Synthetic CanEqual for primitives does not handle Nothing as a subtype

[jcracknell]

As CanEqual is contravariant, this should be valid and is likely the root cause for issues around matching Nil/None under strict equality:

scala> implicitly[CanEqual[String, Nothing]]
val res0: CanEqual[String, Nothing] = scala.CanEqual$derived$@7c6b997b

scala> implicitly[CanEqual[Int, Nothing]]                                                                          
-- Error:
1 |implicitly[CanEqual[Int, Nothing]]
  |                                  ^
  |          Values of types Int and Nothing cannot be compared with == or !=

https://github.com/lampepfl/dotty/blob/370280bcfa96d1ea15e8b0b59f272dbbb3a982b9/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala#L102-L114

[odersky]

I agree this should be fixed. CanEqual[Int, Nothing] instances should be synthesizable.

[SethTisue]

nitpick: in Scala 3, summon is more idiomatic than implicitly

is likely the root cause for issues around matching Nil/None under strict equality

what are those issues — what are you doing with Nil and None that isn't working? we'd like to test that, too, rather than only testing what summon calls work and don't

so far all I've found that fails is:

scala> ??? == 3
-- Error: ----------------------------------------------------------------------
1 |??? == 3
  |^^^^^^^^
  |Values of types Nothing and Int cannot be compared with == or !=
1 error found

while the same thing with String compiles.

[SethTisue]

summon[CanEqual[3, Nothing]] fails just like summon[CanEqual[Int, Nothing]], so that's a test case to also include

it might not be any extra work to cover, since if one side is Nothing, as far as I can see it doesn't even matter what the other side is

[jcracknell]

@SethTisue 3e963c0 introduces CanEqual.canEqualOption[A] forcing a LUB computation as a workaround for this particular issue, which will occur for any type which would require derivation of CanEqual for a primitive and Nothing.

Under 3.0.2 prior to the addition of the workaround this did not work:
https://scastie.scala-lang.org/C3pjrS3vTAWC4tqC9bkSgw

[SethTisue]

ah, I see #13512 now, I had not noticed that link before

since if one side is Nothing, as far as I can see it doesn't even matter what the other side is

and I see there was already some discussion on this at #12419

general reference on CanEqual: https://docs.scala-lang.org/scala3/reference/contextual/multiversal-equality.html

[SethTisue]

PR: #13828