Why am I getting this warning from GHCi?

Question

I'm getting a curious warning when pattern matching, but only when OverloadedStrings is enabled...

$ ghci -Wall
GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> let f x = case (x :: [String]) of {[""] -> "root"; ["product", _] -> "product"; _ -> "unknown"}
Prelude> :q
Leaving GHCi.
$ ghci -Wall -XOverloadedStrings
GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> let f x = case (x :: [String]) of {[""] -> "root"; ["product", _] -> "product"; _ -> "unknown"}

<interactive>:1:10:
    Warning: Pattern match(es) are overlapped
             In a case alternative: [""] -> ...
Prelude> let g x = case (x :: [String]) of {[] -> "root"; ["product", _] -> "product"; _ -> "unknown"}
Prelude> let h x = case (x :: [String]) of {["oops"] -> "root"; ["product", _] -> "product"; _ -> "unknown"}
Prelude> :q
Leaving GHCi.

I don't understand why I get the warning for f with OverloadedStrings, particularly since I don't get the warning for f without OverloadedStrings, and also don't get the warning for g or h, which differ from f only in the first pattern (which in all cases matches only a single particular value).

On the assumption that this is not a bug in GHC, what am I missing?

Answer 1

Here's a slightly simpler example that shows the same issue in GHC 6.12.3:

f :: String -> Bool
f "" = True
f "a" = False

g :: String -> Bool
g "" = True
g "aa" = False

Only g gets the overlap warning with -XOverloadedStrings. I think this has to be a bug.