[Bug 213217] [patch] Passing -isystem <sysroot>/usr/include to clang breaks C++ compilation

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213217

Dimitry Andric <dim at FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dim at FreeBSD.org
             Status|New                         |Closed
         Resolution|---                         |Rejected

--- Comment #2 from Dimitry Andric <dim at FreeBSD.org> ---
Short answer: Don't use -isystem /usr/include.

If you do so for C++ programs, you mess up the include search path order.  If
you must do it for some reason, you must also add -isystem entries for the C++
include directories, and at the front of the list.

For example, a C++ program will use the following search path by default (where
x.y.z is the clang version):

#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/v1
 /usr/bin/../lib/clang/x.y.z/include
 /usr/include
End of search list.

>From libc++ 3.8.0 onwards, if you include a C standard header, such as
<cstddef>, you will get libc++'s wrapper header first.  This header sets up a
few things, then does #include_next<stddef.h>, and with the above search path,
this finds /usr/include/stddef.h.  (We don't install clang's internal stddef.h,
since it is not compatible with our system headers yet.)

However, if you add -isystem /usr/include, you force /usr/include to be the
first in the list, e.g. the search path will become:

ignoring duplicate directory "/usr/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include
 /usr/include/c++/v1
 /usr/bin/../lib/clang/x.y.z/include
End of search list.

If you now include <cstddef>, and it eventually does #include_next<stddef.h>,
it will attempt to search the paths *after* /usr/include/c++/v1, and will not
be able to find the header.

Summary: If for some reason you must completely rebuild the header search path
from scratch, you need to add  -isystem /usr/include/c++/v1 *before* -isystem
/usr/include.  But it is better not to do this at all. :)

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