Constant expressions are easily the part of C I understand the least (or at least, my biggest "known unknown"), so forgive my ignorance.

Should I be declaring everything constexpr? I was recently writing some file format handling code and when it came time to write a const variable to hold some magic numbers I wasn't sure if there was any downside to doing this vs using static const or extern const. I understand a couple of const globals is not a make or break thing, but as a rule of thumb?

There are a million blog posts about "you can do this neat thing with constexpr" but few or none that explore their shortcomings. Do they have any?