-Werror has unintended consequenses in a release. Say you are using transformers/mtl and you want to stay backwards compatible, you can't use Except since it's just in the new versions, but with -Werror you couldn't use Error since it would produce deprecation warnings. CPP isn't an option since that would change the implementation. I don't like the idea of disabling deprecation warninings since that setting has at least module level granularity, if other deprecations pop up I want to see those! By enabling -Werror you are turning a non breaking change (deprecation) into a breaking one and not even an upper bound of <= A.B.C.D would be safe since the number of components are arbitrary.
The granularity of disabled warnings is part of the problem, it'd be really nice if you could disable deprecation warnings for specific identifiers, same with a lot of the other warnings.
- Adam