object Ops: implicit class EitherSeqOps[E, T](private val seq: Seq[Either[E, T]]) extends AnyVal: def sequence: Either[::[E], Seq[T]] = ??? trait BuildException case class CompositeBuildException(ex: ::[BuildException]) extends BuildException trait ActionableDiagnostic trait ActionableHandler[A <: ActionableDiagnostic]: def exec: Either[BuildException, Seq[A]] import Ops._ val test: Either[BuildException, Seq[ActionableDiagnostic]] = // Can be replaced with Seq[Either[BuildException, Seq[ _ <: ActionableDiagnostic]]] , but current version matches better type of missing implicit Seq.empty[ActionableHandler[_]].map(_.exec) .sequence .left.map(_.head) .map(_.flatten) // error

[error] ./test.scala:21:19: [error] No given instance of type Seq[Any] => IterableOnce[B] was found for parameter asIterable of method flatten in trait IterableOps [error] where: B is a type variable with constraint <: ActionableDiagnostic [error] .map(_.flatten) // error [error]