strictEquality with explicit-nulls do not work well together

soronpo · February 21, 2025, 4:33pm

With -Yexplicit-nulls

import language.strictEquality

val x: String | Null = null

val check = x == null //error

Should the explicit nulls feature introduce CanEqual[T | Null, Null] and CanEqual[T | Null, T]?

Could be relevant for the discussion, how to generally handle strict equality for unions:

github.com/scala/scala3

CanEqual is undefined by default for unions?

opened 09:02PM - 04 Dec 23 UTC

soronpo

itype:enhancement area:typeclass-derivation backlog

## Compiler version v3.3.1 ## Minimized code https://scastie.scala-lang….org/94w48wQbRdq9rQ2IKcbNCg ```Scala import language.strictEquality sealed trait Foo derives CanEqual sealed trait Bar derives CanEqual object FooObj extends Foo object BarObj extends Bar val x: Foo | Bar = FooObj @main def main: Unit = x match case FooObj => case BarObj => ``` ## Output ```scala error: Values of types object FooObj and Foo | Bar cannot be compared with == or != ``` ## Expectation I would expect that by default we have something like ```scala given CanEqualUnion[A, B](using CanEqual[A, A], CanEqual[B, B]): CanEqual[A | B, A | B] = CanEqual.derived ``` So no error should happen under strict equality for any union of types that have `CanEqual` defined for them.

Sporarum · February 21, 2025, 4:45pm

This seems required for these two features to interact in a safe way

Is there a way to allow that, but disallow "null" == null (Since "null" is a String | null) ?