Failed to derive mirror in trait with self type

[Anghammar]

Compiler version

3.3.0

Minimized code

trait SuperTrait {
  sealed trait InnerTrait
  case class Foo() extends InnerTrait
}

trait OtherTrait

trait TraitWithSelfType extends SuperTrait { this: OtherTrait =>
  summon[deriving.Mirror.Of[Foo]]
  summon[deriving.Mirror.Of[InnerTrait]]
}

object Implementation extends TraitWithSelfType, OtherTrait {
  summon[deriving.Mirror.Of[Foo]]
  summon[deriving.Mirror.Of[InnerTrait]]
}

Output

-- [E172] Type Error: DerivingMirrorBug:10:40 ----------------------------------
10 |  summon[deriving.Mirror.Of[InnerTrait]]
   |                                        ^
   |No given instance of type deriving.Mirror.Of[TraitWithSelfType.this.InnerTrait] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[TraitWithSelfType.this.InnerTrait]:
   |	* trait InnerTrait is not a generic product because it is not a case class
   |	* trait InnerTrait is not a generic sum because its child class Foo is not accessible to call site value <local TraitWithSelfType>
1 error found

Expectation

summon[deriving.Mirror.Of[InnerTrait]] fails inside of TraitWithSelfType, but no of the other three summons fails. I'm not 100% sure this is actually a bug, but since OtherTrait does not actually do anything it sure seems like it. If it in fact works as intended, I would appreciate a short explanation why.

[bishabosha]

given that we can call Foo() from inside TraitWithSelfType it would seem that it is accessible, so maybe that check needs investigation

[Anghammar]

I just now realised I accidentally used the compiler version 3.3.0, but I have verified that the same issue exists on 3.3.1.

[EugeneFlesselle]

@bishabosha when checking if we have access through inheritance, we first get the classSymbol of the SingletonType.
But in the case above, (TraitWithSelfType.this : OtherTrait & TraitWithSelfType).parentSymbol == NoSymbol because of the ambiguous superclasses.

I tried this fix, but this this neg test also passes as a result:

class Outer2 {
  sealed trait Item
  case object A extends Item
}
def testOuter2 =
  // discussion point: should we allow this, or only allow singleton prefix?
  val m_Outer2_Item = summon[Mirror.Of[Outer2#Item]] // error: Item is not accessible from m_Outer2_Item

[bishabosha]

should we allow this, or only allow singleton prefix?

Prefixes should only be valid paths so yeah all singleton types

[michelou]

@bishabosha Candidate for a backport ?