Commit 4260466929 added the "r has at
least two elements" constraint to the `Member' t r ('S n)` instance but
`Member t effs` with a general (unknown) effs isn't enough to deduce
that effs is non-empty, so this doesn't typecheck:
f :: Eff (IO ': effs) () -> Eff effs ()
f = undefined
g :: (Member Maybe effs) => Eff effs ()
g = undefined
h :: (Member Maybe effs) => Eff effs ()
h = f g
GHC complains that it could not deduce: `effs ~ (r : rs)` arising from
the use of `g`. It can be worked around by using this instead:
h :: (Member Maybe (e ': es)) => Eff (e ': es) ()
h = f g
but I don't think this is a good user experience, and it's a regression
from 0.2.3.0 that would normally require a major version bump to 0.3 as
it breaks existing code. Therefore a fix should go in quickly and should
get into lts-7 asap.
This commit adds the "effs is non-empty" constraint to `Member t effs`
so the above typechecks again.
