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4046. Effects of inserting into or erasing from flat container adaptors when an exception is thrown need to be more permissive

Section: 23.2.7.2 [associative.reqmts.except], 23.6.8.1 [flat.map.overview], 23.6.9.1 [flat.multimap.overview], 23.6.11.1 [flat.set.overview], 23.6.12.1 [flat.multiset.overview] Status: New Submitter: Jiang An Opened: 2024-01-23 Last modified: 2024-03-15

Priority: 2

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Discussion:

The issue was originally discovered while implementing flat_(multi_)set in MSVC STL (microsoft/STL#4059).

Currently, the effects of inserting a single element into or erasing elements from a flat container adaptor when an exception is thrown, are implicitly specified in 23.2.7.2 [associative.reqmts.except].

However, given erasing elements from a vector or a deque generally needs to assign elements, which is not guaranteed to be non-throwing, it seems unreasonable to require erase of flat container adaptors not to throw in such a case. Likewise, as inserting a single value into deque is not guaranteed to be effectless when an exception is thrown, so flat container adaptors are unlike to provide such a guarantee.

Perhaps we should conditionally relax the requirements when the underlying containers can't provide them.

[2024-03-15; Reflector poll]

Set priority to 2 after reflector poll.

P2767 §3.5 is related/conflicting.

Proposed resolution: