NoType when deriving an abstract type

[bertlebee]

Compiler version

3.1.0

Minimized code

object OpaqueTypes:
  opaque type OpaqueType[A] = List[A]
  object OpaqueType:
    def derived[A]: OpaqueType[A] = Nil

import OpaqueTypes.OpaqueType
case class Boom[A](value: A) derives OpaqueType

Output

-- Error: foo.scala:7:37 -------------------------------------------------------
7 |case class Boom[A](value: A) derives OpaqueType
  |                                     ^^^^^^^^^^
  |                                     <notype> is not a class type
1 error found

Expectation

compiles or provides more a useful error

In case it's not clear from the code, I'm trying to derive an instance of an opaque type.

[bishabosha]

I have updated the code with the error shown in scala 3.1

[bishabosha]

minimised to being a plain abstract type:

object FooModule:
  type Foo[A]
  object Foo:
    def derived[A]: Foo[A] = Nil.asInstanceOf[Foo[A]]

import FooModule.Foo
case class Boom[A](value: A) derives Foo

[bertlebee]

Wow, thanks for the quick response and fix! I take it from the new error message we're unable to use the derivation functionality with opaque types, so I'll have to use a wrapper type instead. Is there a reason this is unsound or is it just not possible?

[odersky]

Nobody has thought through the ramifications of allowing something different than a class type. Also it would be irregular wrt to extends which also requires classes.

[bertlebee]

Thanks for the explanation. This is way above my pay grade so I'll create a feature request and leave it for more brilliant minds to decide if it's sound and worth implementing.