[PATCH] Implement std::to_address for C++2a

[Jonathan Wakely]

On 28/11/17 12:30 -0500, Glen Fernandes wrote:
>On Tue, Nov 28, 2017 at 9:24 AM, Jonathan Wakely wrote:
>> Thanks, Glen, I've committed this to trunk, with one small change to
>> fix the copyright dates in the new test, to be just 2017.
>
>Thanks!
>
>> Because my new hobby is finding uses for if-constexpr, I think we
>> could have used the detection idiom to do it in a single overload, but
>> I don't see any reason to prefer that over your implementation:
>
>I was thinking about using if-constexpr with std::is_detected_v but
>wondered if it wouldn't be appropriate to use the latter until it
>transitions from TS to IS. (But now that you've pointed it out, I
>guess an implementation detail like __detected_or_t can live on
>forever, even if the detection idiom facilities do not get adopted).
>
>> However, more importantly, both this form and yours fails for the
>> following test case, in two ways:
>>
>> struct P {
>>  using F = void();
>>
>>  F* operator->() const noexcept { return nullptr; }
>> };
>>
>> I'm not sure if this is a bug in our std::pointer_traits, or if the
>> standard requires the specialization of std::pointer_traits<P> to be
>> ill-formed (see [pointer.traits.types] p1). We have a problem if it
>> does require it, and either need to relax the requirements on
>> pointer_traits, or we need to alter the wording for to_address so that
>> it doesn't try to use pointer_traits when the specialization would be
>> ill-formed.
>
>Could both be avoided? That is: I don't know if we need to relax it,
>or make to_address tolerate it, if the intent is to require the user
>to make P a valid pointer-like type  such that pointer_traits<P> is
>not ill-formed (by 1. providing an element_type member or 2.
>specializing pointer_traits<P>, since P is not a template<class T,
>class...> template). Current implementations of __to_address or
>__to_raw_pointer that are in use by our library facilities already
>have this requirement implicitly (those that use typename
>pointer_traits<P>::element_type* as the return type, instead of C++14
>auto), so users working with non-raw pointers would already be doing 1
>or 2.

OK, that seems reasonable. In that case I think adding a note to the
standard might be useful, to clarify that for the first overload, even
if pointer_traits<Ptr>::to_address(p) is not well-formed, the
specialization pointer_traits<Ptr> must be well-formed (because the
check for to_address(p) can trigger errors outside the immediate
context).

So my test should be changed to have element_type (or specialize
pointer_traits), like so:

#include <memory>

struct P {
  using element_type = void();

  element_type* operator->() const noexcept { return nullptr; }
};

int main()
{
  P p;
  std::to_address(p);
}

That compiles, and the bug is that is should fail the static assertion.

>> Secondly, if I remove that static_assert from <bits/ptr_traits.h> then
>> the test compiles, which is wrong, because it calls std::to_address on
>> a function pointer type. That should be ill-formed. The problem is
>> that the static_assert(!is_function_v<_Ptr>) is in std::to_address and
>> the implementation actually uses std::__to_address. So I think we want
>> the !is_function_v<_Ptr> check to be moved to the __to_address(_Ptr*)
>> overload.
>
>Ah, yes. I'll move the static_assert into that overload (enabled in
>C++2a or higher mode, since it uses is_function_v).

Great, thanks.

(Using is_function<_Tp>::value as in your original patch would allow
the assertion to be done for all modes that define __to_address)