Improve error reporting for missing members (#19800)

Fixes: #19731

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

@@ -702,8 +702,8 @@ object RefChecks {
 
         val missingMethods = grouped.toList flatMap {
           case (name, syms) =>
-            val withoutSetters = syms filterNot (_.isSetter)
-            if (withoutSetters.nonEmpty) withoutSetters else syms
+            syms.filterConserve(!_.isSetter)
+              .distinctBy(_.signature) // Avoid duplication for similar definitions (#19731)
         }
 
         def stubImplementations: List[String] = {
@@ -714,7 +714,7 @@ object RefChecks {
 
           if (regrouped.tail.isEmpty)
             membersStrings(regrouped.head._2)
-          else (regrouped.sortBy("" + _._1.name) flatMap {
+          else (regrouped.sortBy(_._1.name.toString()) flatMap {
             case (owner, members) =>
               ("// Members declared in " + owner.fullName) +: membersStrings(members) :+ ""
           }).init
@@ -733,7 +733,7 @@ object RefChecks {
           return
         }
 
-        for (member <- missing) {
+        for (member <- missingMethods) {
           def showDclAndLocation(sym: Symbol) =
             s"${sym.showDcl} in ${sym.owner.showLocated}"
           def undefined(msg: String) =

tests/neg/i19731.check

@@ -0,0 +1,27 @@
+-- Error: tests/neg/i19731.scala:4:6 -----------------------------------------------------------------------------------
+4 |class F1 extends Foo: // error
+  |      ^
+  |      class F1 needs to be abstract, since def foo(): Unit in class F1 is not defined 
+-- Error: tests/neg/i19731.scala:7:6 -----------------------------------------------------------------------------------
+7 |class F2 extends Foo: // error
+  |      ^
+  |      class F2 needs to be abstract, since:
+  |      it has 2 unimplemented members.
+  |      /** As seen from class F2, the missing signatures are as follows.
+  |       *  For convenience, these are usable as stub implementations.
+  |       */
+  |        def foo(): Unit = ???
+  |        def foo(x: Int): Unit = ???
+-- Error: tests/neg/i19731.scala:16:6 ----------------------------------------------------------------------------------
+16 |class B1 extends Bar: // error
+   |      ^
+   |      class B1 needs to be abstract, since:
+   |      it has 2 unimplemented members.
+   |      /** As seen from class B1, the missing signatures are as follows.
+   |       *  For convenience, these are usable as stub implementations.
+   |       */
+   |        // Members declared in B1
+   |        def foo(x: Int): Unit = ???
+   |        
+   |        // Members declared in Bar
+   |        def foo(): Unit = ???

tests/neg/i19731.scala

@@ -0,0 +1,17 @@
+trait Foo:
+  def foo(): Unit
+
+class F1 extends Foo: // error
+  def foo(): Unit
+
+class F2 extends Foo: // error
+  def foo(): Unit
+  def foo(x: Int): Unit
+
+
+trait Bar:
+  def foo(): Unit
+  def foo(x: Int): Unit
+
+class B1 extends Bar: // error
+  def foo(x: Int): Unit