[cmake-developers] [CMake 0015970]: CMake changes order of include directories when a directory is specified both as a SYSTEM and non-SYSTEM include

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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https://cmake.org/Bug/view.php?id=15970 
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Reported By:                jbohren
Assigned To:                
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Project:                    CMake
Issue ID:                   15970
Category:                   CMake
Reproducibility:            always
Severity:                   major
Priority:                   normal
Status:                     new
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Date Submitted:             2016-02-11 13:44 EST
Last Modified:              2016-02-11 13:44 EST
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Summary:                    CMake changes order of include directories when a
directory is specified both as a SYSTEM and non-SYSTEM include
Description: 
I ran into a problem recently where a package I was building set a SYSTEM
include directory which was already a non-SYSTEM include directory imported from
a dependency. When this happened, CMake dropped the non-SYSTEM directory in
favor of the SYSTEM one, causing my build to fail.

According to GCC [1], it gives precedence to non-SYSTEM include directories, and
specifying both for the same path produces a warning.

[1] https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html
> All directories named by -isystem are searched after all directories named by
-I, no matter what their order was on the command line. If the same directory is
named by both -I and -isystem, the -I option is ignored. GCC provides an
informative message when this occurs if -v is used.

With CMake, if both are specified, it drops the non-SYSTEM include and fails
silently. For example, when building this project with any recent version of
CMake on Linux, the build command is:

```bash
/usr/bin/c++  -isystem /path/to/overlay/devel/include  -I/opt/ros/indigo/include
 -o CMakeFiles/foo.dir/foo.cpp.o -c /home/jbohren/scratch/isystem/foo.cpp
```

But with the three `incude_directory` options given in `CMakeLists.txt`, I would
expect it to read:

```bash
/usr/bin/c++  -I/path/to/overlay/devel/include  -I/opt/ros/indigo/include 
-isystem /path/to/overlay/devel/include  -o CMakeFiles/foo.dir/foo.cpp.o -c
/home/jbohren/scratch/isystem/foo.cpp
```

Even the following would maintain the order of the search paths, since the
non-SYSTEM directories are all scanned before the SYSTEM ones:


```bash
/usr/bin/c++  -I/path/to/overlay/devel/include  -I/opt/ros/indigo/include  -o
CMakeFiles/foo.dir/foo.cpp.o -c /home/jbohren/scratch/isystem/foo.cpp
```


Steps to Reproduce: 
Build this trivial CMake project to observe the effect:
https://github.com/jbohren/isystem
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Issue History 
Date Modified    Username       Field                    Change               
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2016-02-11 13:44 jbohren        New Issue                                    
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