Sample project to demonstrate issues ignoring warnings in clang module headers

Stackoverflow question:

http://stackoverflow.com/questions/38889998/how-to-ignore-warnings-in-headers-that-are-part-of-a-clang-c-module

Issues:

• Pragma information does not seem to be preserved in clang's internal/precompiled representation of a module header
• -Wno-system-headers does not inhibit warnings from system headers that have been pulled in as precompiled modules
• Using the [system] module attribute will suppress warnings even when including the header via a "user" style include, e.g. when building the library sources themselves, which means warnings we want to see will also be suppressed.

Building

mkdir build && cd build
cmake /path/to/ClangModuleWarnings
make

You should see two errors:

In module 'mylib' imported from /path/to/mylib/mylib.cpp:2:
/path/to/mylib/include/mylib/mylib.h:14:16: error: comparing floating point with == or != is unsafe
      [-Werror,-Wfloat-equal]
    return lhs == rhs;
           ~~~ ^  ~~~

This error should have been ignored via the pragmas, since mylib.cpp is including the header as a non-system "user" header and -Wno-system-headers will not take effect.

and:

In module 'mylib' imported from /path/to/main.cpp:4:
/path/to/mylib/include/mylib/mylib.h:14:16: error: comparing floating point with == or != is unsafe
      [-Werror,-Wfloat-equal]
    return lhs == rhs;
           ~~~ ^  ~~~

This error should have been ignored by -Wno-system-header since the header is included as a system header via the CMake include_directories path which specifies the SYSTEM option which causes it to be added via the -isystem argument.

Commenting out the CMakeLists.txt lines that enable clang modules fixes both these errors.

Note there is a warning in mylib.h that we want to see when buliding mylib.cpp, since it includes the header via a "user" style include, but not when clients import it via a "system" style include:

While building module 'mylib' imported from /path/to/mylib/mylib.cpp:2:
In file included from <module-includes>:1:
/path/to/mylib/include/mylib/mylib.h:23:6: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
    *i = 123;

This is suppressed if we add the [system] attribute to the mylib module, which we don't want.

Be careful: you will want to clear your module cache after any tweaks or changes you make otherwise you may end up getting incorrectly (un)successful builds due to stale caches.