Deriving TagKK[F[Either]] with TagK[F] in context (scala 2.13 only)

[road21]

This code is not compiling in scala 2.13.16:

import izumi.reflect.{TagK, TagKK, HKTag}

type Mono[F[_], x, y] = F[Either[x, y]]

def foo[F[_]: TagK]: TagKK[Mono[F, *, *]]] =
  implicitly // compiler error: could not find implicit value for parameter e: izumi.reflect.TagKK[[β$0$, γ$1$]F[scala.util.Either[β$0$,γ$1$]]]

The same code is compiling in scala 3 without compiler errors:

//> using scala 3.6.2
//> using dep "dev.zio::izumi-reflect:3.0.0"
import izumi.reflect.{TagK, TagKK, HKTag}

type Mono[F[_], x, y] = F[Either[x, y]]

def foo[F[_]: TagK]: TagKK[[x, y] =>> Mono[F, x, y]] = 
  implicitly // ok

As a workaround type Mono[F[_], x, y] >: F[Either[x, y]] <: F[Either[x, y]] works.

Scastie: https://scastie.scala-lang.org/road21/j3t7ODHHQzqjYW0hPysctg/30

[neko-kai]

@road21 Interesting, this looks like a type inference issue on Scala 2, not a bug in the macro itself. If the TagKK type is inlined to its underlying HKTag form, it works:

import izumi.reflect.{TagK, TagKK}

type Mono[F[_], x, y] = F[Either[x, y]]

def test[F[_]: TagK]: TagKK[Mono[F, *, *]] = implicitly[HKTag[{ type Arg[A, B] = Mono[F, A, B] }]]

println(test[Option].tag.debug())
// ⚙️ begin : λ %1,%2 → scala.Option[+scala.util.Either[+1,+2]]

Additionally, the trace on explicit macro invocation is weird:

def test[F[_]: TagK]: TagKK[Mono[F, *, *]] = izumi.reflect.HKTag.hktagFromTagMacro

could not find implicit value for izumi.reflect.Tag[A]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in A? e.g. def x[T: Tag, F[_]: TagK] = ...


<trace>: 
  deriving Tag for A, dealiased: A:
  could not find implicit value for Tag[A]: A is a type parameter without an implicit Tag!
      def test[F[_]: TagK]: TagKK[Mono[F, *, *]] = izumi.reflect.HKTag.hktagFromTagMacro

What is A here? It seems to be an artefact of type inference.

It would be great to minimize this so as not to require the macro and report to Scala 2 upstream...

[road21]

Thank you very much for the quick response!
I will try to minimize it