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Date:      Wed, 05 Oct 2016 10:43:11 +0000
From:      bugzilla-noreply@freebsd.org
To:        freebsd-toolchain@FreeBSD.org
Subject:   [Bug 213217] [patch] Passing -isystem <sysroot>/usr/include to clang breaks C++ compilation
Message-ID:  <bug-213217-29464-Sxp3qMIZw4@https.bugs.freebsd.org/bugzilla/>
In-Reply-To: <bug-213217-29464@https.bugs.freebsd.org/bugzilla/>
References:  <bug-213217-29464@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D213217

Dimitry Andric <dim@FreeBSD.org> changed:

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

--- Comment #2 from Dimitry Andric <dim@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 (w=
here
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 pa=
th,
this finds /usr/include/stddef.h.  (We don't install clang's internal stdde=
f.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 n=
ot
be able to find the header.

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

--=20
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