False positive unused for class only used by calling companion apply

[eejbyfeldt]

Compiler version

3.8.0-RC1, 3.8.1-RC1-bin-20251117-5ccea40-NIGHTLY

Works correctly in 3.7.4.

Minimized code

//> using scala 3.8.0-RC1
//> using options -Wunused:all

object test {
  private case class Impl()
  inline def makeImpl(): Any = Impl()
}

Output

[warn] ./unused_class_inline.scala:5:22
[warn] unused private member
[warn]   private case class Impl()
[warn]  

Expectation

No warning.

[som-snytt]

Sneaky.

    inline def makeImpl(): Any = test.inline$Impl.apply():Any
    final def inline$Impl: test.Impl.type = test.Impl

[som-snytt]

In 3.7.4, case class was taken as trivially referencing itself via companion methods.

The underlying issue is that the check should record usages in the types of (all) trees.

[SethTisue]

Here's a variant that doesn't involve inline:

//> using scala 3.nightly
//> using option -Wunused:all

class C:
  def foo(): Unit =
    case class Loop()
    println(Loop())

[warn] ./C.scala:6:16
[warn] unused local definition
[warn]     case class Loop()
[warn]                ^^^^

the problem goes away if you remove case

[som-snytt]

@SethTisue thanks I added the local test

It's a high bar for ticket title grammar that fits.