title: Is it legal to use #elif with #ifdef?
body: <p>A simple question that Google doesn't help me with. Is it legal in C++ to use <code>#elif</code> clause in the context of <code>#ifdef</code>? It seems to compile and work as expected with all the major compilers in the c++11 mode (MSVC 2015/2017, clang, GCC), but I'm not certain whether it is standard-compliant.</p>