Why aren't C++14 standard-defined literals in the global namespace by default?

Question

C++14 includes standard-defined literals for, amongst other things, std::string and various timespans from the header.

To us

Answer 1

Look at paper N2765. UDLs are hooked into the regular name lookup process. As string literals have common string types, there's a large chance of a collision if you ignored namespaces.