The hSetNewlineMode and hSetEncoding functions from GHC/IO/Handle.hs do not check that the Handle is in an open mode. It is possible to use them on closed handles. hSetEncoding on a closed Handle triggers a segfault. Similarly, the operations are also both possible on semi-closed handles, and given the way hGetContents is implemented, this will affect the result of hGetContents which is clearly against the intention of the hGetContents/semi-closed stuff.

Both functions use the withAllHandles__ helper. Unlike similar helpers like wantReadableHandle_ this one doesn't do any handle mode checking.

Additionally, hSetBuffering and hSetBinaryMode mode also use the withAllHandles__ pattern and don't obviously check for an open handle but I've not verified this.