The following code is rejected by libc++ because the second iterator argument to vector(iterator, iterator) is not deduced (https://godbolt.org/z/q8f9Y7):

#include <iterator>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> v({}, std::istream_iterator<int>{});
}

The the following code, however, is accepted (https://godbolt.org/z/nM4Yh4):

#include <iterator>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> v;
    v.assign({}, std::istream_iterator<int>{});
}

I believe this to be a conformance bug.

I stumbled on this in my own code when I noticed that a typo in the following code was being accepted:

std::vector<int> v(other.cbegin(), other.end()); // note .end(), not .cend()

Since the second argument is not deduced, and the first argument is a const iterator, the second argument is implicitly converted to a const iterator. This is a symptom of the same issue, though I think this particular case could arguably be considered a conforming extension.