Can't use `derives` to derive an `opaque type`, yields "is not a class type"

[mrdziuban]

Compiler version

3.4.0

Minimized code

object types {
  opaque type Test[A] = Unit

  object Test {
    def derived[A]: Test[A] = ()
  }
}

case class Foo() derives types.Test

Output

-- [E170] Type Error: ----------------------------------------------------------
9 |case class Foo() derives types.Test
  |                         ^^^^^^^^^^
  |                         types.Test is not a class type

Expectation

Given the docs on how derives clauses are desugared, I wouldn't expect it to matter that types.Test is not a class type. I expected the generated definition to look like this, which compiles if written by hand:

given types.Test[Foo] = types.Test.derived

[odersky]

Here's the start of the doc page:

Type class derivation is a way to automatically generate given instances for type classes which satisfy some simple conditions. A type class in this sense is any trait or class with a single type parameter determining the type being operated on, and the special case CanEqual.

So this is a language enhancement.

[joan38]

+1 to this. I got hit by the same "issue", or lack of opaque type type class support.

Here is a real world usecase

[sjrd]

This looks like a language change that requires a SIP. As @odersky mentioned, the implementation follows the spec/documentation.