Inspecting the implementations of `hIsReadable` and `hIsWritable` in `ghc-internal-9.1201.0`, I found that these implementations are incorrect for duplex handles. Both use a shortcut for the `DuplexHandle` case: they just return `True` for duplex handles. This seems plausible at first sight, given that duplex handles always have a reading and a writing part and thus should be usable for reading and writing. However, this shortcut misses the check for whether the handle is closed or semi-closed, which is performed in the `FileHandle` case. I propose to let `hIsReadable` and `hIsWritable` treat duplex handles similarly to file handles. Beware, however, that the solution is not to simply drop the special treatment of duplex handles, at least not for `hIsWritable`, because the `withHandle_` operation, currently used for the `FileHandle` case, always operates on the reading part of a duplex handle.