Re: [v3 PATCH] Implement LWG 2825, LWG 2756 breaks class template argument deduction for optional.

[Ville Voutilainen <ville dot voutilainen at gmail dot com>]

On 31 January 2017 at 00:41, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
> On 31 January 2017 at 00:06, Tim Song <t.canens.cpp@gmail.com> wrote:
>> On Mon, Jan 30, 2017 at 9:36 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>>>
>>> On 30/01/17 13:28 +0000, Jonathan Wakely wrote:
>>> >On 30/01/17 13:47 +0200, Ville Voutilainen wrote:
>>> >>Tested on Linux-x64.
>>> >
>>> >OK, thanks.
>>>
>>> To be clear: this isn't approved by LWG yet, but I think we can be a
>>> bit adventurous with deduction guides and add them for experimental
>>> C++17 features. Getting more usage experience before we standardise
>>> these things will be good, and deduction guides are very new and
>>> untried. If we find problems we can remove them again, and will have
>>> invaluable feedback for the standards committee.
>>>
>>
>> My brain compiler says that this may cause problems with
>>
>> std::optional<int> o1;
>> std::optional o2 = o1; // wanted optional<int>, deduced optional<optional<int>>
>>
>> Trunk GCC deduces optional<int>, but I don't think it implements
>> P0512R0 yet, which prefers explicit guides to implicit ones before
>> considering partial ordering. This example is very similar to the
>> example in https://timsong-cpp.github.io/cppwp/over.match.best#1.6.
>
>
> I'll see about constraining the guide tomorrow.

I don't actually need to constrain it, I could just add a guide like

template <typename _Tp> optional(optional<_Tp>) -> optional<_Tp>;

However, I'm not convinced I need to. The preference to an explicit
guide is, at least based
on that paper, a tie-breaker rule. If the copy/move constructors are
better matches than the guide,
those should be picked over a guide. Jason?