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?