error on concrete val in universal trait

see neg/anytrait.scala

Author: adriaanm

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1340,7 +1340,13 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
               notAllowed(s"redefinition of $name method. See SIP-15, criterion 4.")
             else if (stat.symbol != null && stat.symbol.isParamAccessor)
               notAllowed("additional parameter")
+            // concrete accessor (getter) in trait corresponds to a field definition (neg/anytrait.scala)
+            // TODO: only reject accessors that actually give rise to field (e.g., a constant-type val is fine)
+            else if (!isValueClass && stat.symbol.isAccessor && !stat.symbol.isDeferred)
+              notAllowed("field definition")
             checkEphemeralDeep.traverse(rhs)
+          // for value class or "exotic" vals in traits
+          // (traits don't receive ValDefs for regular vals until fields phase -- well, except for early initialized/lazy vals)
           case _: ValDef =>
             notAllowed("field definition")
           case _: ModuleDef =>