False unused local definition passing NamedTuple as Tuple

[eejbyfeldt]

Compiler version

3.7.4-RC2, 3.8.0-RC1-bin-20251026-5c51b7b-NIGHTLY

Works as expected on 3.7.3, 3.7.4-RC1

Minimized code

object test {
  def f(t: Tuple): Nothing = ???
  val _ = (inputTuple: NamedTuple.NamedTuple[Tuple, Tuple]) => f(inputTuple)
}

Output

$ scala-cli compile --server=false -S 3.7.4-RC2 -Wall as_expr_3.7.4_rc2.scala 
-- [E198] Unused Symbol Warning: /home/ejbyfeldt/dev/scala_playground/as_expr_3.7.4_rc2.scala:3:65 
3 |  val _ = (inputTuple: NamedTuple.NamedTuple[Tuple, Tuple]) => f(inputTuple)
  |                                                                 ^
  |                                                   unused local definition
1 warning found

Expectation

No warning should be generated.

[som-snytt]

The lint doesn't warn for InlineProxy but the first proxy is an opaque alias proxy which doesn't have the flag. Maybe it suffices to exclude Synthetic symbols.

($proxy1 is used in the type of x$proxy1, but that is an inferred type which is ignored by the linter.)

[[syntax trees at end of                  inlining]] // /home/amarki/snips/i24263.scala
package <empty> {
  final lazy module val test: test = new test()
  @SourceFile("i24263.scala") final module class test() extends Object() {
    this: test.type =>
    private def writeReplace(): AnyRef =
      new scala.runtime.ModuleSerializationProxy(classOf[test.type])
    def f(t: Tuple): Nothing = ???
    (inputTuple: NamedTuple.NamedTuple[Tuple, Tuple]) =>
      test.f(
        {
          val $proxy1:

              NamedTuple.type{
                type AnyNamedTuple = Any;
                  type NamedTuple[N <: Tuple,V <: Tuple] = V
              }

           =
            NamedTuple.$asInstanceOf[

                NamedTuple.type{
                  type AnyNamedTuple = Any;
                    type NamedTuple[N <: Tuple,V <: Tuple] = V
                }

            ]
          val x$proxy1:
            (inputTuple : NamedTuple.NamedTuple[Tuple, Tuple]) &
              $proxy1.NamedTuple[Tuple, Tuple]
           =
            inputTuple.$asInstanceOf[
              (inputTuple : NamedTuple.NamedTuple[Tuple, Tuple]) &
                $proxy1.NamedTuple[Tuple, Tuple]
            ]
          x$proxy1:Tuple
        }
      )
    :(NamedTuple.NamedTuple[Tuple, Tuple] => Nothing) @unchecked match
      {
        case _ => ()
      }
  }
}

[Gedochao]

Last good release: 3.8.0-RC1-bin-20251008-6194563-NIGHTLY
First bad release: 3.8.0-RC1-bin-20251009-eb67a9c-NIGHTLY
3d55eda is the first bad commit
(I know it's already fixed, just an FYI @hamzaremmal)