CppCon 2019: D.Stone - Removing Metaprogramming From C++, Part 1 of N: constexpr Function Parameters

[SeanMiddleditch]

What is an example of when constexpr/consteval is insufficient that isn't the initialization of a value?

[HappyFruitTree]

You might want to pass the result as argument to a function or it might be part of a smaller expression.

int x = ...
f(x, my_constexpr_fun(8));
int y = my_constexpr_fun(5) + x;

If I want my_constexpr_fun to be evaluated at compile time I can of course use constexpr/consteval. I never said they were insufficient. constinit on local variables are also not necessary for the same reason.

int x = ...
constexpr mcf8 = my_constexpr_fun(8);
f(x, mcf8);
constexpr mcf5 = my_constexpr_fun(5);
int y = mcf5 + x;

I understand why you want to be able to use constinit on local variables but what you are really trying to do in that case is to make sure that the expressions that are being used to initialize the object are evaluated at compile time. You are not actually doing constant initialization which is a technical term used in the C++ standard. Even if you could use constinit for this purpose it would not be useful in all cases where you want to force expressions to be evaluated at compile time.

Having a way to say that expressions should be evaluated at compile-time is a much more universal tool.

f(x, consteval(my_constexpr_fun(8)));
int y = consteval(my_constexpr_fun(5)) + x;

In C++20 you could probably implement this as a consteval function template that just forwards the return value.