unionBuildInfo should not always use nub
In libraries/Cabal/Distribution/PackageDescription.hs, the unionBuildInfo function combines, for example, ldOptions from two BuildInfos, using nub to eliminate duplicate arguments. This is wrong for ldOptions and some other elements. For example, if ldOptions contains:
-Xlinker -R -Xlinker /dir1 -Xlinker -R -Xlinker /dir2
Then the nubbed result is:
-Xlinker -R /dir1 /dir2
which doesn't work at all. There may be some redundancy that could be eliminated, but for arguments passed to external programs, it can only be done safely by understanding the semantics of the arguments. Similarly for extraLibs,
-lfoo -lbar -lfoo may be necessary if
libbar reference each other's symbols.
Using nub is wrong for at least cppOptions, ccOptions, ldOptions, and extraLibs. But I think it is necessary for some other BuildInfo components, like hsSourceDirs, so it can't be removed completely; that results in an error while building GHC:
Preprocessing library unix-184.108.40.206... Generating Makefile unix-220.127.116.11... Setup: makefile: can't cope with multiple hs-source-dirs yet, sorry
combine could take an extra arguments, which would be nub in some cases or id in others.