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.
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>> Consider the following program:
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>> #include<bit>
>> decltype(std::bit_cast<void*>(^^::))x;
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>> 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.

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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>> 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?

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.
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> 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.

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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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