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### 3577. Merging an (unordered) associative container with itself

Section: 24.2.7.1 [associative.reqmts.general], 24.2.8.1 [unord.req.general] Status: New Submitter: Joaquín M López Muñoz Opened: 2021-08-04 Last modified: 2021-08-20

Priority: 3

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Discussion:

For the expression `a.merge(a2)`, it is not explicitly stated whether `a2` can be the same object as `a`. libstdc++-v3 and libc++ seemingly assume this is not allowed, as the following code produces an infinite loop with both standard library implementations:

>     #include <set>

>     int main()
>     {
>       std::multiset<int> c={0, 0};
>       c.merge(c);
>     }

A strict reading of postconditions seems to ban the case where `a` and `a2` are the same:

- 24.2.7.1 [associative.reqmts.general]: "Iterators referring to the transferred elements […] now behave as iterators into `a`, not into `a2`": if `a` and `a2` are the same, a transferred iterator can't be both an iterator to `a` and not an iterator to `a2`.

- 24.2.8.1 [unord.req.general]: "Iterators referring to the transferred elements and all iterators referring to `a` will be invalidated, but iterators to elements remaining in `a2` will remain valid": if `a` and `a2` are the same, an iterator can't both be invalidated and remain valid.

Even if a provision is made that, when `a` and `a2` are the same, no elements are transferred by convention, 24.2.8.1 [unord.req.general] would still implicitly ban the case, as all iterators would be invalidated but the iterators to the remaining elements (again, all iterators) would remain valid, which is contradictory.

For context, analogous operations for `std::list` take inconsistent approaches:

- `splice(const_iterator position, list& x)` requires that source and destination be not the same.

- `splice(const_iterator position, list& x, const_iterator i)` implicitly allows `addressof(x) == this`, as the case `position == i` is taken care of.

- `std::list::merge` explicitly allows the case `addressof(x) == this` (resulting in a no-op).

[2021-08-20; Reflector poll]

Set priority to 3 after reflector poll. Tim Song commented: "I think the current PR of LWG2414 bans this code, but we might want to have consistency with `list::merge` instead."

Proposed resolution: