Possible bug in match type reduction

Compiler version

[eje@localhost poc-coulomb-scala3]$ ./mill show coulomb.scalaVersion
[1/1] show 
"3.0.0-RC2"

Minimized code

the following code also resides here: https://github.com/erikerlandson/poc-coulomb-scala3/blob/match-type-bug/coulomb/src/coulomb/infra/sigops.scala

package repro

trait SNil
trait %:[H, T]
trait Zero
trait Fail

type FilterZ[S] = S match
    case SNil => SNil
    case Zero %: t => FilterZ[t]
    case h %: t => h %: FilterZ[t]
    case Any => Fail

Output

Does not traverse the type and remove Zero as expected:

scala> import repro.*

scala> type T = FilterZ[String %: Zero %: SNil]
// defined alias type T = repro.FilterZ[String %: repro.Zero %: repro.SNil]

Expectation

My reading of the match-type doc led me to hope that it could filter out elements that match the corresponding match-type case:

// ideally, some variation on this, where `Zero` gets filtered out
scala> import repro.*

scala> type T = FilterZ[String %: Zero %: SNil]
// defined alias type T = String %: repro.SNil

[OlivierBlanvillain]

It triggers, it’s just not printed as such in the repl:

object Test {
  type T = FilterZ[String %: Zero %: SNil]
  implicitly[T =:= String %: SNil] // compiles
}

[erikerlandson]

I suppose, unless you ever want to use FilterZ with non final elements, in which case I would also make Zero final or the compiler might not be able able to conclude that your elements are disjoint from Zero

I don’t have total control over what kind of types will appear in these expressions, so I think match types will not be a safe solution for my needs here. I can make use of my existing context/implicit based logic instead.

Thanks for explaining the issues with using traits!