using namespace linalg::ostream_overloads sometimes does not work as planed.
For example doctest library(https://github.com/onqtam/doctest) does not found operator defined like this.

It might be a good idea to change it in the documentation for this way:

#include "linalg.h"
namespace linalg { using linalg::ostream_overloads::operator <<; }

It works better because c++ adl mechanism lookup function in argument`s namespaces first. So, in other places it can ignore overloaded operator for various reasons.

(In fact, it's not entirely clear to me why the overloads are generally hidden. )

Thanks for your work.