SI-4134: abstract override crasher if lacking super impl

The example from the ticket is committed as a neg test.
The problem is that a super.m on an abstract override
member m has no concrete implementation, that is, the
trait T is not mixed in after a class C with a concrete m.

The error is noticed at phase mixin when the super accessor
is added to the concrete mixer. (Pun alert?) When super.m
is rebound, no concrete matching symbol is found up the
linearization.

Previously, it was asserted that such a symbol should
be found, but since this is our first opportunity to
detect that there is none, an error should be emitted
instead. The new message is of the form:

Member method f of mixin trait T2 is missing a concrete super implementation.

Additionally, a couple of flag tests were changed to use isAbstractOverride.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index 5a7c015f09..68a722aab4 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -662,7 +662,7 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
           }
 
           // Check the remainder for invalid absoverride.
-          for (member <- rest ; if ((member hasFlag ABSOVERRIDE) && member.isIncompleteIn(clazz))) {
+          for (member <- rest ; if (member.isAbstractOverride && member.isIncompleteIn(clazz))) {
             val other = member.superSymbol(clazz)
             val explanation =
               if (other != NoSymbol) " and overrides incomplete superclass member " + infoString(other)
@@ -756,11 +756,10 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
 
       // 4. Check that every defined member with an `override` modifier overrides some other member.
       for (member <- clazz.info.decls)
-        if ((member hasFlag (OVERRIDE | ABSOVERRIDE)) &&
-            !(clazz.thisType.baseClasses exists (hasMatchingSym(_, member)))) {
+        if (member.isAnyOverride && !(clazz.thisType.baseClasses exists (hasMatchingSym(_, member)))) {
           // for (bc <- clazz.info.baseClasses.tail) Console.println("" + bc + " has " + bc.info.decl(member.name) + ":" + bc.info.decl(member.name).tpe);//DEBUG
           unit.error(member.pos, member.toString() + " overrides nothing");
-          member resetFlag OVERRIDE
+          member resetFlag (OVERRIDE | ABSOVERRIDE)  // Any Override
         }
     }