Partial fix for #2296: instead of VerifyError we issue a compile-time error (implementation restriction). To properly generate accessors in traits we need to move most (all?) logic related to protected accessors to phase mixin, use abstract accessors all the way up to mixin, and generate the code in the mixin class. In other words, exactly what is done for super accessors right now. Review by extempore.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@22219 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

Author: dragos

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

@@ -411,10 +411,15 @@ abstract class SuperAccessors extends transform.Transform with transform.TypingT
         unit.error(pos, "Implementation restriction: " + msg)
       }
 
+      def accessibleThroughSubclassing: Boolean =
+        (validCurrentOwner
+            && currentOwner.enclClass.thisSym.isSubClass(sym.owner)
+            && !currentOwner.enclClass.isTrait)
+      
       val res = /* settings.debug.value && */
       ((sym hasFlag PROTECTED) 
        && !sym.owner.isPackageClass
-       && (!validCurrentOwner || !(currentOwner.enclClass.thisSym isSubClass sym.owner))
+       && !accessibleThroughSubclassing
        && (enclPackage(sym.owner) != enclPackage(currentOwner))
        && (enclPackage(sym.owner) == enclPackage(sym.accessBoundary(sym.owner))))
 
@@ -424,9 +429,12 @@ abstract class SuperAccessors extends transform.Transform with transform.TypingT
         if (host.isPackageClass) false
         else if (host.thisSym != host) {
           if (host.thisSym.tpe.typeSymbol.hasFlag(JAVA))
-            errorRestriction(currentOwner.enclClass + " accesses protected " + sym 
-                             + " from self type " + host.thisSym.tpe)
+            errorRestriction("%s accesses protected %s from self type %s.".format(currentOwner.enclClass, sym, host.thisSym.tpe))
           false
+        } else if (host.isTrait && sym.hasFlag(JAVA)) {
+            errorRestriction(("%s accesses protected %s inside a concrete trait method. " +
+                    "Add an accessor in a class extending %s to work around this bug.").format(currentOwner.enclClass, sym, sym.enclClass))
+            false
         } else res
       } else res
     }