See these issues: #3549 haskell/cabal#4863

Released Cabal has a bug that makes it so that showGenericPackageDescription drops quite a few fields. Should we build stack-1.6 against a version of the Cabal library which has a fix? If we do that, should modify the warning message I added here: 9f06ccf

Noted by phadej here:

By

Any way of pick, you have to make sure to reject cabal-version: 2.1 and later, as their spec is open atm.
I meant you should reject .cabal files with too new Cabal spec version, there are features in master which may not be in their final form.