On Sun, Oct 2, 2022 at 5:25 PM Edward Catmur <ecatmur_at_[hidden]> wrote:

>
>
> On Sun, 2 Oct 2022 at 22:58, Barry Revzin <barry.revzin_at_[hidden]> wrote:
>
>>
>>
>> On Sun, Oct 2, 2022 at 4:47 PM Edward Catmur via Std-Discussion <
>> std-discussion_at_[hidden]> wrote:
>>
>>> On Sun, 2 Oct 2022 at 22:00, Aleksander Maciej Miera via Std-Discussion <
>>> std-discussion_at_[hidden]> wrote:
>>>
>>>> Hello,
>>>>
>>>> While fiddling with template metaprogramming I managed to run into an
>>>> issue related to some std:: containers and type traits. As far as I
>>>> have
>>>> researched it, it is a known (although not obvious at first)
>>>> shortcoming.
>>>>
>>>> Consider the following code snippet:
>>>>
>>>> #include <type_traits>
>>>> #include <memory>
>>>> #include <vector>
>>>>
>>>> struct Copyable{};
>>>> using MoveOnly = std::unique_ptr<Copyable>;
>>>>
>>>> static_assert(!std::is_copy_constructible_v<std::vector<MoveOnly>>);
>>>>
>>>> (BTW, the vector is used here as an example, but the list and
>>>> forward_list also behave the same way for the same reason)
>>>>
>>>> Counter-intuitively, the static assert fails. This stems from the fact
>>>> that the vector's copy constructor is not a function template; thus it
>>>> cannot be SFINAEd (everything can be a verb if one tries hard enough
>>>> ;))
>>>> away, because it's always declared.
>>>>
>>>
>>> Since C++20, the copy constructor can be constrained:
>>>
>>> vector(vector const&) requires std::copyable<T>;
>>>
>>> I think this would solve your issue. This would require the containers
>>> general requirements to lift this requirement (on `X u(a)`, that `T` is
>>> Cpp17CopyInsertable, etc.) from a precondition to a constraint. There are
>>> probably quite a few clauses that would need to be considered in such an
>>> effort.
>>>
>>
>> The issue is that vector<T> supports T being incomplete. So if you did
>> that, then:
>>
>> struct Node;
>> vector<Node> nodes;
>>
>> Would instantiate the copy constructor which would error on Node being
>> incomplete. So you'd have to come up with a way for vector<Incomplete> to
>> still work.
>>
>
> Isn't that ill-formed already as it instantiates the default and allocator
> ctors? Or have I misunderstood [vector.overview]/4?
>

Fine:

struct Node {
    vector<Node> nodes;
};