Report only non-overridden unimplemented members (scala#21337)

Previously, when a concrete class A had unimplemented members that are
overridden, all overrides would be reported as unimplemented in the
error message. This would produce error messages that are not accurate,
and that suggest stubs that are not correct.

This patch fixes the issue by reporting in the error message only the
unimplemented members that are not overridden by other unimplemented
members.

Fixes scala#21335

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

@@ -698,6 +698,15 @@ 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
@@ -720,7 +729,8 @@ 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