base: Use displayException in top-level exception handler

Happily this also allows us to eliminate a special case for Deadlock
exceptions.

Implements [CLC #198](https://github.com/haskell/core-libraries-committee/issues/198).

libraries/base/changelog.md

@@ -2,6 +2,7 @@
 
 ## 4.20.0.0 *TBA*
   * Export `foldl'` from `Prelude` ([CLC proposal #167](https://github.com/haskell/core-libraries-committee/issues/167))
+  * The top-level handler for uncaught exceptions now displays the output of `displayException` rather than `show`  ([CLC proposal #198](https://github.com/haskell/core-libraries-committee/issues/198))
   * Add `permutations` and `permutations1` to `Data.List.NonEmpty` ([CLC proposal #68](https://github.com/haskell/core-libraries-committee/issues/68))
   * Add a `RULE` to `Prelude.lookup`, allowing it to participate in list fusion ([CLC proposal #175](https://github.com/haskell/core-libraries-committee/issues/175))
   * Implement `stimes` for `instance Semigroup (Endo a)` explicitly ([CLC proposal #4](https://github.com/haskell/core-libraries-committee/issues/4))

libraries/ghc-internal/src/GHC/Internal/Conc/Sync.hs

@@ -109,9 +109,6 @@ import GHC.Internal.Foreign.C.String
 import GHC.Internal.Foreign.Storable
 import GHC.Internal.Foreign.StablePtr
 
-import GHC.Internal.Data.Typeable
-import GHC.Internal.Data.Maybe
-
 import GHC.Internal.Base
 import {-# SOURCE #-} GHC.Internal.IO.Handle ( hFlush )
 import {-# SOURCE #-} GHC.Internal.IO.StdHandles ( stdout )
@@ -952,11 +949,9 @@ uncaughtExceptionHandler :: IORef (SomeException -> IO ())
 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
    where
       defaultHandler :: SomeException -> IO ()
-      defaultHandler se@(SomeException ex) = do
+      defaultHandler se = do
          (hFlush stdout) `catchAny` (\ _ -> return ())
-         let msg = case cast ex of
-               Just Deadlock -> "no threads to run:  infinite loop or deadlock?"
-               _                  -> showsPrec 0 se ""
+         let msg = displayException se
          withCString "%s" $ \cfmt ->
           withCString msg $ \cmsg ->
             errorBelch cfmt cmsg

libraries/ghc-internal/src/GHC/Internal/IO/Exception.hs

@@ -105,7 +105,8 @@ blockedIndefinitelyOnSTM = toException BlockedIndefinitelyOnSTM
 data Deadlock = Deadlock
 
 -- | @since base-4.1.0.0
-instance Exception Deadlock
+instance Exception Deadlock where
+    displayException _ = "no threads to run:  infinite loop or deadlock?"
 
 -- | @since base-4.1.0.0
 instance Show Deadlock where