On Thu, 21 May 2026 at 14:38, Mital Ashok via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Thu, 21 May 2026 at 16:02, Andrey Semashev via Std-Proposals wrote:
> > > This is useful in ternary conditional expressions, like `o.is_valid()
> > > ? *o : std::unreachable<O::value_type>()`.
> > How is this different from just `*o`?
>
> std::unreachable is mainly for optimisations. In case of the specific
> pattern `B ? E : std::unreachable<T>()`, it is similar to
> `[[assume(B)]]; E`, but it doesn't require a separate statement. For
> example, if `*o` had a check for is_value, it can be optimised out.
>

But [[assume(o.has_value())]] is clearer and more expressive.


>
> Another use case is something like `x < 0 ? f() : x < 5 ? g() : x < 10
> ? h() : std::unreachable<int>()`, which is partially documenting
> (often in this case the last conditional is just omitted),


Right, so as Andrey said, you just wouldn't write the last conditional
operator.


> but may
> have some performance implications if `h()` has a check for x < 10
> that the compiler fails to otherwise prove


[[assume(h < 10)]] before the complicated conditional expressions seems to
work nicely here too.