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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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

Contributor guide

Research direction
Investigate how implicit resolution matches `given [T]: CanEqual[T, T]` for two different types. The given instance provides equality for any type T with itself, but for `Test(1) == Test2(2)`, the compiler resolves `CanEqual[Test, Test2]`. Examine how type inference and implicit search interact, particularly how T is inferred and whether the compiler incorrectly unifies the types. Look into the Scala 3 specification on given instances and implicit resolution, and check the compiler source code for CanEqual resolution and type inference. The bug report includes a Scastie link for reproduction.
Tech stack
scala
Domain
backendcompilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Scalaimplicit resolution concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20