Fay is currently experimenting with replacing the base package with a fay-base package, but this is leading to build failures in fay-jquery which exposes its Paths_fay_jquery module. Cabal tries to build that module with the same flags as the other modules, but it's importing lots of modules from the base package that fay-base isn't exporting.
Besides Fay, this is not the first time a problem has appeared due to Cabal building this generated module with the same flags as the library; these bugs are all a result of this: #907 and #1000, #994, #1137.
The module itself is written with assumptions about how the .cabal file is written, assumptions that quickly break down in different setups.
👍 I just ran into this problem by combining OverloadedStrings and RebindableSyntax. See this Gist for a small, complete example.