Don't warn retainedBody (#22510)

Fixes #22507 

Easy name-based check for private members.

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -8,7 +8,7 @@ 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}
-import dotty.tools.dotc.core.NameKinds.{ContextBoundParamName, ContextFunctionParamName, WildcardParamName}
+import dotty.tools.dotc.core.NameKinds.{BodyRetainerName, ContextBoundParamName, ContextFunctionParamName, WildcardParamName}
 import dotty.tools.dotc.core.StdNames.nme
 import dotty.tools.dotc.core.Symbols.{ClassSymbol, NoSymbol, Symbol, defn, isDeprecated, requiredClass, requiredModule}
 import dotty.tools.dotc.core.Types.*
@@ -511,6 +511,7 @@ object CheckUnused:
       if ctx.settings.WunusedHas.privates
         && !sym.isPrimaryConstructor
         && sym.is(Private, butNot = SelfName | Synthetic | CaseAccessor)
+        && !sym.name.is(BodyRetainerName)
         && !sym.isSerializationSupport
         && !(sym.is(Mutable) && sym.isSetter && sym.owner.is(Trait)) // tracks sym.underlyingSymbol sibling getter
       then

Diff for: tests/warn/t22507.scala

@@ -0,0 +1,14 @@
+
+//> using options -Werror -Wunused:privates
+
+case class BinaryFen(value: Array[Byte]) extends AnyVal:
+
+  def read: Unit =
+    val reader = new Iterator[Byte]:
+      val inner                            = value.iterator
+      override inline def hasNext: Boolean = inner.hasNext // nowarn
+      override inline def next: Byte       = if hasNext then inner.next else 0.toByte // nowarn
+
+    if reader.hasNext then
+      val b: Byte = reader.next
+      println(b)