I have this simplified module: module Ganeti.LensPlain where import Control.Lens (LensLike, traverseOf) import Data.Functor.Compose (Compose(..)) traverseOf2 :: LensLike (Compose f g) s t a b -> (a -> f (g b)) -> s -> f (g t) traverseOf2 k f = getCompose . traverseOf k (Compose . f) With ghc-9.2 and later I get this kind error: src/Ganeti/LensPlain.hs:8:32: error: [GHC-25897] • Couldn't match kind ‘k2’ with ‘*’ When matching types Compose f g0 :: * -> * Compose f g :: k2 -> * Expected: s -> Compose f g t Actual: s -> Compose f g0 t0 ‘k2’ is a rigid type variable bound by the type signature for: traverseOf2 :: forall {k1} {k2} (f :: k1 -> *) (g :: k2 -> k1) s (t :: k2) a (b :: k2). LensLike (Compose f g) s t a b -> (a -> f (g b)) -> s -> f (g t) at src/Ganeti/LensPlain.hs:7:1-79 • In the second argument of ‘(.)’, namely ‘traverseOf k (Compose . f)’ In the expression: getCompose . traverseOf k (Compose . f) In an equation for ‘traverseOf2’: traverseOf2 k f = getCompose . traverseOf k (Compose . f) • Relevant bindings include f :: a -> f (g b) (bound at src/Ganeti/LensPlain.hs:8:15) k :: LensLike (Compose f g) s t a b (bound at src/Ganeti/LensPlain.hs:8:13) traverseOf2 :: LensLike (Compose f g) s t a b -> (a -> f (g b)) -> s -> f (g t) (bound at src/Ganeti/LensPlain.hs:8:1) | 8 | traverseOf2 k f = getCompose . traverseOf k (Compose . f) | ^^^^^^^^^^^^^^^^^^^^^^^^^^ GHC-9.0 accepts the code. Also GHC-9.2 accepts it when the module is compiled as part of a larger Cabal project! I thought that (.) would cause the problem, but manually inlining it does not help. I compiled with: $ ghc-9.6.6 -package lens -c src/Ganeti/LensPlain.hs