Make an exception for universal traits

They can have a value class as self type, after all.

Author: odersky

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

@@ -3,6 +3,7 @@ package transform
 
 import core._
 import Contexts._, Types._, MegaPhase._, ast.Trees._, Symbols._, Decorators._, Flags._
+import SymUtils.*
 
 /** Transform references of the form
  *
@@ -30,7 +31,7 @@ class ExplicitSelf extends MiniPhase {
 
   private def castQualifier(tree: RefTree, cls: ClassSymbol, thiz: Tree)(using Context) =
     val selfType = cls.classInfo.selfType
-    if selfType.classSymbols.exists(_.isValueClass) then
+    if selfType.classSymbols.exists(_.isValueClass) && !cls.isUniversalTrait then
       report.error(em"self type $selfType of $cls may not be a value class", thiz.srcPos)
     cpy.Select(tree)(thiz.cast(AndType(selfType, thiz.tpe)), tree.name)
 

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

@@ -64,6 +64,9 @@ object SymUtils:
 
     def isSuperAccessor(using Context): Boolean = self.name.is(SuperAccessorName)
 
+    def isUniversalTrait(using Context): Boolean =
+      self.is(Trait) && self.asClass.parentSyms.head == defn.AnyClass
+
     /** Is this a type or term parameter or a term parameter accessor? */
     def isParamOrAccessor(using Context): Boolean =
       self.is(Param) || self.is(ParamAccessor)