Explicit Typeable for older GHC.

src/Panic.hs

@@ -5,11 +5,12 @@ module Panic (
 
 import Base (Show)
 import Data.Text (Text)
+import Data.Typeable (Typeable)
 import Control.Exception as X
 
 -- | Uncatchable exceptions thrown and never caught.
 data FatalError = FatalError { msg :: Text }
-  deriving (Show)
+  deriving (Show, Typeable)
 
 instance Exception FatalError