template or member function selection

The constructor template's parameter is declared as forwarding reference. When being passed an lvalue such as line, the template parameter X is deduced as std::string&, and after reference collapsing the parameter type is std::string&, i.e. an lvalue-reference to non-const std::string. It's an exact match and wins in overload resolution.

On the other hand, the constructor taking const std::string& requires const-qualifying the argument, and the constructor taking std::string&& can't be used with lvalues.