Travis doesn't *really* test cabal builds.

[Blaisorblade]

Or rather, it tests them but ignores the errors. See the failures here:
https://travis-ci.org/commercialhaskell/stack/jobs/149459127
https://travis-ci.org/commercialhaskell/stack/jobs/149736106#L435

Those ones are just harmless warnings turned into errors by -Werror.

That's because the travis script composes commands with ; instead of && in the script section. One could fix that also with set -e.

However, the stack build also gives similar warnings, but without -Werror:
https://travis-ci.org/commercialhaskell/stack/jobs/149459130

That's because of cabal configure --enable-tests --enable-benchmarks -v2 --ghc-options="-O0 -Werror" in .travis.yml picks -Werror for cabal, but not for Travis.
My plan:

• add -Werror to stack builds
• fix the build script to propagate failures
• create an issue about those warnings and/or investigating them myself. In particular, the fix might not need conditional compilation: the warning did not exist before 8.0, so hopefully the constraints are also redundant on previous versions.