Compile-time Programming

What can the compiler do for us before the program runs? We watch a constexpr function get evaluated during the build, let if constexpr throw away the branch a type can't use, ask type traits questions about types, and use one to choose memcpy or an element loop, measuring the real difference on a Mac with and without optimisation. We finish with static_assert and C++20's constinit, which make the compiler check our assumptions.