The following code, when compiled with -Wall, gives no warnings:

module Main where

main :: IO ()
main = print $ foo (10 :: Int)
  where
    foo x
        | True = (20 :: Int)
        | x == 10 = (30 :: Int)

I would expect that since the pattern guard | True obviates the following guards, that it would warn about unreachable code. There was a bug in my code today which I tracked down to a code transposition that would have been easily spotted with such a warning.