NFD

Section 17.6.4.2.1 of the ISO C++ standard says:

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a
namespace within namespace std unless otherwise specified. A program may add a template specialization
for any standard library template to namespace std only if the declaration depends on a user-defined type
and the specialization meets the standard library requirements for the original template and is not explicitly
prohibited.

The fallback definition of to_string in common.hpp violates this requirement.