Typeclass instance selection depends on the optimisation level

(See the attached files for a minimal case.)

A file A defines a typeclass, and gives an incoherent instance for all types a, and exports a function relying on said typeclass. A file B defines some data types, makes them specific instances of that class, and uses the function defined in A. Which instance ends up being picked for B depends on the optimisation level those files are compiled with.

//A.hs//

class A a where
  someValue :: a -> Maybe Int

instance {-# INCOHERENT #-} A a where
  someValue = const Nothing

getInt :: A a => a -> Int
getInt x = fromMaybe 0 $ someValue x

//B.hs//

data B = B Int

instance A B where
  someValue (B x) = Just x

getBInt :: Int
getBInt = getInt $ B 42

//Main.hs//

main = print getBInt

• *Upon compiling with -O0, this prints 42; upon compiling with -O2, this prints 0.**

(Interestingly, if the redundant class constraint is removed from getInt, then it always prints 0.)