On 21/05/2026 13:32, Mital Ashok via Std-Proposals wrote:
> This should be a non-breaking extension.
> Add a templated function:
>
> template<class T>
> [[noreturn]] T unreachable() {
> unreachable();
> }
>
> This is useful in ternary conditional expressions, like `o.is_valid()
> ? *o : std::unreachable<O::value_type>()`.
>
> It can also be used in some weird cases where you want to ODR-use
> std::declval in a constant expression (e.g., when used as an unknown
> reference)
>
> T is returned instead of add_rvalue_reference_t<T> intentionally
> because of the use case in conditional expressions where we don't want
> to materialise a temporary the other side (so std::unreachable<int>()
> is a prvalue for example). This deviates from std::declval but has
> similar semantics to std::reference_con{struct,vert}s_from_temporary
> (T is prvalue, T& is lvalue, T&& is xvalue), with the unfortunate
> exclusion of array types, but that should not be a big use case.
>
> The above is one of the things that can be discussed about this
> facility. I was mostly wondering if there is anything else I need to
> consider before writing R0 of the proposal

Are you using this to reduce the binary size on an embedded platform?

I don't use __builtin_unreachable(), but I would if the compiler's control-flow analysis would warn that actually it is reachable! That would be really useful, could be maybe used by P4021 compile_assert(). Sharing what each vendor's compiler knows is very helpful.

myfunc(2);


int myfunc(int x)
{
    if (x == 0)
        return 1;

    __builtin_unreachable();
}