Conflicting base type arguments error in RC2

[gzoller]

Compiler version

RC2

Minimized code

case class IntersectionTypeAdapter[L, R](
    info: RType,
    leftTypeAdapter:  TypeAdapter[L],
    rightTypeAdapter: TypeAdapter[R])(implicit taCache: TypeAdapterCache)
  extends TypeAdapter[L & R]:

Output

[error] -- Error: /Users/gregoryzoller/git/junk/src/main/scala/co.blocke.junk/Junk.scala:17:11
[error] 17 |case class IntersectionTypeAdapter[L, R](
[error]    |           ^^^^^^^^^^^^^^^^^^^^^^^
[error]    |class IntersectionTypeAdapter cannot be instantiated since it has conflicting base type arguments L and R

Expectation

No errors expected. This code worked for all Scala 3 until RC2

[odersky]

Can you provide a self contained example?

This looks like a problem caused by some tightenings of the rules to prevent a soundness hole.

[gzoller]

Surely. Here's a further-minimized, stand-alone example:

trait MyBase[A]{ 
  def foo: String
}

case class BothThing[L, R]() extends MyBase[L & R]:
  def foo: String = "blather"

This produces:

[error] 15 |case class BothThing[L, R]()
[error]    |           ^^^^^^^^^
[error]    |class BothThing cannot be instantiated since it has conflicting base type arguments L and R
[error] one error found

[adamgfraser]

I think we are seeing a version of this in trying to upgrade ZIO to 3.0.0-RC2. Minimized example:

trait Has[A]

trait Console
trait Clock

type ConsoleWithClock = Has[Console] with Has[Clock]

class Spec[R <: Has[_]]

object MySpec1 extends Spec[Has[Console] with Has[Clock]] // does not compile
object MySpec2 extends Spec[ConsoleWithClock] // okay