Consider setter of effectively private var (scala#23211)

Anonymous classes get a setter for a var member:
as usual, don't warn about the setter but only about the var getter.

Additionally, check whether the setter is referenced before claiming
that the var is unset.

Fixes scala#23200

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -7,7 +7,7 @@ import dotty.tools.dotc.config.ScalaSettings
 import dotty.tools.dotc.core.Contexts.*
 import dotty.tools.dotc.core.Flags.*
 import dotty.tools.dotc.core.Names.{Name, SimpleName, DerivedName, TermName, termName}
-import dotty.tools.dotc.core.NameOps.{isAnonymousFunctionName, isReplWrapperName, isContextFunction}
+import dotty.tools.dotc.core.NameOps.{isAnonymousFunctionName, isReplWrapperName, isContextFunction, setterName}
 import dotty.tools.dotc.core.NameKinds.{
   BodyRetainerName, ContextBoundParamName, ContextFunctionParamName, DefaultGetterName, WildcardParamName}
 import dotty.tools.dotc.core.StdNames.nme
@@ -543,7 +543,13 @@ object CheckUnused:
       if sym.isLocalToBlock then
         if ctx.settings.WunusedHas.locals && sym.is(Mutable) && !infos.asss(sym) then
           warnAt(pos)(UnusedSymbol.unsetLocals)
-      else if ctx.settings.WunusedHas.privates && sym.isAllOf(Private | Mutable) && !infos.asss(sym) then
+      else if ctx.settings.WunusedHas.privates
+        && sym.is(Mutable)
+        && (sym.is(Private) || sym.isEffectivelyPrivate)
+        && !sym.isSetter // tracks sym.underlyingSymbol sibling getter, check setter below
+        && !infos.asss(sym)
+        && !infos.refs(sym.owner.info.member(sym.name.asTermName.setterName).symbol)
+      then
         warnAt(pos)(UnusedSymbol.unsetPrivates)
 
     def checkPrivate(sym: Symbol, pos: SrcPos) =
@@ -552,7 +558,10 @@ object CheckUnused:
         && !sym.isOneOf(SelfName | Synthetic | CaseAccessor)
         && !sym.name.is(BodyRetainerName)
         && !sym.isSerializationSupport
-        && !(sym.is(Mutable) && sym.isSetter && sym.owner.is(Trait)) // tracks sym.underlyingSymbol sibling getter
+        && !( sym.is(Mutable)
+           && sym.isSetter // tracks sym.underlyingSymbol sibling getter
+           && (sym.owner.is(Trait) || sym.owner.isAnonymousClass)
+        )
         && !infos.nowarn(sym)
       then
         warnAt(pos)(UnusedSymbol.privateMembers)

tests/warn/i23200.check

@@ -0,0 +1,8 @@
+-- [E198] Unused Symbol Warning: tests/warn/i23200.scala:8:8 -----------------------------------------------------------
+8 |    var x: Int = 42 // warn
+  |        ^
+  |        unset private variable, consider using an immutable val instead
+-- [E198] Unused Symbol Warning: tests/warn/i23200.scala:40:6 ----------------------------------------------------------
+40 |  var x: Int = 42 // warn local var
+   |      ^
+   |      unset local variable, consider using an immutable val instead

tests/warn/i23200.scala

@@ -0,0 +1,41 @@
+//> using options -Wunused:all
+
+trait Foo
+trait Bar
+
+def `anon not updated` =
+  new Foo {
+    var x: Int = 42 // warn
+    val _ = new Bar:
+      println(x)
+    //x = 27
+    //x_=(27)
+  }
+def `anon yes updated` =
+  new Foo {
+    var x: Int = 42 // nowarn
+    val _ = new Bar:
+      println(x)
+    x = 27
+    //x_=(27)
+  }
+def `anon yes updated from nested context` =
+  new Foo {
+    var x: Int = 42 // nowarn
+    val _ = new Bar:
+      println(x)
+      x = 27
+    //x_=(27)
+  }
+def `anon yes updated in daring use of setter` =
+  new Foo {
+    var x: Int = 42 // nowarn
+    val _ = new Bar:
+      println(x)
+    //x = 27
+    x_=(27)
+  }
+
+def f: Unit =
+  var x: Int = 42 // warn local var
+  println(x)