Type matched class cannot be referenced using `classOf`

[WojciechMazur]

Compiler version

3.2.0-RC1-bin-20220429-400427d-NIGHTLY

Works with 3.1.2

The first bad commit is 54dc0c1 from #14761

Minimized code

//> using scala "3.2.0-RC1-bin-20220429-400427d-NIGHTLY"

// Last working version
// //> using scala "3.1.2"

import scala.reflect.ClassTag
// https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala-3/org/json4s/ext/package.scala#L4-L8
type Aux[A] = { type Value = A }
type EnumValue[A <: Enumeration] = A match {
  case Aux[a] => a
}

// https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala/org/json4s/ext/EnumSerializer.scala#L25-L26
class EnumSerializer[E <: Enumeration: ClassTag](enumeration: E) {
  val EnumerationClass = classOf[EnumValue[E]]
}

Output

[error] ./test.scala:15:34: EnumValue[E] is not a class type
[error]   val EnumerationClass = classOf[EnumValue[E]]
[error]                                  ^^^^^^^^^^^^```

Expectation

Should correctly allow getting an instance of Class for Enum.Value

[nicolasstucki]

The first bad commit is 54dc0c1 from #14761

[griggt]

The first bad commit is 54dc0c1 from #14761

Hmm, that commit shows as good for me. I bisected it to d1957e5 from #13780.

[odersky]

It looks indeed plausible that #13780 is the PR that makes it fail.

I added a summon[EnumValue[A]] and got:

Note: a match type could not be fully reduced:
   |
   |  trying to reduce  EnumValue[E]
   |  failed since selector  E
   |  does not match  case Aux[a] => a
   |  and cannot be shown to be disjoint from it either.

[odersky]

I am not sure the match type is supposed to reduce. Maybe use a TypeTest? But I don't know enough about the details here.

[odersky]

@OlivierBlanvillain, do you have an opinion about this issue?

[OlivierBlanvillain]

I am not sure the match type is supposed to reduce.

I'm also not sure. This code is asking the compiler to show that E <: Aux[a], knowing that E <: Enumeration and type Aux[A] = { type Value = A }. Looking at the definitions, Value is an abstract inner class of Enumeration, so it feels like E could be a subtype of the { type Value = A } refinement, but I don't know how those two features interact (inner classes vs refinement types)...