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

[Glen Fernandes <glen dot fernandes at gmail dot com>]

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.

> 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).

Glen