foo.cc: In function 'std::__cxx11::string sec1(std::__cxx11::string)':
foo.cc:10:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0;i < str.length();i++)
                ~~^~~~~~~~~~~~~~
foo.cc: In function 'std::__cxx11::string sec2(std::__cxx11::string, int)':
foo.cc:22:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 1;i <= str.length(),i++)
                ~~^~~~~~~~~~~~~~~
foo.cc:22:18: warning: value computed is not used [-Wunused-value]
foo.cc:22:37: error: expected ';' before ')' token
  for(int i = 1;i <= str.length(),i++)
                                     ^
foo.cc: In function 'std::__cxx11::string sec3(std::__cxx11::string, int)':
foo.cc:34:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 1;i <= str.length();i++)
                 ~~^~~~~~~~~~~~~~~
foo.cc: In function 'int main()':
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++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
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++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
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++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
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++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
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++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' 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++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const std::__cxx11::basic_string<_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++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' 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++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' 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/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' 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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' 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++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:51:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
                                               ^
foo.cc:51:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str1) == sec2(str2,i) == sec3(str3,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.cc:56:33: 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(str3,i) == sec3(str2,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:56:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
                                               ^
foo.cc:56:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str1) == sec2(str3,i) == sec3(str2,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.cc:61:33: 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(str2) == sec2(str1,i) == sec3(str3,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:61:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
                                               ^
foo.cc:61:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str2) == sec2(str1,i) == sec3(str3,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.cc:66:33: 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(str2) == sec2(str3,i) == sec3(str1,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:66:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
                                               ^
foo.cc:66:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str2) == sec2(str3,i) == sec3(str1,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.cc:71:33: 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(str3) == sec2(str1,i) == sec3(str2,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:71:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
                                               ^
foo.cc:71:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str3) == sec2(str1,i) == sec3(str2,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.cc:76:33: 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(str3) == sec2(str2,i) == sec3(str1,i))
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/iosfwd:40:0,
                 from /usr/include/c++/7/ios:38,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::fpos<_StateT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_pair.h:448:5: 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++/7/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::pair<_T1, _T2>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:302:5: 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++/7/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:340:5: 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++/7/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1121:5: 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++/7/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:1127:5: 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++/7/bits/stl_iterator.h:1127:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::move_iterator<_IteratorL>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/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++/7/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:41:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/allocator.h:152:5: note: candidate: template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::allocator<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6009:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6009:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6017:5: note: candidate: template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6017:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6031:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6031:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const _CharT*' and 'bool'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/basic_string.h:6043:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6043:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/ios_base.h:46:0,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/system_error:292:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_code&)
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:292:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:297:3: note: candidate: bool std::operator==(const std::error_code&, const std::error_condition&)
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:297:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_code&'
/usr/include/c++/7/system_error:304:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_code&)
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/7/system_error:304:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
/usr/include/c++/7/system_error:311:3: note: candidate: bool std::operator==(const std::error_condition&, const std::error_condition&)
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/7/system_error:311:3: note:   no known conversion for argument 1 from '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}' to 'const std::error_condition&'
In file included from /usr/include/c++/7/bits/locale_facets.h:48:0,
                 from /usr/include/c++/7/bits/basic_ios.h:37,
                 from /usr/include/c++/7/ios:44,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note: candidate: template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/7/bits/streambuf_iterator.h:204:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const std::istreambuf_iterator<_CharT, _Traits>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/ext/new_allocator.h:155:5: note: candidate: template<class _Tp> bool __gnu_cxx::operator==(const __gnu_cxx::new_allocator<_Tp>&, const __gnu_cxx::new_allocator<_Tp>&)
     operator==(const new_allocator<_Tp>&, const new_allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/7/ext/new_allocator.h:155:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const __gnu_cxx::new_allocator<_Tp>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:869:5: note: candidate: template<class _Iterator, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:869:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/stl_iterator.h:862:5: note: candidate: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator==(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
     ^~~~~~~~
/usr/include/c++/7/bits/stl_iterator.h:862:5: note:   template argument deduction/substitution failed:
foo.cc:76:47: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>' and '__gnu_cxx::__enable_if<true, bool>::__type {aka bool}'
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
                                               ^
foo.cc:76:17: warning: suggest parentheses around comparison in operand of '==' [-Wparentheses]
   if(sec1(str3) == sec2(str2,i) == sec3(str1,i))
      ~~~~~~~~~~~^~~~~~~~~~~~~~~

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