Hello GHC-Devs, I am currently giving #11475 a try. However I hit a few problems and wanted to ask for a bit of advice about how to proceed. I already can handle the following: case x of A y -> e1 _ -> ....(case x of B -> r2 C -> r3) ... My first problem then was the following construct: case (case ... of ... -> Left x) of Left y -> ... Or more simple: case Left x of Left y -> ... So I have to check which constructors the scrutinee may return. I know the simplifier has to implement such logic somewhere but did not want to use the implementation for two reasons: - I didn't know where to find it - If GHC uses the same test to prune impossible alternatives and to check if they were pruned correctly, extending the linter is useless. A bug in the test code will cause also cause a bug in the linter. So I implemented such a test myself, but now I am stuck at something like this: foo a b = a : b bar = ... case foo a b of (:) x y -> ... I cannot see through foo, so I assume it may return (:) and []. The simplifier on the other hand is able to see through foo and did remove the [] case, causing my current implementation to throw an incorrect lint error. How can I get a list/set of all possible AltCons returned by a function/constant? Or a list of the impossible ones? I tried to get some information from the unfolding of a variable, but it seems like unfoldings are only attached to top-level names. Jonas