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It's not entirely clear, but I think this is a C++ only feature that is only available when using the GCC C++ front end (g++). The linked document includes the ambiguous "Support is only available in C++ for i386 targets", but other sources (such as the GCC 4.9.2 documentation) have clarified this to "With the GNU C++ front end, for target i386, you may specify multiple versions of a function".

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Function-Multiv...

indeed, my bad! to my defense, the code looks a lot like C code ;-)

That's because gcc is trying to compile it as C, not C++. You need to call the file "test.cpp" or explicitly use g++ instead of gcc to compile it. (You'll also need to "#include <assert.h>" or it won't compile.)

It only works with c++, your command failed for me, try renaming the file:

gcc test.cpp -g

or use the c++ compiler explicitly:

g++ test.c -g