I expect the first few years of this being used will almost exclusively consist of people replacing instances of it with `#elif defined FOO` for use with older

I expect the first few years of this being used will almost exclusively consist of people replacing instances of it with `#elif defined FOO` for use with older compilers or in code (e.g. headers) that also needs to be compilable with a C++ compiler. Sure, in isolation it's more orthogonal. But it's hard to see this as a real value add when amortised against the compatibility headache, when the arguably cleaner alternative of `#if defined FOO ... #elif defined BAR ... #endif` is already available. (Incidentally (1) you can choose to also spell out `#ifdef FOO` as `#if defined FOO` when it's one of a list of alternatives like this, so they all look similar; (2) there's no need for excess parentheses with `#if defined` either.)