Coercible solver incomplete with non-variable transitivity
If I say
import Data.Coerce import Data.Proxy foo :: (Coercible (a b) (c d), Coercible (c d) (e f)) => Proxy (c d) -> a b -> e f foo _ = coerce
I get an error, because GHC isn't smart enough to figure out transitivity in this case. It can do it with bare variables, but not variable applications. It's possible there's a way to do this, but we'll wait for someone to shout before investing effort.
Do shout if this lack of functionality is ruining your day!