Presumably, you're compiling this program outside of a git repository, and so the Template Haskell splice $(gitHash) evaluates to "UNKNOWN" at compile time. Here is an even more minimal example, which does not depend on gitrev: f :: () f = case "UNKNOWN" of "UNKNOWN" -> () _ -> () This, when compiled with GHC 8.6.1 or later, gives the same warning: Test.hs:4:7: warning: [-Woverlapping-patterns] Pattern match is redundant In a case alternative: _ -> ... | 4 | _ -> () | ^^^^^^^^^^^^^^^ This, I would argue, is a correct error message. The string literal "UNKNOWN" is, well, statically known to be "UNKNOWN", so GHC knows that that catch-all case cannot be reached. (See [1] for the commit which introduced this check.) Granted, this makes things slightly inconvenient for your purposes, since in your actual program, $(gitHash) might evaluate to *different* things at compile time depending on the directory you compile it in. I'd recommend rewriting the commitInfo function to pattern guards: commitInfo :: Maybe String commitInfo | hash == "UNKNOWN" = Nothing | otherwise = Just hash where hash = $(gitHash) Since that does not trigger -Woverlapping-patterns. (Granted, it's possible that GHC could become much smarter in the future and be able to detect that that `otherwise` case is unreachable when $(gitHash) evaluates to "UNKNOWN" at compile time. But GHC certainly isn't that smart today!) Ryan S. ----- [1] http://git.haskell.org/ghc.git/commit/1f88f541aad1e36d01f22f9e71dfbc247e6558...