Expand type member in type mismatches

[kubukoz]

scala-cli https://raw.githubusercontent.com/kubukoz/demos/scala3-dependent-error/main.scala

//> using scala "3.1.3"

trait Request {
  type Out
}

object demo {

  def foo(r: Request)(v: r.Out) = ()

  val get: Request { type Out = String } = ???
  val put: Request { type Out = Unit } = ???

  foo(get)(())
}

In the code above, the compilation error you get with Scala 3.1.3 is:

Found:    Unit
Required: demo.get.Out

Scala 2.13.8 gives you more information because it dealiases the Out type:

type mismatch;
 found   : Unit
 required: demo.get.Out
    (which expands to)  String

I think it's a very useful thing to have, so I hope it can be added back.