*Re.: Full details here: Root cause of haddock / clang failure https://gist.github.com/mzero/d4ba11c567977111749a* Iain Nicol had submmited a set of commits back in May which addressed the "double pre-process" issue with Haddock. No one had figured out the connection to this set of issues with clang back then. I back-ported them to 1.18 last night https://github.com/haskell/cabal/pull/2010, verfied that these patches do indeed fix the issue, and Johan has committed them to the tree. My plan is to ship haskell platform with Cabal-1.18.1.4 (forthcoming), which is just these patches beyond the Cabal-1.18.1.3 that comes with GHC 7.8.3. This will just be an additional version of the Cabal package, in the package db that ships with the platform. I won't re-build 7.8.3 with a patched Cabal. I don't think there will be any major issues(*) with users getting a package db with two Cabal versions (cabal-install will be built against the later one). - Mark (*) If some proejct depends on package ghc and Cabal, then they will get 1.18.1.3 (as that is what ghc, in 7.8.3 is compiled against in the official bindist). If that project then invokes the haddock operation ... it will double pre-process input if EnableExtension CPP is in allExtensions of the project they are building. I think we're safe...