[libcxx-dev] List begin iterator invalidation

[Alexander Torstling via libcxx-dev]

Hello!

I've stumbled upon a peculiarity with lists which I haven't been able to
explain so far.

Consider the following code:

#include <list>
#include <cstdio>

int main() {
    std::list<int> l;
    std::list<int>::iterator it = l.begin();
    l.push_back(0);
    l.insert(it, 1);
    for(const int &i: l) {
        printf("%d", i);
    }
}

This will print "01". I fail to understand why the 1 is inserted at the
end. I would expect that the begin iterator would stay valid across the
`push_back`. For the record, g++ seems to have the same behavior.

I've already created a stack overflow question around this (
https://stackoverflow.com/questions/59974821/validity-of-list-begin-iterator-after-insertion)
and asked in the llvm IRC channel about it. So far without having an
explanation. So now I'm turning to you.

Thank you in advance

Alexander Torstling
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