GHC warns about non-exhaustive pattern match of Word8

import Data.Word

foo :: Word8 -> Bool
foo 0x00 = True
foo 0x01 = True
foo 0x02 = True
foo 0x03 = True
...
foo 0xFF = True

Is exhaustive, one can test that with

test = map foo [ minBound .. maxBound ]

However, GHC seems to disagree an issues a warning.

Complete file is in attachment.

demo.hs