warnOnTwoConfigs is overzealous on Windows

[Bodigrim]

cabal/cabal-install/src/Distribution/Client/Config.hs

Lines 729 to 744 in c4e1268

	-- | Issue a warning if both @$XDG_CONFIG_HOME/cabal/config@ and
	-- @~/.cabal@ exists.
	warnOnTwoConfigs :: Verbosity -> IO ()
	warnOnTwoConfigs verbosity = do
	defaultDir <- getAppUserDataDirectory "cabal"
	dotCabalExists <- doesDirectoryExist defaultDir
	xdgCfg <- getXdgDirectory XdgConfig ("cabal" </> "config")
	xdgCfgExists <- doesFileExist xdgCfg
	when (dotCabalExists && xdgCfgExists) $
	warn verbosity $
	"Both "
	<> defaultDir
	<> " and "
	<> xdgCfg
	<> " exist - ignoring the former.\n"
	<> "It is advisable to remove one of them. In that case, we will use the remaining one by default (unless '$CABAL_DIR' is explicitly set)."

This is wrong on Windows:

• getAppUserDataDirectory "cabal" returns C:/Users/<user>/AppData/Roaming/cabal.
• getXdgDirectory XdgConfig ("cabal" </> "config"), unless XDG environment variables are set (and they are not by default), returns C:/Users/<user>/AppData/Roaming/cabal/config, in the very same folder.

Thus both paths exist and everyone is shown a very confusing warning:

Both C:/Users/<user>/AppData/Roaming/cabal 
and C:/Users/<user>/AppData/Roaming/cabal/config exist - ignoring the former.
It is advisable to remove one of them. 
In that case, we will use the remaining one by default (unless '$CABAL_DIR' is explicitly set)."

I cannot remove one of them, they are the same location.

This is easy to fix, but I'm unpleasantly surprised that I'm the first person to test Cabal HEAD on Windows. If I may suggest, it would be nice to review that XDG business works outside of Linux paradise.

[fgaz]

Looks like this also affects 3.10 :-/ #8971

[Mikolaj]

This is easy to fix, but I'm unpleasantly surprised that I'm the first person to test Cabal HEAD on Windows. If I may suggest, it would be nice to review that XDG business works outside of Linux paradise.

Yes, we are short on the precious resource of Windows developers and testers. We'll ask especially for WIndows testing once we announce unofficial 3.10.2.0 alpha binaries.