variances are not preserved for 5+ type parameters

[memo33]

From zio/zio-http#3264:

For classes with 5 or more type parameters, the variances seem to be randomly shuffled among the type parameters.

class Foo[-A,B,-C,+D,+E]

izumi.reflect.Tag[Foo[Int, String, Double, Float, Long]].tag

Output is random:

val res0: izumi.reflect.macrortti.LightTypeTag = ::Foo[-Int,+String,=Double,+Float,-Long]
val res0: izumi.reflect.macrortti.LightTypeTag = ::Foo[-Int,+String,-Double,=Float,+Long]
val res0: izumi.reflect.macrortti.LightTypeTag = ::Foo[+Int,-String,=Double,+Float,-Long]

(Tested with izumi-reflect 2.3.10, Scala 3.6.3.)

[pshirshov]

If it's really random, it makes me think that there is a compiler bug. We've seen similar things before. Probably you should open an upstream issue.

[pshirshov]

Another possibility is some subtle bug in our binary tag encoding, but I very much doubt it because there is no concurrent access involved and the code is more or less straightforward. I would try to add some debug logging and see what happens.

[neko-kai]

@memo33 What about Scala 2? Does this reproduce on Scala 2?

EDIT: It does NOT reproduce on Scala 2

[neko-kai]

This IS a bug in Scala 3 Reflection API - specifically Symbol.typeMembers uses a Set underneath and the ordering is non-deterministic for large sets. This is not an API misuse issue, because there's no other way, that I found, using public API to get at type params of an applied concrete type. I'll check tomorrow if maybe I can retrieve a type lambda of a type constructor through its ParamRef retrieved through a .typeMembers call - no other ideas how to restore ordering using public API yet.

EDIT: No, .typeMembers returns TypeRefs, not ParamRefs.

Opened upstream issue: scala/scala3#22472

/cc @nicolasstucki @smarter

[pshirshov]

Ouch. So, I was kinda right. I think it's an API design issue, it returns an ordered collection interface which is backed by an unordered collection.

[memo33]

Awesome. Thanks a lot for the quick fix. It works for me.