HLint doesn't understand that identifiers with the same name can have different types and values, which can introduce subtle runtime bugs

Consider this program:

import Data.Maybe (isJust)

threeTenths :: RealFrac a => Maybe a
threeTenths = if 1/10 + 1/5 == correctAnswer then Just correctAnswer else Nothing
  where correctAnswer = 3/10

main :: IO ()
main = print $ isJust (fmap fromRational threeTenths)

It prints True. Running hlint on it will suggest using isJust threeTenths instead of isJust (fmap fromRational threeTenths). However, implementing that suggestion will cause it to instead print False.

Start with the Haskell reproducer in the issue and inspect the HLint suggestion that replaces isJust (fmap fromRational threeTenths) with isJust threeTenths. Determine where this suggestion is generated and verify that the fix does not change the program's result; done means this unsafe suggestion is no longer produced and the reproducer is covered by a regression test.