Language: C++

#include <iostream>
#include <cmath>
using namespace std;

#define debug(var)  do{ std::cout << #var << " : "; view(var); }while(0)
using ll = long long;
using ld = long double;
inline int num (const std::string& S, int idx) noexcept { return S.at(idx) - '0'; }
inline int ctoi(char c) noexcept { return c - '0'; }
inline bool isNumber(char c) noexcept { return ('0'<c && c<'9'? true : false); }
inline int safeCtoi(char c) { if(isNumber(c)) { return ctoi(c); } else { throw 1; } }
inline char itoc(int i) noexcept { return i + '0'; }
inline unsigned int bitMask(int i) noexcept { return (1 << i); }
inline bool isTrue(unsigned int bit, int i) noexcept { return bit & bitMask(i); }
inline bool isEven(int num) noexcept { return !num%2; }
inline bool isOdd(int num) noexcept { return num%2; }
template<typename T> inline short sgn(T num) noexcept { return (num==0? 0 : (num>0? 1 : -1) ); }
template<typename T> inline bool isZero(T num) noexcept { return (num==0? true : false); }
template<typename T> inline bool isPositive(T num) noexcept { return (num>0? true : false); }
template<typename T> inline bool isNegative(T num) noexcept { return (num<0? true : false); }
template<typename T> void view(T e) noexcept { cout << e; }
template<typename T> void view(std::vector<T> v) noexcept { for(const auto& e : v) cout << e << " "; }
template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }


int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	double r;
	cin >> r;
	cout << 2 * M_PI * r << endl;
	return 0;
}


Compiler output:
a.cc:22:27: error: variable or field 'view' declared void
   22 | template<typename T> void view(std::vector<T> v) noexcept { for(const auto& e : v) cout << e << " "; }
      |                           ^~~~
a.cc:22:37: error: 'vector' is not a member of 'std'
   22 | template<typename T> void view(std::vector<T> v) noexcept { for(const auto& e : v) cout << e << " "; }
      |                                     ^~~~~~
a.cc:3:1: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
    2 | #include <cmath>
  +++ |+#include <vector>
    3 | using namespace std;
a.cc:22:45: error: expected primary-expression before '>' token
   22 | template<typename T> void view(std::vector<T> v) noexcept { for(const auto& e : v) cout << e << " "; }
      |                                             ^
a.cc:22:47: error: 'v' was not declared in this scope
   22 | template<typename T> void view(std::vector<T> v) noexcept { for(const auto& e : v) cout << e << " "; }
      |                                               ^
a.cc:23:27: error: variable or field 'view' declared void
   23 | template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }
      |                           ^~~~
a.cc:23:37: error: 'vector' is not a member of 'std'
   23 | template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }
      |                                     ^~~~~~
a.cc:23:37: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
a.cc:23:49: error: 'vector' is not a member of 'std'
   23 | template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }
      |                                                 ^~~~~~
a.cc:23:49: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
a.cc:23:57: error: expected primary-expression before '>>' token
   23 | template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }
      |                                                         ^~
a.cc:23:60: error: 'vv' was not declared in this scope
   23 | template<typename T> void view(std::vector<std::vector<T>> vv) noexcept { for(const auto& v : vv) view(v); }
      |                                                            ^~