False unused local definition for lambda and inline

[eejbyfeldt]

Compiler version

3.7.4-RC2 and 3.8.0-RC1-bin-20251027-520668f-NIGHTLY

Minimized code

//> using options -Wall -Werror

object test {
  inline def f(testFun: => Any) = testFun

  f {
    val i = 1
    summon[i.type <:< Int]
  }
}

Output

-- [E198] Unused Symbol Warning: /home/ejbyfeldt/dev/scala_playground/only_tpe.scala:5:8 
5 |    val i = 1
  |        ^
  |        unused local definition
1 warning found

Expectation

No warning is produced.

[eejbyfeldt]

Possibly related to #24248 ?

[Gedochao]

Possibly related to #24248 ?

Yup, bisects to the same change, so it's likely...

[Gedochao]

Last good release: 3.8.0-RC1-bin-20251008-6194563-NIGHTLY
First bad release: 3.8.0-RC1-bin-20251009-eb67a9c-NIGHTLY
912e89b is the first bad commit (#24043)
cc @som-snytt

[som-snytt]

The reference is available after typer:

[[syntax trees at end of                     typer]]
PackageDef(Ident(<empty>), List(
  ValDef(test, Ident(test$), Apply(Select(New(Ident(test$)), <init>), List())),
  TypeDef(test$,
    Template(DefDef(<init>, List(List()), TypeTree(), Thicket(List())),
      List(Apply(Select(New(TypeTree()), <init>), List())),
      ValDef(_, SingletonTypeTree(Ident(test)), Thicket(List())), List(
      DefDef(f,
        List(List(ValDef(testFun, ByNameTypeTree(Ident(Any)), Thicket(List()))))
          ,
      TypeTree(), Typed(Ident(testFun), TypeTree())),
      Apply(Ident(f), List(
        Block(List(ValDef(i, TypeTree(), Literal(1))),
          Inlined(
            Apply(
              TypeApply(Ident(summon), List(
                AppliedTypeTree(Ident(<:<), List(SingletonTypeTree(Ident(i)),
                  Ident(Int)))
              )),
            List(TypeApply(Ident(refl), List(TypeTree())))),
            List(
              ValDef(x$proxy1, TypeTree(),
                TypeApply(Ident(refl), List(TypeTree())))
            ),
          Inlined(Thicket(List()), List(), Ident(x$proxy1)))
        )
      ))
    ))
  )
))

though not after inlining:

[[syntax trees at end of                  inlining]]
PackageDef(Ident(<empty>), List(
  ValDef(test, Ident(test$), Apply(Select(New(Ident(test$)), <init>), List())),
  TypeDef(test$,
    Template(DefDef(<init>, List(List()), TypeTree(), Thicket(List())),
      List(Apply(Select(New(TypeTree()), <init>), List())),
      ValDef(_, SingletonTypeTree(Ident(test)), Thicket(List())), List(
      DefDef(writeReplace, List(List()), TypeTree(),
        Apply(Select(New(TypeTree()), <init>), List(
          Literal(classOf[<empty>.this.test.type])))
      ),
      DefDef(f,
        List(List(ValDef(testFun, ByNameTypeTree(Ident(Any)), Thicket(List()))))
          ,
      TypeTree(), Typed(Ident(testFun), TypeTree())),
      Inlined(
        Apply(Ident(f), List(
          Block(List(ValDef(i, TypeTree(), Literal(1))),
            Inlined(Ident(Predef$),
              List(
                ValDef(x$proxy1, TypeTree(),
                  TypeApply(Ident(refl), List(TypeTree())))
              ),
            Inlined(Thicket(List()), List(), Ident(x$proxy1)))
          )
        )),
        List(
          DefDef(testFun$proxy1, List(), TypeTree(),
            Block(List(ValDef(i, TypeTree(), Literal(1))),
              Inlined(Ident(Predef$),
                List(
                  ValDef(x$proxy1, TypeTree(),
                    TypeApply(Ident(refl), List(TypeTree())))
                ),
              Inlined(Thicket(List()), List(), Ident(x$proxy1)))
            )
          )
        ),
        Typed(Inlined(Thicket(List()), List(), Ident(testFun$proxy1)),
          TypeTree())
      )
    ))
  )
))

That should be OK. (But suggests that it's necessary to walk both trees, unfortunately.)

[som-snytt]

It was actually the unused i introduced for testFun$proxy1 that was reported.

The PR conservatively does not warn for definitions introduced by inlining. Something like -Wmacros from Scala 2 would be a new feature to help debug those trees.

The PR doesn't include adding the -uniqid to the warning; Scala 2 is more verbose for that purpose.