Comment by WalterBright

> A good way to think about constant expressions is that `constexpr` is a hint that lets the compiler know that the variable or function CAN be evaluated at compile time, but it is not required

In D, CTFE (Compile Time Function Evaluation) is guaranteed to run at compile time. CTFE happens whenever there's a constant-expression in the grammar, such as the initialization of an enum:

    enum A = func(3);

func(3) evaluates at compile time.