/in/foo.cc: In function 'std::__cxx11::string sec1(std::__cxx11::string)':
/in/foo.cc:10:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0;i < str.length();i++)
                ~~^~~~~~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string sec2(std::__cxx11::string, int)':
/in/foo.cc:22:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 1;i <= str.length(),i++)
                ~~^~~~~~~~~~~~~~~
/in/foo.cc:22:18: warning: value computed is not used [-Wunused-value]
/in/foo.cc:22:37: error: expected ';' before ')' token
  for(int i = 1;i <= str.length(),i++)
                                     ^
/in/foo.cc: In function 'std::__cxx11::string sec3(std::__cxx11::string, int)':
/in/foo.cc:34:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 1;i <= str.length();i++)
                 ~~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:51:33: error: no match for 'operator==' (operand types are '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' and 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}')
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/6/iosfwd:40:0,
                 from /usr/include/c++/6/ios:38,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/postypes.h:216:5: note: candidate: template<class _StateT> bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&)
     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
     ^~~~~~~~
/usr/include/c++/6/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_pair.h:435:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
     ^~~~~~~~
/usr/include/c++/6/bits/stl_pair.h:435:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:292:5: note: candidate: template<class _Iterator> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator==(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:292:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:349:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
     operator==(const reverse_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:349:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:1113:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)
     operator==(const move_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:1113:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:1119:5: note: candidate: template<class _Iterator> bool std::operator==(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)
     operator==(const move_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:1119:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/string:41:0,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/allocator.h:146:5: note: candidate: template<class _T1, class _T2> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_T2>&)
     operator==(const allocator<_T1>&, const allocator<_T2>&)
     ^~~~~~~~
/usr/include/c++/6/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
/in/foo.cc:51:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/6/string:41:0,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool