Incompleteness in the Coercible constraint solver

$ ghci -ignore-dot-ghci
GHCi, version 8.5.20180105: http://www.haskell.org/ghc/  :? for help
Prelude> newtype WF f a = WF (f a)
Prelude> import Data.Coerce
Prelude Data.Coerce> :set -XFlexibleContexts 
Prelude Data.Coerce> :t coerce :: Coercible (cat a b) (a -> f b) => cat a b -> (a -> WF f b)

<interactive>:1:1: error:
    • Couldn't match representation of type ‘cat1 a1 b1’
                               with that of ‘a1 -> WF f1 b1’
        arising from a use of ‘coerce’
    • In the expression:
          coerce ::
            Coercible (cat a b) (a -> f b) => cat a b -> (a -> WF f b)

I'm not sure if I've filed this before or if it's even a bug.

But we know that Coercible (a -> f b) (a -> WF f b) so why doesn't this work?

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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