https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to chuck cranor from comment #3) > I think you'll find most build systems that do "-isystem /usr/include" > instead of "-I /usr/include" are only using "-isystem" for the change > in the warning behavior. The change in the include path order is not > wanted... Then they should stop (mis)using -isystem, since it's clearly documented to affect the order directories are searched: If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the "#include_next" directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. The corollary is that you shouldn't use it unless you really need to change the search order for system directories!
