type resolution with path-dependent types fails (works in Scala 2)

[yurique]

Compiler version

3.3.4, 3.6.2

Minimized code

trait Module {
  trait Reader[T]
}

class Builder[M <: Module](val m: M) {

  def json[Receive](implicit
                    reader: m.Reader[Receive], // path-dependent type here
                   ): Unit = {}

}

object mod extends Module

object Main {

  implicit val stringReader: mod.Reader[String] = new mod.Reader[String] {}

  def builder[M <: Module](m: M): Builder[M] = new Builder[M](m)

  def main(args: Array[String]): Unit = {
    builder(mod).json[String](stringReader) // the expected type of the reader parameter is Nothing  
   
  }

}

Output

$ scala-cli --scala 3.6.2 test.sc
Compiling project (Scala 3.6.2, JVM (17))
[error] ./test.sc:22:31
[error] Found:    (test$_.Main.stringReader : test$_.this.mod.Reader[String])
[error] Required: Nothing
[error]     builder(mod).json[String](stringReader) // the expected type of the reader parameter is Nothing
[error]                               ^^^^^^^^^^^^
Error compiling project (Scala 3.6.2, JVM (17))
Compilation failed

$ scala-cli --scala 2.13.15 test.sc
Compiling project (Scala 2.13.15, JVM (17))
Compiled project (Scala 2.13.15, JVM (17))

Expectation

I was expecting this to compile in Scala 3, since it compiles and works in Scala 2.13.
But I don't know if this is supposed to work, or if I'm doing it wrong, or if there is a workaround.

[KacperFKorban]

This would be a workaround. i.e. Adding a type parameter with an upper bound.

//> using scala 3.nightly

trait Module {
  trait Reader[T]
}

class Builder[M <: Module](val m: M) {
  def json[Receive, R <: m.Reader[Receive]](using reader: R): Unit = {}
}

object mod extends Module

given stringReader: mod.Reader[String] = new mod.Reader[String] {}

def main(args: Array[String]): Unit = {
  new Builder(mod).json(using stringReader) // the expected type of the reader parameter is Nothing  
}

[yurique]

@KacperFKorban thank you for the suggestion!

The problem with this is I'll have to add that extra type parameter (R) and it will make it hard to use.

The real code where I encountered this has two type parameters, so it's even worse: https://github.com/tulz-app/laminext/blob/series/0.16.x/modules/websocket-upickle/src/main/scala/io/laminext/websocket/upickle/WebSocketReceiveBuilderUpickleOps.scala#L12

The usage would have to change from

val ws = 
  WebSocket
    .url("wss://echo.websocket.events")
    .upickle(OptionPickler)
    .json[DataToSend, DataToReceive]
    .build()

to

val ws = 
  WebSocket
    .url("wss://echo.websocket.events")
    .upickle(OptionPickler)
    .json[DataToSend, DataToReceive, OptionPickler.Reader[DataToSend], OptionPickler.Writer[DataToReceive]]
    .build()

which is far from perfect :)