Backport "Report only non-overridden unimplemented members" to LTS

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

@@ -600,6 +600,15 @@ object RefChecks {
               && withMode(Mode.IgnoreCaptures)(mbrDenot.matchesLoosely(impl, alwaysCompareTypes = true)))
           .exists
 
+      /** Filter out symbols from `syms` that are overridden by a symbol appearing later in the list.
+       *  Symbols that are not overridden are kept. */
+      def lastOverrides(syms: List[Symbol]): List[Symbol] =
+        val deduplicated =
+          syms.foldLeft(List.empty[Symbol]):
+            case (acc, sym) if acc.exists(s => isOverridingPair(s, sym, clazz.thisType)) => acc
+            case (acc, sym) => sym :: acc
+        deduplicated.reverse
+
       /** The term symbols in this class and its baseclasses that are
        *  abstract in this class. We can't use memberNames for that since
        *  a concrete member might have the same signature as an abstract
@@ -622,7 +631,8 @@ object RefChecks {
 
         val missingMethods = grouped.toList flatMap {
           case (name, syms) =>
-            syms.filterConserve(!_.isSetter)
+            lastOverrides(syms)
+              .filterConserve(!_.isSetter)
               .distinctBy(_.signature) // Avoid duplication for similar definitions (#19731)
         }
 

tests/neg/i21335.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg/i21335.scala:7:6 -----------------------------------------------------------------------------------
+7 |class Z1 extends Bar1 // error
+  |      ^
+  |      class Z1 needs to be abstract, since override def bar(): Bar1 in trait Bar1 is not defined 
+-- Error: tests/neg/i21335.scala:12:6 ----------------------------------------------------------------------------------
+12 |class Z2 extends Bar2 // error
+   |      ^
+   |      class Z2 needs to be abstract, since def bar(): Bar2 in trait Bar2 is not defined 

tests/neg/i21335.scala

@@ -0,0 +1,12 @@
+trait Foo:
+  def bar(): Foo
+
+trait Bar1 extends Foo:
+  override def bar(): Bar1
+
+class Z1 extends Bar1 // error
+
+trait Bar2 extends Foo:
+  def bar(): Bar2
+
+class Z2 extends Bar2 // error