Backport "Ignore parameter of accessors, fixes #16955" (#17270)

Backports #16957

Author: Kordyjan

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

@@ -621,7 +621,7 @@ object CheckUnused:
         (sym.is(Param) || sym.isAllOf(PrivateParamAccessor | Local, butNot = CaseAccessor)) &&
         !isSyntheticMainParam(sym) &&
         !sym.shouldNotReportParamOwner &&
-        (!sym.exists || !sym.owner.isAllOf(Synthetic | PrivateLocal))
+        (!sym.exists || !(sym.owner.isAllOf(Synthetic | PrivateLocal) || sym.owner.is(Accessor)))
 
       private def shouldReportPrivateDef(using Context): Boolean =
         currScopeType.top == ScopeType.Template && !memDef.symbol.isConstructor && memDef.symbol.is(Private, butNot = SelfName | Synthetic | CaseAccessor)

tests/neg-custom-args/fatal-warnings/i15503e.scala

@@ -52,3 +52,6 @@ package foo.test.trivial:
     def f77(x: Int) = foo // error
   }
   object Y
+
+package foo.test.i16955:
+  class S(var r: String) // OK

tests/neg-custom-args/fatal-warnings/i15503i.scala

@@ -126,6 +126,49 @@ package foo.test.possibleclasses:
     def a = k + y + s + t + z
   }
 
+package foo.test.possibleclasses.withvar:
+  case class AllCaseClass(
+    k: Int, // OK
+    private var y: Int // OK /* Kept as it can be taken from pattern */
+  )(
+    s: Int, // error /* But not these */
+    var t: Int, // OK
+    private var z: Int // error
+  )
+
+  case class AllCaseUsed(
+    k: Int, // OK
+    private var y: Int // OK
+  )(
+    s: Int, // OK
+    var t: Int, // OK
+    private var z: Int // OK
+  ) {
+    def a = k + y + s + t + z
+  }
+
+  class AllClass(
+    k: Int, // error
+    private var y: Int // error
+  )(
+    s: Int, // error
+    var t: Int, // OK
+    private var z: Int // error
+  )
+
+  class AllUsed(
+    k: Int, // OK
+    private var y: Int // OK
+  )(
+    s: Int, // OK
+    var t: Int, // OK
+    private var z: Int // OK
+  ) {
+    def a = k + y + s + t + z
+  }
+
+
+
 package foo.test.from.i16675:
   case class PositiveNumber private (i: Int) // OK
   object PositiveNumber: