Surely GHC is too clever by half in asserting that just 1 of two impossible branches is redundant in the code

case 0::Int of 0 -> putStrLn "A" 1 -> putStrLn "B" _ -> putStrLn "C"

The assertion suggests that GHC code optimization would leave some useless vestige of the third case, The long-range solution to Thielmann's quandry is a bug report. Doug