<deque>: Support incomplete value type

[zhihaoy]

Describe the bug

Certain standard sequence containers, such as list and vector, allow the value type to be an incomplete type to support recursive data structures. Although it's not a part of the standard (yet), Microsoft STL's deque implementation should consider supporting this. libstdc++ and libc++ since ABI version 2 (contributed by Google) have this support.

See also: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4510.html

Godbolt test case

https://godbolt.org/z/7fEbPnTc3

#include <deque>

struct Entry
{
    std::deque<Entry> messages;
    // ...
};

Expected behavior

Compiles without error.

STL version

Any.

Additional context

It could be done in vNext, as a part of #147.

[fsb4000]

I'm sorry. The issue has been discussed before and will not be fixed: #108

We even had a ready fix: #2506

[zhihaoy]

Let me check the reason:

We talked about this at the weekly maintainer meeting and have consensus to close this PR. If the Standard is changed in the future to require deque<Incomplete> then merging such a change will be possible.

I was told "we (MSVC STL) cannot support n3890 because we don't want to break ABI," which is why I removed deque from n4510. Supporting the change in major implementations may be a better way than forming a loop here.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting, and our view is unchanged: supporting incomplete value types in deque would be a non-Standard extension, which is explicitly a non-goal of our project.

[frederick-vs-ja]

I've moved the requested changes to this non-official branch. IIUC the changes won't break ABI, and it's possibly OK to write a new paper to support deque<Incomplete>.