Cannot derive Mirror.SumOf for inherited trait

[oscar-broman]

Compiler version

Latest version tested is 3.1.2-RC1-bin-20211102-82172ed-NIGHTLY

Minimized code

import scala.deriving.Mirror
// Works:
sealed trait HasFoo { sealed trait Foo; case object B extends Foo; summon[Mirror.SumOf[Foo]] }

// Doesn't work:
final class UsesFoo extends HasFoo { summon[Mirror.SumOf[Foo]] }

Output

no implicit argument of type deriving.Mirror.SumOf[UsesFoo.this.Foo] was found for parameter x of method summon in object Predef

Expectation

It should derive the mirror just fine.