If you’re dealing with errors like this compiling a C++ project that uses CMake:

/QOpenSys/pkgs/lib/gcc/powerpc-ibm-aix6.1.0.0/10/include/c++/bits/memoryfwd.h:63:3: error: template with C linkage
   63 |   template<typename>
      |   ^~~~~~~~

…it’s because CMake is passing includes with -isystem instead of -I. On some platforms (i.e. AIX), GCC will assume any header that comes from -isystem is C only (as it may be some funny vendor C++ dialect), and will automatically implicitly wrap them in extern "C", which is why you see the error in spite of not seeing extern "C" anywhere in the source.

For CMake, the easiest way to fix thisis to remove SYSTEM from usages of target_include_directories. However, this doesn’t seem to guarantee it; I’ve had things marked with just INTERFACE end up using -isystem. In that case, then you can add this property, and anything that comes after will always use -I:

set(CMAKE_NO_SYSTEM_FROM_IMPORTED TRUE)

Note that it may also be possible to rebuild GCC to remove this assumption as well.