MonadError syntax for OptionT uses incorrect instance

[wedens]

This PR introduced the problem: #2299

MonadError[OptionT[F, ?], Unit] is given higher priority than MonadError[F, E] => MonadError[OptionT[F, ?], E].

And, of course, syntax for MonadError always resolves the first instance making it useless for any E other than Unit.

[SystemFw]

👍 to changing it back, http4s is affected, since you can't call handleErrorWith on HttpApp anymore (it's Kleisli of OptionT), basically for the purpose of using MonadError of Unit, which is close to a toy instance.

cc @rossabaker @ChristopherDavenport

[kailuowang]

+1 to fixing this.
We have two options:
1, We can simply make the Unit instance explicit
2. We can switch the two and use this trick to bump the priority of MonadError[F, E] => MonadError[OptionT[F, ?], E], (otherwise they probably will be of the same priority and causing ambiguity issue.) We also need to add 2 Mima exceptions (such changes were proven to be fine) and ideally one test to https://github.com/typelevel/cats/blob/master/binCompatTest/src/main/scala/catsBC/MimaExceptions.scala to make sure it works fine.

Possibly we can merge this fix to 1.5.x branch and be in time for the 1.5.x release

[ChristopherDavenport]

Definitely, 👍 to this being fixed.

[barambani]

I'm having a look if that's all right.