As of commit b9c1aa925, trying to build git-annex on either Ubuntu or macOS fails with:

Annex/TransferrerPool.hs:56:24: error:
    Not in scope: type constructor or class `MonadFail'
   |         :: (MonadIO m, MonadFail m, MonadMask m)
   |                        ^^^^^^^^^

This can be fixed with the following patch:

diff --git a/Annex/TransferrerPool.hs b/Annex/TransferrerPool.hs
index 973f75629..0de145461 100644
--- a/Annex/TransferrerPool.hs
+++ b/Annex/TransferrerPool.hs
@@ -28,6 +28,7 @@ import Utility.ThreadScheduler
 import Control.Concurrent
 import Control.Concurrent.Async
 import Control.Concurrent.STM hiding (check)
+import Control.Monad.Fail (MonadFail)
 import Control.Monad.IO.Class (MonadIO)
 import Text.Read (readMaybe)
 import Data.Time.Clock.POSIX

fixed --Joey