The difference between constexpr and C++20 consteval:

• constexpr: Function can be evaluated at compile-time if arguments are constant expressions, but can also run at runtime if given runtime variables.
• consteval (Immediate Function): Function must evaluate at compile-time. If called with runtime variables, the compiler produces a hard compilation error!

consteval int ComputeLookupTable(int index) {
    return index * index + 37;
}
constexpr int val = ComputeLookupTable(5); // Evaluated directly in compiler frontend!

consteval is incredible for precomputing mathematical lookup tables, sine/cosine waves, and cryptographic constants during compilation.

Results in zero runtime initialization cost when the executable boots.