C++ - const vs constexpr vs consteval vs constinit

TL;DR

const:
- runtime constant.
- can only be used for non-static member functions, not functions in general.
constexpr: Since C++11
- MAY be evaluated at compile-time, but may be at runtime.
  - A constexpr function can also be called at runtime like a regular function if its arguments are not constant expressions or if the result is not needed in a context that requires a constant expression.
- can be used for both variables and functions.
consteval functions: Since C++20
- a.k.a. immediate functions;
- MUST be evaluated at compile-time: always produce a compile-time expression and always visible only at compile-time.
- consteval can only be applied to the declaration of a function or function template.
constinit: Since C++20,
- asserts that a variable has static initialization (zero initialization or constant initialization).
- when the declared variable is a reference, constinit is equivalent to constexpr.
- purpose: to prevent the "static initialization fiasco," where the order of initialization of static variables across translation units is undefined.