Re: [PATCH] PR77528 add default constructors for container adaptors

[Tim Song <t dot canens dot cpp at gmail dot com>]

On Wed, Jan 11, 2017 at 8:30 AM, Jonathan Wakely <jwakely@redhat.com> wrote:

>>> Re the new DMI, my brain compiler says that _Sequence c = _Sequence();
>>> breaks anything with an explicit copy/move constructor pre-C++17, but
>>> I also don't think we care about those, right?
>>
>>
>> I dislike them,
>
>
> I meant to add "but we try to support them where plausible".
>
> If the standard requires CopyConstructible then we don't need to care
> about explicit copy constructors. But if it only requires
> is_copy_constructible then that does work with explicit copy ctors.
> And if it says one thing, but means the other, then we just have to
> guess what's intended! :-)
>
>

Clause 23 is ... not a model of clarity. It depends on how strongly
you read the "any sequence container" phrasing, I suppose. Table 83
requires X u = a; to work for containers, but it also requires a == b
to work.

There's also the problem of Compare (which I don't see any requirement
w/r/t CopyConstructible and like on). It does say things like
"initializes comp with x", but doesn't say what kind of
initialization...