Backport "Exclude synthetic opaque proxy from lint" to 3.7.4 (#24287)

Backports #24264 to the 3.7.4. PR submitted by the release tooling.

Author: WojciechMazur

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

@@ -642,7 +642,7 @@ object CheckUnused:
 
  def checkLocal(sym: Symbol, pos: SrcPos) =
  if ctx.settings.WunusedHas.locals
- && !sym.is(InlineProxy)
+ && !sym.isOneOf(InlineProxy | Synthetic)
  && !sym.isCanEqual
  then
  if sym.is(Mutable) && infos.asss(sym) then

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)
+}