/in/foo.cc:4:13: error: 'double y1' redeclared as different kind of symbol
 double x1=0,y1=0,x2=0,y2=0;
             ^~
In file included from /usr/include/features.h:364:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/c++config.h:507,
                 from /usr/include/c++/6/iostream:38,
                 from /in/foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:251:1: note: previous declaration 'double y1(double)'
 __MATHCALL (y1,, (_Mdouble_));
 ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:7:12: error: no match for 'operator>>' (operand types are 'std::basic_istream<char>::__istream_type {aka std::basic_istream<char>}' and 'double(double) throw ()')
     cin>>x1>>y1>>x2>>y2;
     ~~~~~~~^~~~
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:120:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(__istream_type& (*__pf)(__istream_type&))
       ^~~~~~~~
/usr/include/c++/6/istream:120:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:124:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__ios_type& (*)(std::basic_istream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>; std::basic_istream<_CharT, _Traits>::__ios_type = std::basic_ios<char>] <near match>
       operator>>(__ios_type& (*__pf)(__ios_type&))
       ^~~~~~~~
/usr/include/c++/6/istream:124:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:131:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(ios_base& (*__pf)(ios_base&))
       ^~~~~~~~
/usr/include/c++/6/istream:131:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'std::ios_base& (*)(std::ios_base&)' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:168:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(bool& __n)
       ^~~~~~~~
/usr/include/c++/6/istream:168:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: warning: the address of 'double y1(double)' will always evaluate as 'true' [-Waddress]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: invalid initialization of non-const reference of type 'bool&' from an rvalue of type 'bool'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:172:7: note: candidate: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char; _Traits = std::char_traits<char>] <near match>
       operator>>(short& __n);
       ^~~~~~~~
/usr/include/c++/6/istream:172:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'short int' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: cannot bind rvalue '(short int)y1' to 'short int&'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:175:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(unsigned short& __n)
       ^~~~~~~~
/usr/include/c++/6/istream:175:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'short unsigned int' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: cannot bind rvalue '(short unsigned int)y1' to 'short unsigned int&'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:179:7: note: candidate: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>] <near match>
       operator>>(int& __n);
       ^~~~~~~~
/usr/include/c++/6/istream:179:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'int' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: cannot bind rvalue '(int)y1' to 'int&'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:182:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(unsigned int& __n)
       ^~~~~~~~
/usr/include/c++/6/istream:182:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'unsigned int' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: cannot bind rvalue '(unsigned int)y1' to 'unsigned int&'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:186:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match>
       operator>>(long& __n)
       ^~~~~~~~
/usr/include/c++/6/istream:186:7: note:   conversion of argument 1 would be ill-formed:
/in/foo.cc:7:14: error: invalid conversion from 'double (*)(double) throw ()' to 'long int' [-fpermissive]
     cin>>x1>>y1>>x2>>y2;
              ^~
/in/foo.cc:7:14: error: cannot bind rvalue '(long int)y1' to 'long int&'
In file included from /usr/include/c++/6/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/istream:190:7: note: candidate: std::basic_istream<_CharT, _Traits>::__istream_type& std::b