Somewhat annoying to get at rethrow syntax due to invariance

[djspiewak]

MonadError is invariant in E, which is problematic in turn because it means that catsSyntaxMonadErrorRethrow will not infer on Either[E, A] where E <: Throwable, which is vastly far and away the most common case. For example, if you have an expression that produces F[Either[RuntimeException, A]], you might reasonably expect that rethrow would be available syntax on such a value (at least, I did). It won't work though, and it's quite difficult to tease out why this is the case.

We can either make MonadError contravariant in E (which I believe would cause a cascade of problematic changes, so I don't recommend it), or we can loosen the constraints on catsSyntaxMonadErrorRethrow. Something like:

implicit final def catsSyntaxMonadErrorRethrow[F[_], E2, E1 <: E2, A](fea: F[Either[E1, A]])(implicit F: MonadError[F, E2]): MonadErrorRethrowOps[F, E2, A]

Or the latter, which is likely to infer slightly better:

implicit final def catsSyntaxMonadErrorRethrow[F[_], E1, E2, A](fea: F[Either[E1, A]])(implicit F: MonadError[F, E2], ev: E1 <~< E2): MonadErrorRethrowOps[F, E2, A]

[LukaJCB]

Closed by #2880