In case it is helpful, here is the work I did on implementing none and fuseError.

  def none[B](implicit ev: A <:< Option[B]): ZIO[R, Option[E], Unit] =
    self.foldM(
      e => ZIO.fail(Some(e)),
      a => ev(a).fold[ZIO[R, Option[E], Unit]](ZIO.succeed(()))(_ => ZIO.fail(None))
    )

  def fuseError[E1, E2 <: E1](default: E2)(implicit ev: E <:< Option[E1]): ZIO[R, E1, A] =
    self.mapError(e => ev(e).getOrElse(default))

@jdegoes @adamgfraser what are the actual use cases for none and fuseError. I'm trying to come up with a meaningful test cases.

@jczuchnowski none is basically asserting that the optional value in a ZIO is empty and failing if it is not. So you should have:

IO.succeed(None).none === IO(())
IO.succeed(Some(1)).none === IO.fail(Option.empty)
IO.fail("Fail").none === IO.fail(Some("Fail"))

Fuse error is taking a ZIO that could fail with Some(error) or None and adding a default error value to fail with to replace the None case. So you should have:

IO.fail(Some("Fail")).fuseError("Default") === IO.fail("Fail")
IO.fail(None).fuseError("Default") === IO.fail("Default")