Here's a reduced test case:

cpp.hs

{-# LANGUAGE CPP #-}

#include "Typeable.h"

main = return ()

command line

$ ghc-stage2 -c -package base cpp.hs

In file included from cpp.hs:4:0:

/home/patrick/code/ghc/libraries/base/include/Typeable.h:17:2:
     warning: #warning <Typeable.h> is obsolete and will be removed in GHC 7.10 [-Wcpp]
     #warning <Typeable.h> is obsolete and will be removed in GHC 7.10
      ^
compilation IS NOT required
$ ghc-stage2 -c -package base -package containers cpp.hs
compilation IS NOT required
$ ghc-stage2 -c -package containers -package base cpp.hs

Notice that if I pass -package containers to ghc, the cpp warning from Typeable.h (from the base library) doesn't appear. This is because containers also has a Typeable.h in its include path, and in the invocation of the preprocessor, containers' include path precedes base's no matter how I order the -package directives.

This behavior is unintuitive and limiting. To fix this, I think that the ordering of -I directives passed to the preprocessor should be consistent with the ordering of -package directives passed to ghc. For example, in the above test case, a warning should be shown in the 1st and 2nd invocations of ghc but not the 3rd, because in the 3rd invocation containers precedes base.

Does this sound okay?