Check for unimplemented abstract override members

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -530,6 +530,7 @@ object Flags {
 
 // ------- Other flag sets -------------------------------------
 
+  val NotConcrete: FlagSet                   = AbsOverride | Deferred
   val AbstractFinal: FlagSet                 = Abstract | Final
   val AbstractOverride: FlagSet              = Abstract | Override
   val AbstractSealed: FlagSet                = Abstract | Sealed

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

@@ -524,10 +524,10 @@ object RefChecks {
 
       def isImplemented(mbr: Symbol) =
         val mbrType = clazz.thisType.memberInfo(mbr)
-        extension (sym: Symbol) def isConcrete = sym.exists && !sym.is(Deferred)
+        def isConcrete(sym: Symbol) = sym.exists && !sym.isOneOf(NotConcrete)
         clazz.nonPrivateMembersNamed(mbr.name)
           .filterWithPredicate(
-            impl => impl.symbol.isConcrete && mbrType.matchesLoosely(impl.info))
+            impl => isConcrete(impl.symbol) && mbrType.matchesLoosely(impl.info))
           .exists
 
       /** The term symbols in this class and its baseclasses that are
@@ -641,7 +641,11 @@ object RefChecks {
 
                     undefined(s"\n(Note that ${pa.show} does not match ${pc.show}$addendum)")
                   case xs =>
-                    undefined(s"\n(The class implements a member with a different type: ${concrete.showDcl})")
+                    undefined(
+                      if concrete.symbol.is(AbsOverride) then
+                        s"\n(The class implements ${concrete.showDcl} but that definition still needs an implementation)"
+                      else
+                        s"\n(The class implements a member with a different type: ${concrete.showDcl})")
                 }
               case Nil =>
                 undefined("")

tests/neg/i9329.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/i9329.scala:8:6 ------------------------------------------------------------------------------------
+8 |class GrandSon extends Son // error
+  |      ^
+  |    class GrandSon needs to be abstract, since def name: => String is not defined 
+  |    (The class implements abstract override def name: => String but that definition still needs an implementation)

tests/neg/i9329.scala

@@ -0,0 +1,8 @@
+trait Parent {
+  def name: String
+}
+trait Son extends Parent {
+  abstract override def name = ""
+  def parentName = super.name
+}
+class GrandSon extends Son // error