scala/scala3

Documentation help needed: `given [T]: CanEqual[T, T]` resolves `CanEqual`s of different types

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#18,631 opened on Oct 2, 2023

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area:documentationgood first issueitype:enhancement

Description

Compiler version: 3.3.1 Compiler option: -language:strictEquality

The following code compiles fine. But why? Where does it finds CanEqual[Test, Test2] of different types?

case class Test(i: Int)
case class Test2(i: Int)

object Eq:
  given [T]: CanEqual[T, T] = CanEqual.derived

  @main def run(): Unit =
    Test(1) == Test2(2)
    ()

https://scastie.scala-lang.org/X6boEpObQXee5uhESbviuQ

Thanks

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Tech stack
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Domain
documentation
Issue type
documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Scala 3 programming experienceUnderstanding of CanEqual and type classes
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the Scala 3 compiler's implicit resolution for the `CanEqual` type class, specifically how the `given [T]: CanEqual[T, T]` instance allows comparison of different types `Test` and `Test2`. Look at the `CanEqual` trait definition in the standard library and the compiler's implicit search logic in `implicits.scala` and related files. Review comments in the issue for any maintainer hints.