J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.

[...] it's Koenig lookup (aka argument dependent lookup, ADL) in action. Shockingly enough, MSVC actually has ADL implemented for operators (though not functions), and that is what causes the problem.

std::pair<int const, int> is from std
std::ostream is from std
inside the std::ostream_iterator, where operator<< is called, is in std.
-> only std is searched for the operator<<
But the operator<< is in global, and hence is not found.

The above fix is illegal in the strict sense since there are only certain things that you are allowed to add to std (e.g. specializations of std::swap).
A legal solution is to use a proxy class: