Fix missing members reporting for var setters

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

@@ -703,10 +703,16 @@ object RefChecks {
         // to consolidate getters and setters.
         val grouped = missing.groupBy(_.underlyingSymbol.name)
 
+        def isDuplicateSetter(sym: Symbol): Boolean =
+          sym.isSetter && {
+            val field = sym.accessedFieldOrGetter
+            grouped.getOrElse(field.name, Nil).contains(field)
+          }
+
         val missingMethods = grouped.toList flatMap {
           case (name, syms) =>
             lastOverrides(syms)
-              .filterConserve(!_.isSetter)
+              .filterConserve(!isDuplicateSetter(_)) // Avoid reporting override error for both `x` and setter `x_=`
               .distinctBy(_.signature) // Avoid duplication for similar definitions (#19731)
         }
 

tests/neg/i23474.check

@@ -0,0 +1,25 @@
+-- Error: tests/neg/i23474.scala:5:11 ----------------------------------------------------------------------------------
+5 |case class Y(val comment: String) extends Comment // error
+  |           ^
+  |           class Y needs to be abstract, since var comment_=(x$1: String): Unit in trait Comment is not defined 
+  |           (Note that an abstract var requires a setter in addition to the getter)
+-- Error: tests/neg/i23474.scala:7:6 -----------------------------------------------------------------------------------
+7 |class Z extends Comment: // error
+  |      ^
+  |      class Z needs to be abstract, since var comment_=(x$1: String): Unit in trait Comment is not defined 
+  |      (Note that an abstract var requires a setter in addition to the getter)
+-- [E164] Declaration Error: tests/neg/i23474.scala:11:15 --------------------------------------------------------------
+11 |  override def comment: String = "" // error
+   |               ^
+   |               error overriding variable comment in trait Comment of type String;
+   |                 method comment of type => String cannot override a mutable variable
+-- Error: tests/neg/i23474.scala:10:6 ----------------------------------------------------------------------------------
+10 |class X extends Comment: // error
+   |      ^
+   |      class X needs to be abstract, since var comment_=(x$1: String): Unit in trait Comment is not defined 
+   |      (Note that an abstract var requires a setter in addition to the getter)
+-- Error: tests/neg/i23474.scala:13:6 ----------------------------------------------------------------------------------
+13 |class W extends Comment // error
+   |      ^
+   |      class W needs to be abstract, since var comment: String in trait Comment is not defined 
+   |      (Note that variables need to be initialized to be defined)

tests/neg/i23474.scala

@@ -0,0 +1,19 @@
+trait Comment {
+  var comment: String
+}
+
+case class Y(val comment: String) extends Comment // error
+
+class Z extends Comment: // error
+  val comment: String = ""
+
+class X extends Comment: // error
+  override def comment: String = "" // error
+
+class W extends Comment // error
+
+
+class OK:
+  val comment: String = ""
+  def comment_=(x: String): Unit = ()
+