#define FORCE_CONSTEXPR(expr) [&]() \
    { constexpr auto x = (expr); return x; }()

Wrap any expression in FORCE_CONSTEXPR(...) to force the compiler to evaluate it completely at compile time (or fail the compilation if it cannot do so).

This macro has a significant effect at -O0 and -O1. At -O2 or higher, compilers seem good enough to do the constexpr evaluation on their own (essentially as an emergent effect of the inliner, I think).

If you were — God knows why — doing this for real, you’d need a separate macro to handle the global scope:

#define GFORCE_CONSTEXPR(expr) []() \
    { constexpr auto x = (expr); return x; }()

int global = GFORCE_CONSTEXPR(constexpr_sqrt(42.0));

Because Clang (alone out of the Big Three compilers) does not support lambdas with capture-defaults at global scope!