Delete fmapCollect. It's a lie, and will infinitely loop.

The implementation instantiates g rather than f, and is the same as
fmapCollect = fmap. Deriving fmap requires cotraverse, not collect.

src/Data/Distributive.hs

@@ -21,7 +21,6 @@ module Data.Distributive
   ( Distributive(..)
   , cotraverse
   , comapM
-  , fmapCollect
   ) where
 
 import Control.Applicative
@@ -241,11 +240,6 @@ instance Distributive Complex where
     imagP (_ :+ i) = i
 #endif
 
--- | 'fmapCollect' is a viable default definition for 'fmap' given
--- a 'Distributive' instance defined in terms of 'collect'.
-fmapCollect :: forall f a b . Distributive f => (a -> b) -> f a -> f b
-fmapCollect = coerce (collect :: (a -> Identity b) -> f a -> Identity (f b))
-
 #if __GLASGOW_HASKELL__ >= 702
 instance Distributive U1 where
   distribute _ = U1