This can however not be done easily for value_t, since
external_constructor depends on it, as is operator< which was moved
outside basic_json too.
This is not really an issue, since all basic_json classes share the same
enum

Those warnings are not "fixed", but that's intentional.
Truncations are expected when ArithmeticType is not the same type than
number_unsigned_t (or another basic_json template argument)

remove some useless trailing return types and && to help MSVC