Exclude synthetic opaque proxy from lint (scala#24264)

Fixes scala#24263

Author: som-snytt

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

@@ -697,7 +697,7 @@ object CheckUnused:
 
     def checkLocal(sym: Symbol, pos: SrcPos) =
       if ctx.settings.WunusedHas.locals
-        && !sym.is(InlineProxy)
+        && !sym.isOneOf(InlineProxy | Synthetic)
       then
         if sym.is(Mutable) && infos.asss(sym) then
           warnAt(pos)(UnusedSymbol.localVars)

tests/warn/i24263.scala

@@ -0,0 +1,6 @@
+//> using options -Werror -Wunused:all
+
+object test {
+  def f(t: Tuple): Nothing = ???
+  val _ = (inputTuple: NamedTuple.NamedTuple[Tuple, Tuple]) => f(inputTuple)
+}