Why hadrian/build --flavour=validate enables -Werror for out-of-tree libraries.

It forces people to do stuff when GHC needs them, and then it might even not need them.

parsec uses head. The GHC introduced warning on that. I would liked to selectively disable that, but that feature (selectively disabling warnings) is started to work on. So now, as a maintainer of parsec I need to make compromises. (I was perfectly fine with the single head usage).

Please, don't enable -Werror on out-of-tree libraries. It's good that new warnings are introduced, but it's up to maintainers which warnings they care about.

AND WORSE -Wdeprecations is always on, even without -Wall. That's very annoying if {-# WARNING #-}s are added to something widely used.