Fibonacci series directly via formula

#include <kickstart/all.hpp>        // Available at GitHub.
using namespace kickstart::all;

#include <math.h>       // pow, llround
#include <fenv.h>       // fetestexcept, FE_INVALID, feclearexcept, FE_ALL_EXCEPT

auto to_longlong( const double value )
    -> long long
{
    const long long result = ::llround( value );
    if( ::fetestexcept( FE_INVALID ) ) {
        ::feclearexcept( FE_ALL_EXCEPT );
        KS_FAIL_( range_error, ""s << value << " exceeds the range of `long long`." );
    }
    return result;
}

void cpp_main( const string_view& cmd_verb, const vector<string_view>& args )
{
    hopefully( args.size() == 1 )
        or exit_app_with_message( ""s << "Usage: " << cmd_verb << " NUMBER" );

    const int       n           = to_int( args[0] );
    const double    sqrt_5      = sqrt( 5 );
    const double    phi_plus    = (1 + sqrt_5)/2;
    const double    phi_minus   = (1 - sqrt_5)/2;
    const double    fib_n_float = (pow( phi_plus, n ) - pow( phi_minus, n))/sqrt_5;
    const long long fib_n       = to_longlong( fib_n_float );

    out << fib_n << endl;
}

auto main( int n_cmd_parts, char* cmd_parts[] )
    -> int
{ return with_exceptions_displayed( cpp_main, n_cmd_parts, cmd_parts ); }