foo.cc:2:17: error: expected identifier before 'int'
 using namespace int;
                 ^~~
foo.cc:2:17: error: expected ';' before 'int'
foo.cc:2:17: error: declaration does not declare anything [-fpermissive]
foo.cc: In function 'int main()':
foo.cc:5:6: error: 'cout' was not declared in this scope
      cout <<"Hello World!";
      ^~~~
foo.cc:5:6: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~

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