Will consteval allow using static_assert on function arguments?

Question

Currently you cannot use static_assert to verify parameters of a constexpr function, even if all calls to it are indeed constexpr. Tha

Answer 1

Will consteval allow to use static_assert on function arguments?

No. Function arguments have never been, and will continue to not be, usable as constant expressions.

There is a difference between something being constant evaluated and being usable as a constant-expression. consteval ensures that we're in a constant evaluation context, but it does not also cause everything to become constant-expressions.

In order to allow function arguments to be usable as constant expressions, you would need to make everything implicitly a template:

template <int> struct X { };

consteval auto foo(int i) {
    static_assert(i > 10); // in order to allow this...
    return X<i>{};         // ... you'd have to allow this too
}

And now foo(20) and foo(30) return different types. That's a template.

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Important background reading for understanding why this is a fundamental and inherent limitation can be found in Andrew Sutton's Translation and evaluation: A mental model for compile-time metaprogramming:

Having a mental model of compile-time evaluation that physically separates it from the process of translation has been extremely helpful for me. In particular, it has helped me understand what is not possible (e.g., instantiating a template during evaluation). This helps prune the design space for otherwise large and complex language features. Hopefully, others will find this note helpful as well.

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With static_assert specifically though, you can add a workaround just to cause a compilation failure. That's just adding anything at all that can't be used during constant evaluation. Like:

#define CONSTEVAL_STATIC_ASSERT(c, msg) do { if (!(c)) throw msg; } while(false)

as in:

consteval char operator""_bchar(const char text[], const size_t length)
{
    CONSTEVAL_STATIC_ASSERT(length == 8, "Binary char has to have 8 digits!");
    // ...
}