`writeCharArray#` behaves different than `writeWord8Array#` under -fcheck-prim-bounds

As discussed on IRC, there seems to be an issue with bounds checking in this code:

instance Read ShortByteString where
    readsPrec p str = [ (packChars x, y) | (x, y) <- readsPrec p str ]

instance IsString ShortByteString where
    fromString = packChars

packChars :: [Char] -> ShortByteString
packChars cs = packLenChars (List.length cs) cs

packLenChars :: Int -> [Char] -> ShortByteString
packLenChars len cs0 =
    create len (\mba -> go mba 0 cs0)
  where
    go :: MBA s -> Int -> [Char] -> ST s ()
    go !_   !_ []     = return ()
    go !mba !i (c:cs) = do
      writeCharArray mba i c
      go mba (i+1) cs

writeCharArray :: MBA s -> Int -> Char -> ST s ()
writeCharArray (MBA# mba#) (I# i#) (C# c#) =
  ST $ \s -> case writeCharArray# mba# i# c# s of
               s -> (# s, () #)

The documentation of writeCharArray# indicates that it truncates correctly.

The following patch makes the bounds checking error go away:

--- a/Data/ByteString/Short/Internal.hs
+++ b/Data/ByteString/Short/Internal.hs
@@ -442,7 +444,7 @@ packLenChars len cs0 =
     go :: MBA s -> Int -> [Char] -> ST s ()
     go !_   !_ []     = return ()
     go !mba !i (c:cs) = do
-      writeCharArray mba i c
+      writeWord8Array mba i (BS.c2w c)
       go mba (i+1) cs

There was never any test failure with both methods.

---

Repro test case:

1. clone https://github.com/hasufell/bytestring/tree/bounds-checking-repro
2. run cabal run --ghc-options='-fcheck-prim-bounds -fno-ignore-asserts' bytestring-tests with the appropriate GHC supporting bounds checking
3. see it fail
4. revert 6511781751228526b81188971c8baf316f3ad8dc
5. run the test again