Why doesn't my type class satisfy the functional dependency?

I tried to create a method for stripping away the Meta information of a generic Rep. While creating a type class and writing instances for every used Rep type, I came across an error saying that two of the instances (:+:, M1) didn't satisfy the functional dependency. I don't understand this.

This is my code:

{-# LANGUAGE FunctionalDependencies #-}
module Generics where
import GHC.Generics

class WithoutMeta f f' | f -> f' where
  withoutMeta :: f p -> f' p

instance WithoutMeta V1 V1 where
  withoutMeta = id

instance WithoutMeta U1 U1 where
  withoutMeta = id

instance (WithoutMeta f f', WithoutMeta g g') => WithoutMeta (f :+: g) (f' :+: g') where
  withoutMeta (L1 l) = L1 $ withoutMeta l
  withoutMeta (R1 r) = R1 $ withoutMeta r

instance (WithoutMeta f f', WithoutMeta g g') => WithoutMeta (f :*: g) (f' :*: g') where
  withoutMeta (f :*: g) = withoutMeta f :*: withoutMeta g

instance WithoutMeta (K1 i a) (K1 i a) where
  withoutMeta = id

instance WithoutMeta f f' => WithoutMeta (M1 i c f) f' where
  withoutMeta (M1 f) = withoutMeta f

I tried to switch from fundeps to type families and it seems to compile. Type families are more readable to me.

{-# LANGUAGE TypeFamilies #-}

import GHC.Generics
import Data.Kind

class WithoutMeta f where
  type WM f :: Type -> Type
  withoutMeta :: f p -> WM f p

instance WithoutMeta V1 where
  type WM V1 = V1
  withoutMeta = id

instance WithoutMeta U1 where
  type WM U1 = U1
  withoutMeta = id

instance (WithoutMeta f, WithoutMeta g) => WithoutMeta (f :+: g) where
  type WM (f :+: g) = (WM f :+: WM g)
  withoutMeta (L1 l) = L1 $ withoutMeta l
  withoutMeta (R1 r) = R1 $ withoutMeta r

instance (WithoutMeta f, WithoutMeta g) => WithoutMeta (f :*: g) where
  type WM (f :*: g) = (WM f :*: WM g)
  withoutMeta (f :*: g) = withoutMeta f :*: withoutMeta g

instance WithoutMeta (K1 i a) where
  type WM (K1 i a) = (K1 i a)
  withoutMeta = id

instance WithoutMeta f => WithoutMeta (M1 i c f) where
  type WM (M1 i c f) = WM f
  withoutMeta (M1 f) = withoutMeta f

If you want to keep fundeps, you can try turning on UndecidableInstances. Without that, it looks like your code does not satisfy the GHC coverage condition that ensures termination.