@JsonCodec produces warnings on generic case class with -Wunused:params

[oleg-py]

circe 0.13.0
Annotating a generic case class with @JsonCodec:

@io.circe.generic.JsonCodec
case class Foo[A](data: A)
object Test extends App

Produces warnings:

parameter value encode0 in method encodeFoo is never used
parameter value decode0 in method decodeFoo is never used

Scastie: https://scastie.scala-lang.org/bGI7rDGURBWynQNaajI6vg

[travisbrown]

This is a bug in the compiler, which you can reproduce without the macro annotation:

import io.circe.Decoder
import io.circe.generic.semiauto.deriveDecoder

case class Foo[A](data: A)

object Foo {
  implicit def decodeFoo[A](implicit decode0: Decoder[A]): Decoder[Foo[A]] =
    deriveDecoder[Foo[A]]
}

…which gives:

// Exiting paste mode, now interpreting.

         implicit def decodeFoo[A](implicit decode0: Decoder[A]): Decoder[Foo[A]] =
                                            ^
On line 7: warning: parameter value decode0 in method decodeFoo is never used

The decode0 instance is very much not unused.

If someone knows a workaround here, I'd be happy to hear about it, but I hate these macro annotations and am looking forward to the day they'll be gone, so I'm inclined to close this as out-of-scope for Circe.

Thanks for the report, though, @oleg-py—I've seen questions about this before, but I don't think we had a proper issue.

[oleg-py]

The common thing to do with unused implicits is just

object Foo {
  implicit def decodeFoo[A](implicit decode0: Decoder[A]): Decoder[Foo[A]] = {
    val _ = decode0
    deriveDecoder[Foo[A]]
  }
}

thought that won't scale with number of type parameters. You can also have a helper unused function instead:

object Foo {
  implicit def decodeFoo[A](implicit decode0: Decoder[A]): Decoder[Foo[A]] = {
    io.circe.util.unused(decode0)
    deriveDecoder[Foo[A]]
  }
}

That of course has the drawback of polluting the library.

The third way I know of - not sure if it's supported by current Scala macros, but I've used it in better-monadic-for - is to mark methods and/or implicit parameters as synthetic and/or artifact (I don't know the difference except that they both work to silence many warnings).

[nafg]

I only started getting this now on scala 2.13.5. Did something change?

[oleg-py]

2.13.5 has added linting for unused context bounds (this pull).

[GlassAndOneHalf]

The same seems to happen with deriveEncoder & deriveDecoder from the semiauto package. For now I have suppressed the warnings using @nowarn.

[hamnis]

This is a compiler issue, not circe.

[hvesalai]

Did anybody report this to the scala developers?