`-Wunused:all` and nested type class derivation

[amumurst]

I tried adding the new unused flag (from #16157) to my work codebase and was surprised that it warned about unused import when used with the json library circes derivation of json Encoders from case classes. ( case class Foo(i: Int) derives Encoder.AsObject). A minimal reproducer independent of circe is below. I see no error when trying to derive the "top level" typeclass and not a subtype.

Compiler version

With nightly: 3.3.0-RC1-bin-20230112-be10bc6-NIGHTLY

Minimized code

//> using scala "3.3.0-RC1-bin-20230112-be10bc6-NIGHTLY"
//> using option "-Wunused:all"

object myPackage:
   trait CaseClassName[A]:
      def name: String

   object CaseClassName:
      trait CaseClassByStringName[A] extends CaseClassName[A]
      import scala.deriving.Mirror
      object CaseClassByStringName:
         inline final def derived[A](using inline A: Mirror.Of[A]): CaseClassByStringName[A] = 
            new CaseClassByStringName[A]:
               def name: String = A.toString

import myPackage.CaseClassName
case class CoolClass(i: Int) derives CaseClassName.CaseClassByStringName
println(summon[CaseClassName[CoolClass]].name)

Output

[warn] ./unused-typeclass-derivation.sc:16:18: unused import
[warn] import myPackage.CaseClassName
[warn]                  ^^^^^^^^^^^^^

Expectation

The import is needed and used 2 places (at the summon and at the derives), so no warn should be provided.

[PaulCoral]

Thanks for reporting all this -Wunused issue, @amumurst !

I see no error when trying to derive the "top level" typeclass and not a subtype.

Can you give me a code example of this. It would help me a lot debuging this. I do not really understand what you meant here.

[amumurst]

Just that this does not give a warning where there is no subtype.

object myPackage:
   trait CaseClassName[A]:
      def name: String

   object CaseClassName:
      import scala.deriving.Mirror
      inline final def derived[A](using inline A: Mirror.Of[A]): CaseClassName[A] = 
         new CaseClassName[A]:
            def name: String = A.toString

import myPackage.CaseClassName
case class CoolClass(i: Int) derives CaseClassName
println(summon[myPackage.CaseClassName[CoolClass]].name)

But I see now that its not really that relevant. This also fails, so its more about derives X.Y than anything specific to subtyping

More minimized

With this setup

object myPackage:
   object Foo:
      trait CaseClassName[A]:
         def name: String

      object CaseClassName:
         inline final def derived[A](using inline A:  scala.deriving.Mirror.Of[A]): CaseClassName[A] = 
            new CaseClassName[A]:
               def name: String = A.toString

This fails

import myPackage.Foo
case class CoolClass(i: Int) derives Foo.CaseClassName
println(summon[Foo.CaseClassName[CoolClass]].name)

But this works

import myPackage.Foo.CaseClassName
case class CoolClass(i: Int) derives CaseClassName
println(summon[CaseClassName[CoolClass]].name)

[ivan-klass]

Nested term selection is not required to be after derives to reproduce, if import from an object directly, false-positive is still there:

package example

import io.circe.Encoder.{AsObject => ToJson}

case class Foo(bar: String, baz: Int) derives ToJson

[warn] 3 |import io.circe.Encoder.{AsObject => ToJson}
[warn]   |                         ^^^^^^^^^^^^^^^^^^
[warn]   |                         unused import

[amumurst]

Aha, my minimised example picked it up as used since it was used in the summon 🤦 @PaulCoral

[ivan-klass]

Disclaimer: I'm not an expert here but just a curious dotty user impatient to -Wunused flag which now is still unusable because of false-positives. And here is some hints I've found:

• derived is represented in ast with DerivingTemplate which is a subclass of Template with non-empty .derived: List[untpd.Tree]
• Template case just delegates to traverseChildren with an updated context
• DerivingTemplate is not traversed with traverseChildren because of intentional "if guard" here

I'm not sure if CheckUnused is a right place to explicitly traverse DerivingTemplate to workaround traverseChildren behavior, but I'll try to compose a draft PR

[szymon-rd]

@ivan-klass thank you for your observations, I am looking into it right now.