Language: C++

using System;
using System.Linq;

namespace AtCoderBiginnerContest154
{
	class Program
	{
		static void Main(string[] args)
		{
			var target = ReadString();
			var result = "";
			for(var i = 0; i < target.Length; i++)
			{
				result += "x";
			}
			Console.WriteLine(result);
		}

		public static int ReadInt()
			=> int.Parse(Console.ReadLine());

		public static int[] ReadIntArray()
			=> Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray();

		public static long ReadLong()
			=> long.Parse(Console.ReadLine());

		public static long[] ReadLongArray()
			=> Console.ReadLine().Split().Select(x => long.Parse(x)).ToArray();

		public static string ReadString()
			=> Console.ReadLine();

		public static string[] ReadStringArray()
			=> Console.ReadLine().Split();
	}
}


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.Linq;
      |       ^~~~~~
a.cc:8:34: error: 'string' has not been declared
    8 |                 static void Main(string[] args)
      |                                  ^~~~~~
a.cc:8:43: error: expected ',' or '...' before 'args'
    8 |                 static void Main(string[] args)
      |                                           ^~~~
a.cc:19:23: error: expected ':' before 'static'
   19 |                 public static int ReadInt()
      |                       ^~~~~~~
      |                       :
a.cc:20:28: error: invalid pure specifier (only '= 0' is allowed) before 'int'
   20 |                         => int.Parse(Console.ReadLine());
      |                            ^~~
a.cc:19:35: error: initializer specified for static member function 'static int AtCoderBiginnerContest154::Program::ReadInt()'
   19 |                 public static int ReadInt()
      |                                   ^~~~~~~
a.cc:22:23: error: expected ':' before 'static'
   22 |                 public static int[] ReadIntArray()
      |                       ^~~~~~~
      |                       :
a.cc:22:34: error: expected unqualified-id before '[' token
   22 |                 public static int[] ReadIntArray()
      |                                  ^
a.cc:25:23: error: expected ':' before 'static'
   25 |                 public static long ReadLong()
      |                       ^~~~~~~
      |                       :
a.cc:26:28: error: invalid pure specifier (only '= 0' is allowed) before 'long'
   26 |                         => long.Parse(Console.ReadLine());
      |                            ^~~~
a.cc:25:36: error: initializer specified for static member function 'static long int AtCoderBiginnerContest154::Program::ReadLong()'
   25 |                 public static long ReadLong()
      |                                    ^~~~~~~~
a.cc:28:23: error: expected ':' before 'static'
   28 |                 public static long[] ReadLongArray()
      |                       ^~~~~~~
      |                       :
a.cc:28:35: error: expected unqualified-id before '[' token
   28 |                 public static long[] ReadLongArray()
      |                                   ^
a.cc:31:23: error: expected ':' before 'static'
   31 |                 public static string ReadString()
      |                       ^~~~~~~
      |                       :
a.cc:31:31: error: 'string' does not name a type
   31 |                 public static string ReadString()
      |                               ^~~~~~
a.cc:34:23: error: expected ':' before 'static'
   34 |                 public static string[] ReadStringArray()
      |                       ^~~~~~~
      |                       :
a.cc:34:31: error: 'string' does not name a type
   34 |                 public static string[] ReadStringArray()
      |                               ^~~~~~
a.cc:36:10: error: expected ';' after class definition
   36 |         }
      |          ^
      |          ;
a.cc: In static member function 'static void AtCoderBiginnerContest154::Program::Main(int*)':
a.cc:10:25: error: 'var' was not declared in this scope
   10 |                         var target = ReadString();
      |                         ^~~
a.cc:11:29: error: expected ';' before 'result'
   11 |                         var result = "";
      |                             ^~~~~~
a.cc:12:33: error: expected ';' before 'i'
   12 |                         for(var i = 0; i < target.Length; i++)
      |                                 ^
a.cc:12:40: error: 'i' was not declared in this scope
   12 |                         for(var i = 0; i < target.Length; i++)
      |                                        ^
a.cc:12:44: error: 'target' was not declared in this scope
   12 |                         for(var i = 0; i < target.Length; i++)
      |                                            ^~~~~~
a.cc:14:33: error: 'result' was not declared in this scope
   14 |                                 result += "x";
      |                                 ^~~~~~
a.cc:16:25: error: 'Console' was not declared in this scope
   16 |                         Console.WriteLine(result);
      |                         ^~~~~~~
a.cc:16:43: error: 'result' was not declared in this scope
   16 |                         Console.WriteLine(result);
      |                                           ^~~~~~