[Bug c++/100956] Unused variable warnings ignore "if constexpr" blocks where variables are conditionally used

[mattreecebentley at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100956

--- Comment #2 from Matt Bentley <mattreecebentley at gmail dot com> ---
Thank you - I'm aware GCC might optimize it out (and failed to test with
GCC10), at least in O2 mode, but other compilers might not, hence the code.