[dig1]

This makes sense for straightforward tests, but static_assert is not all you need in general, because some things has be executed in runtime, after series of steps or after some timeframe. Good luck reproducing or testing these in compile-time.

[mr_00ff00]

Could you not put a series of steps or something that mocks time into constexpr? A comment mentioned above that in C++20, almost all features are available at compile time now.

[ovao]

With few limitations, yes. In C++20, you could for example test for constant evaluation[1], using that as a mechanism to fall back to a real clock during runtime.

[1]: https://en.cppreference.com/w/cpp/types/is_constant_evaluate...