`GHC.Utils.TmpFs.newTempDir` creates a new `ghc_N` subdirectory in a temp directory, but this directory cannot be removed. The result is a warning at the end of the GHC run. It's not possible to clean this `ghc_N` subdirectory using `addFilesToClean`: - `addFilesToClean` uses `removeFile` and fails for directories - the to-clean list is kept in a `Set FilePath` and doesn't preserve the order in which the files were added, so a dir could potentially be removed before the files contained in it. I've come up with two ideas to fix it - when removing a top-level ghc temp dir, automatically remove empty `ghc_N` subdirs inside it (in `cleanTempDirs`) - keep track of all created `ghc_N` by `newTempDir` and remove them as the first step in `cleanTempDirs` Any preferences or other ideas?