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Try this program:

module Main where

import System.IO

main = do
    roundtrip_enc <- mkTextEncoding "UTF16//ROUNDTRIP"
    h <- openFile "out.temp" WriteMode
    hSetEncoding h roundtrip_enc
    hPutStr h "Hi\xEFE8Hi"

It fails with:

hSetEncoding: invalid argument (Invalid argument)

If you change UTF16 to UTF-16 (so we use the builtin encoding rather than iconv) it works, but the output file only contains the first Hi.

I think part of what is going on here is that iconv does not generate EILSEQ for identity transformations such as that between a UTF-16 text file and our UTF-16 CharBuffers. Since we never get that exception, we can't fix up the lone surrogates we use to encode roundtrip characters.