Text builder instance writes to wrong offset

[goertzenator]

Consider the following program:

import Data.Double.Conversion.Convertable
import Data.Text
import qualified Data.Text.Lazy.Builder as TLB

main :: IO ()
main = do
  let
    x1 :: Text = "I have " <> toExponential 2 (3.14 :: Double) <> " cakes."
    x2 :: TLB.Builder = "I have " <> toExponential 2 (3.14 :: Double) <> " cakes."
  putStrLn $ show x1
  putStrLn $ show x2

The output is ...

"I have 3.14e0 cakes."
"3.14e0 \NUL\NUL\NUL\NUL\NUL\NUL cakes."

The Text instance of Convertable produces correct output, but the Builder instance does not. In some other tests I did the Builder output always gets written to the beginning instead of the correct location.

[goertzenator]

The issue might be here: https://github.com/haskell/double-conversion/blob/master/Data/Double/Conversion/Internal/TextBuilder.hs#L51

-  return $ writeN (fromIntegral size) $ \mArr _ -> A.copyI (fromIntegral size) mArr 0 tempArr 0
+  return $ writeN (fromIntegral size) $ \mArr offset -> A.copyI (fromIntegral size) mArr offset tempArr 0