On Mon, Mar 23, 2026 at 9:52 PM Jonathan Wakely <cxx_at_[hidden]> wrote:
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> On Tue, 24 Mar 2026, 01:19 Halalaluyafail3, <luigighiron_at_[hidden]> wrote:
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>> On Mon, Mar 23, 2026 at 9:06 PM Jonathan Wakely <cxx_at_[hidden]> wrote:
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>> > On Tue, 24 Mar 2026, 00:34 Halalaluyafail3 via Std-Proposals, <std-proposals_at_[hidden]> wrote:
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>> >> 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:
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>> >> #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.
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>> > 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.
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>> > In an uninstantiated template the condition is not necessarily checked.
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>> 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?
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> Do you really think trying to draw general conditions from the wording for those two findings makes sense?
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> 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.
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> Maybe they're special cases, which are not good examples of general purpose wording.

"Mandates: the conditions that, if not met, render the program ill-formed." from
my reading does require odr-use for the program to be ill-formed.

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>> >> 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.
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>> > Why?
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>> 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.
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>> >> 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?
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>> > 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?
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>> Not to say that requiring this would be useful, but it seems like intent is not
>> currently reflected in how Mandates is specified.
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>> >> 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?
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>> > I don't see a defect here.
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>> >