Consider the following program. {-# LANGUAGE TypeApplications #-} import Data.IntMap (IntMap, mapKeysMonotonic, Key) import Data.Coerce (coerce) newtype MyMap = MyMap (IntMap Bool) doesn'tWork :: (Key -> Key) -> MyMap -> MyMap doesn'tWork = coerce mapKeysMonotonic works :: (Key -> Key) -> MyMap -> MyMap works = coerce (mapKeysMonotonic @Bool) mapKeysmonotonic has type (Key -> Key) -> IntMap a -> IntMap a and we attempt to coerce it to type (Key -> Key) -> MyMap -> MyMap since MyMap is a newtype around IntMap Bool this ends up generating the constraint Coercible a Bool and there is no way to specify that constraint given that a doesn't otherwise appear in the top-level type. Hence doesn'tWork will not type check. If I force the type of a to be Bool using a TypeApplication then it does type check (works). But is this a missing feature of GHC's Coercible system? Since a is not otherwise constrained is it not valid for GHC to choose it to be Bool ? Coercible instances contain now observable content so no observable behaviour could depend on the particular choice of Coercible a Bool instance, and choosing a ~ Bool is the natural one. Is there some violation of type safety or otherwise broken behaviour that could arise from choosing arbitrary Coercible instances on type variables that are not otherwise constrained? Tom