbol, pos: SrcPos) = Validator.validateDefinition(sym) match {
       case Some(VarianceError(tvar, required)) =>
-        def msg = i"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym"
+        def msg =
+          val enumAddendum =
+            if sym.owner.isAllOf(EnumCase) && sym.owner.isClass then
+              i"\nenum case ${sym.owner} may require explicit type parameters to resolve this issue"
+            else
+              ""
+          i"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum"
         if (migrateTo3 &&
             (sym.owner.isConstructor || sym.ownersIterator.exists(_.isAllOf(ProtectedLocal))))
           report.migrationWarning(