Language: C++

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;

namespace A
{
    class StartingPoint
    {
        static void Main(string[] args)
        {
            if (args?.Any() == true)
            {
                var assembly = Assembly.GetEntryAssembly();
                var path = "../../../../In.txt";
                var stream = new FileStream(path, FileMode.Open);
                var streamReader = new StreamReader(stream, Encoding.UTF8);
                var str = streamReader.ReadToEnd();
                var textReader = new StringReader(str);
                Console.SetIn(textReader);
            }

            try
            {
                foreach (var s in new Program().Solve())
                {
                    Console.WriteLine(s);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                if (args?.Any() != true)
                    throw e;
            }

            if (args?.Any() == true)
                Console.ReadKey();
        }
    }

    class Program
    {
        public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
        public List<int> ScanInts(long count) => ScanInts((int)count);
        public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
        public List<long> ScanLongs(long count) => ScanLongs((int)count);
        public int ScanInt => int.Parse(Str);
        public long Scan => long.Parse(Str);
        public double ScanDouble => (double)Scan;

        StringBuilder sb = new StringBuilder();

        public string Str => ScanStr();
        public string ScanStr()
        {
            sb.Clear();

            int r = 0;
            bool? b = null;
            do
            {
                r = Console.Read();
                b = IsBreak(r);
            } while (b == true);

            if (b == null)
                throw new Exception("input error: unexpected end of stream");

            do
            {
                sb.Append((char)r);
                r = Console.Read();
                b = IsBreak(r);
            } while (b == false);

            return sb.ToString();
        }

        bool? IsBreak(int c)
        {
            switch (c)
            {
                case -1:
                    return null;
                case (int)' ':
                case (int)'\n':
                case (int)'\r':
                    return true;
                default:
                    return false;
            }
        }

        public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
        public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);

        public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
        public IEnumerable<long> Loop(long from, long to)
        {
            for (long i = from; i <= to; i++)
                yield return i;
        }

        public string YesNo(bool isYes) => isYes ? "Yes" : "No";

        public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);


        public IEnumerable<string> Solve()
        {
            var res = new string(ScanStr().Select(x => 'x').ToArray());
            yield return res.ToString();
        }
    }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.IO;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Linq;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.Reflection;
      |       ^~~~~~
a.cc:6:7: error: expected nested-name-specifier before 'System'
    6 | using System.Text;
      |       ^~~~~~
a.cc:12:26: error: 'string' has not been declared
   12 |         static void Main(string[] args)
      |                          ^~~~~~
a.cc:12:35: error: expected ',' or '...' before 'args'
   12 |         static void Main(string[] args)
      |                                   ^~~~
a.cc:42:6: error: expected ';' after class definition
   42 |     }
      |      ^
      |      ;
a.cc: In static member function 'static void A::StartingPoint::Main(int*)':
a.cc:14:17: error: 'args' was not declared in this scope
   14 |             if (args?.Any() == true)
      |                 ^~~~
a.cc:14:22: error: expected primary-expression before '.' token
   14 |             if (args?.Any() == true)
      |                      ^
a.cc:14:36: error: expected ':' before ')' token
   14 |             if (args?.Any() == true)
      |                                    ^
a.cc:14:36: error: expected primary-expression before ')' token
a.cc:16:17: error: 'var' was not declared in this scope
   16 |                 var assembly = Assembly.GetEntryAssembly();
      |                 ^~~
a.cc:17:21: error: expected ';' before 'path'
   17 |                 var path = "../../../../In.txt";
      |                     ^~~~
a.cc:18:21: error: expected ';' before 'stream'
   18 |                 var stream = new FileStream(path, FileMode.Open);
      |                     ^~~~~~
a.cc:19:21: error: expected ';' before 'streamReader'
   19 |                 var streamReader = new StreamReader(stream, Encoding.UTF8);
      |                     ^~~~~~~~~~~~
a.cc:20:21: error: expected ';' before 'str'
   20 |                 var str = streamReader.ReadToEnd();
      |                     ^~~
a.cc:21:21: error: expected ';' before 'textReader'
   21 |                 var textReader = new StringReader(str);
      |                     ^~~~~~~~~~
a.cc:22:17: error: 'Console' was not declared in this scope
   22 |                 Console.SetIn(textReader);
      |                 ^~~~~~~
a.cc:22:31: error: 'textReader' was not declared in this scope
   22 |                 Console.SetIn(textReader);
      |                               ^~~~~~~~~~
a.cc:27:26: error: 'var' was not declared in this scope
   27 |                 foreach (var s in new Program().Solve())
      |                          ^~~
a.cc:27:17: error: 'foreach' was not declared in this scope
   27 |                 foreach (var s in new Program().Solve())
      |                 ^~~~~~~
a.cc:32:20: error: 'Exception' does not name a type
   32 |             catch (Exception e)
      |                    ^~~~~~~~~
a.cc:34:17: error: 'Console' was not declared in this scope
   34 |                 Console.WriteLine(e);
      |                 ^~~~~~~
a.cc:34:35: error: 'e' was not declared in this scope
   34 |                 Console.WriteLine(e);
      |                                   ^
a.cc:35:21: error: 'args' was not declared in this scope
   35 |                 if (args?.Any() != true)
      |                     ^~~~
a.cc:35:26: error: expected primary-expression before '.' token
   35 |                 if (args?.Any() != true)
      |                          ^
a.cc:35:40: error: expected ':' before ')' token
   35 |                 if (args?.Any() != true)
      |                                        ^
a.cc:35:40: error: expected primary-expression before ')' token
a.cc:39:17: error: 'args' was not declared in this scope
   39 |             if (args?.Any() == true)
      |                 ^~~~
a.cc:39:22: error: expected primary-expression before '.' token
   39 |             if (args?.Any() == true)
      |                      ^
a.cc:39:36: error: expected ':' before ')' token
   39 |             if (args?.Any() == true)
      |                                    ^
a.cc:39:36: error: expected primary-expression before ')' token
a.cc:40:17: error: 'Console' was not declared in this scope
   40 |                 Console.ReadKey();
      |                 ^~~~~~~
a.cc: At global scope:
a.cc:46:15: error: expected ':' before 'List'
   46 |         public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
      |               ^~~~~
      |               :
a.cc:46:16: error: 'List' does not name a type
   46 |         public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
      |                ^~~~
a.cc:47:15: error: expected ':' before 'List'
   47 |         public List<int> ScanInts(long count) => ScanInts((int)count);
      |               ^~~~~
      |               :
a.cc:47:16: error: 'List' does not name a type
   47 |         public List<int> ScanInts(long count) => ScanInts((int)count);
      |                ^~~~
a.cc:48:15: error: expected ':' before 'List'
   48 |         public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
      |               ^~~~~
      |               :
a.cc:48:16: error: 'List' does not name a type
   48 |         public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
      |                ^~~~
a.cc:49:15: error: expected ':' before 'List'
   49 |         public List<long> ScanLongs(long count) => ScanLongs((int)count);
      |               ^~~~~
      |               :
a.cc:49:16: error: 'List' does not name a type
   49 |         public List<long> ScanLongs(long count) => ScanLongs((int)count);
      |                ^~~~
a.cc:50:15: error: expected ':' before 'int'
   50 |         public int ScanInt => int.Parse(Str);
      |               ^~~~
      |               :
a.cc:51:15: error: expected ':' before 'long'
   51 |         public long Scan => long.Parse(Str);
      |               ^~~~~
      |               :
a.cc:52:15: error: expected ':' before 'double'
   52 |         public double ScanDouble => (double)Scan;
      |               ^~~~~~~
      |               :
a.cc:54:9: error: 'StringBuilder' does not name a type
   54 |         StringBuilder sb = new StringBuilder();
      |         ^~~~~~~~~~~~~
a.cc:56:15: error: expected ':' before 'string'
   56 |         public string Str => ScanStr();
      |               ^~~~~~~
      |               :
a.cc:56:16: error: 'string' does not name a type
   56 |         public string Str => ScanStr();
      |                ^~~~~~
a.cc:57:15: error: expected ':' before 'string'
   57 |         public string ScanStr()
      |               ^~~~~~~
      |               :
a.cc:57:16: error: 'string' does not name a type
   57 |         public string ScanStr()
      |                ^~~~~~
a.cc:82:13: error: expected unqualified-id before '?' token
   82 |         bool? IsBreak(int c)
      |             ^
a.cc:97:15: error: expected ':' before 'IEnumerable'
   97 |         public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
      |               ^~~~~~~~~~~~
      |               :
a.cc:97:16: error: 'IEnumerable' does not name a type
   97 |         public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
      |                ^~~~~~~~~~~
a.cc:98:15: error: expected ':' before 'IEnumerable'
   98 |         public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);
      |               ^~~~~~~~~~~~
      |               :
a.cc:98:16: error: 'IEnumerable' does not name a type
   98 |         public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);
      |                ^~~~~~~~~~~
a.cc:100:15: error: expected ':' before 'IEnumerable'
  100 |         public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
      |               ^~~~~~~~~~~~
      |               :
a.cc:100:16: error: 'IEnumerable' does not name a type
  100 |         public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
      |                ^~~~~~~~~~~
a.cc:101:15: error: expected ':' before 'IEnumerable'
  101 |         public IEnumerable<long> Loop(long from, long to)
      |               ^~~~~~~~~~~~
      |               :
a.cc:101:16: error: 'IEnumerable' does not name a type
  101 |         public IEnumerable<long> Loop(long from, long to)
      |                ^~~~~~~~~~~
a.cc:107:15: error: expected ':' before 'string'
  107 |         public string YesNo(bool isYes) => isYes ? "Yes" : "No";
      |               ^~~~~~~
      |               :
a.cc:107:16: error: 'string' does not name a type
  107 |         public string YesNo(bool isYes) => isYes ? "Yes" : "No";
      |                ^~~~~~
a.cc:109:15: error: expected ':' before 'HashSet'
  109 |         public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);
      |               ^~~~~~~~
      |               :
a.cc:109:16: error: 'HashSet' does not name a type
  109 |         public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);
      |                ^~~~~~~
a.cc:112:15: error: expected ':' before 'IEnumerable'
  112 |         public IEnumerable<string> Solve()
      |               ^~~~~~~~~~~~
      |               :
a.cc:112:16: error: 'IEnumerable' does not name a type
  112 |         public IEnumerable<string> Solve()
      |                ^~~~~~~~~~~
a.cc:117:6: error: expected ';' after class definition
  117 |     }
      |      ^
      |      ;
a.cc:50:29: error: expected primary-expression before '>' token
   50 |         public int ScanInt => int.Parse(Str);
      |                             ^
a.cc:50:31: error: expected primary-expression before 'int'
   50 |