On Tue, 24 Mar 2026, 01:19 Halalaluyafail3, <luigighiron_at_[hidden]> wrote:

> On Mon, Mar 23, 2026 at 9:06 PM Jonathan Wakely <cxx_at_[hidden]> wrote:
> >
> >
> >
> > On Tue, 24 Mar 2026, 00:34 Halalaluyafail3 via Std-Proposals, <
> std-proposals_at_[hidden]> wrote:
> >>
> >> N5032 Section 16.3.2.4 "Detailed specifications"
> [structure.specifications]
> >> Paragraph 3:
> >> > Mandates: the conditions that, if not met, render the program
> ill-formed.
> >>
> >> Consider the following program:
> >>
> >> #include<bit>
> >> decltype(std::bit_cast<void*>(^^::))x;
> >>
> >> Assume sizeof(void*)==sizeof(^^::) is true. Is this program ill-formed?
> It would
> >> seem that is the case, but existing implementations do not reject this.
> >
> >
> > You haven't instantiated the function template. The return type is
> clearly void* so checking it without odr-using the function might works,
> but is kinda dumb.
> >
> > In an uninstantiated template the condition is not necessarily checked.
>
> I do not see how the current wording supports such an interpretation. Note
> that
> std::declval is specified using the mandate "This function is not
> odr-used", is
> this redundant and could be reduced to "false is true" similar to the
> preconditions of std::unreachable?
>

Do you really think trying to draw general conditions from the wording for
those two findings makes sense?

Those tell functions cannot be evaluated ever in a valid program. There are
no specializations of declval that are usable, ever. And any call to
unreachable is a bug.

Maybe they're special cases, which are not good examples of general purpose
wording.





> >
> >> The
> >> example that follows the quoted text suggests that two possible
> implementations
> >> of mandates are static_asserts and deleted overloads. For uses of
> mandates
> >> outside of templates it seems like deleting the function is the only
> way to
> >> implement the mandate.
> >
> >
> > Why?
>
> Since a static_assert failing there would immediately make the program
> ill-formed. The only way to delay that until its use would be =delete.
>
> >
> >> For uses of mandates inside of templates both options
> >> would be valid, and create differences in the validity of programs such
> the
> >> program above. Was it intended for both possibilities to be valid, or
> was it
> >> intended for this program to assuredly be ill-formed or well-formed?
> >
> >
> > I don't think it matters. If it mattered and the condition was supposed
> to be SFINAE testable, it would use Constraints not Mandates.
> >
> > Why would you ever need the code above to be portable?
>
> Not to say that requiring this would be useful, but it seems like intent
> is not
> currently reflected in how Mandates is specified.
>
> >
> >
> >>
> >> It seems like the uses of Mandates for the purposes of specifying
> deletion could
> >> be split up into a new "Deleted When" element, if the intent was to
> require one
> >> form be used. For example text_encoding uses mandates to require
> CHAR_BIT==8,
> >> that could realistically only be implemented as a deleted overload.
> Would it
> >> make sense to open a LWG issue for this?
> >
> >
> > I don't see a defect here.
> >
> >
> >
>