Potential footgun in `ptr_map`

[Noxitu]

There is an easy risk of creating use-after-free errors with current implementation of iterators. This simple code:

auto it = map.find(key);
const auto &data = it->second;

The problem comes from iterator::operator-> returning temporary (proxy), where second is a value, and not reference. This means that as soon as the assignment to data ends, temporary is freed and our reference points to destroyed object. This will be also true for directly doing map.find(key)->second.

A full example on godbolt with enabled address sanitizer: https://godbolt.org/z/M9h43Pdd1

This problem is dangerous, because it doesn't generate any warning even with decently aggressive flags (on clang-15, -Wall -Wextra -Wpedantic). Moreover, same code for std::map works correctly.

I have no idea if this can be solved on the code level (i.e. if there is a value that could be referenced in that proxy instead of making a copy), or if only solution is just making sure documentation warns about this.