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derived with explicit term parameter creates confusing error at derives-site #15987

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Sporarum opened this issue Sep 7, 2022 · 0 comments
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area:reporting Error reporting including formatting, implicit suggestions, etc area:typeclass-derivation itype:enhancement Spree Suitable for a future Spree

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@Sporarum
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Sporarum commented Sep 7, 2022

Compiler version

scalacOptions ++= Seq(
  "-deprecation",
  "-encoding", "UTF-8",
  "-feature",
  "-unchecked"
)

scalaVersion := "3.2.0-RC1"
libraryDependencies += "org.scastie" %% "runtime-scala" % "1.0.0-SNAPSHOT"

addSbtPlugin("org.scastie" % "sbt-scastie" % "1.0.0-SNAPSHOT")

(3.2.0-RC1 is the latest version available on scastie)

Minimized code

trait Show[A]:
  extension(a: A) def show: String

object Show:
  def derived[A, B](x: B): Show[A] = ???

case object obj derives Show

https://scastie.scala-lang.org/Bw7NrMnbQdyKaakA8D5VQQ

Output

Found:    Playground.Show[A]
Required: String

where:    A is a type variable

Expectation

Something like:

derived instance Show[Person] failed to generate:
method `derived` from object Show takes explicit term parameters

Or:

Show cannot be used after `derives` as its companion object does not contain a method `derived` with signature:
  def derived[A, ...](using ...)...(using ...): Show[A]
Found:
  def derived[A, B](x: B): Show[A]
@Sporarum Sporarum added itype:bug stat:needs triage Every issue needs to have an "area" and "itype" label labels Sep 7, 2022
@mbovel mbovel added itype:enhancement area:reporting Error reporting including formatting, implicit suggestions, etc area:typeclass-derivation Spree Suitable for a future Spree and removed itype:bug stat:needs triage Every issue needs to have an "area" and "itype" label labels Sep 12, 2022
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Labels
area:reporting Error reporting including formatting, implicit suggestions, etc area:typeclass-derivation itype:enhancement Spree Suitable for a future Spree
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