Language: C++

#include<iostream>
#include<cmath>
#include<algorithm>
#include<string>
using namespace std;

int x1,y1,z1,w1,x2,y2,z2,w2;

int main(){
	while(1){
		cin>>n;
		if(n==0){break;}
		for(int i=0;i<n;i++){
			cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
			cout<<x1*x2-y1*y2-z1*z2-w1*w2;
			cout<<" ";
			cout<<x2*y1+x1*y2+z1*w2-w1*z2;
			cout<<" ";
			cout<<x1*z2+z1*x2+w1*y2-y1*z2;
			cout<<" ";
			cout<<w1*x2-z1*y2+y1*z2+x1*w2;
			cout<<endl;
		}
	}
	
	return 0;
}

Compiler output:
a.cc:7:8: error: 'int y1' redeclared as different kind of entity
    7 | int x1,y1,z1,w1,x2,y2,z2,w2;
      |        ^~
In file included from /usr/include/features.h:523,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/c++config.h:683,
                 from /usr/include/c++/14/bits/requires_hosted.h:31,
                 from /usr/include/c++/14/iostream:38,
                 from a.cc:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:257:1: note: previous declaration 'double y1(double)'
  257 | __MATHCALL (y1,, (_Mdouble_));
      | ^~~~~~~~~~
a.cc: In function 'int main()':
a.cc:11:22: error: 'n' was not declared in this scope; did you mean 'yn'?
   11 |                 cin>>n;
      |                      ^
      |                      yn
a.cc:14:32: error: no match for 'operator>>' (operand types are 'std::basic_istream<char>::__istream_type' {aka 'std::basic_istream<char>'} and 'double(double) noexcept')
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                         ~~~~~~~^~~~
      |                            |     |
      |                            |     double(double) noexcept
      |                            std::basic_istream<char>::__istream_type {aka std::basic_istream<char>}
In file included from /usr/include/c++/14/iostream:42:
/usr/include/c++/14/istream:170:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  170 |       operator>>(bool& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:170:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: cannot bind non-const lvalue reference of type 'bool&' to a value of type 'double (*)(double) noexcept'
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
/usr/include/c++/14/istream:174:7: 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)
  174 |       operator>>(short& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:174:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'short int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(short int)y1' to 'short int&'
/usr/include/c++/14/istream:177:7: 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>; __istream_type = std::basic_istream<char>]' (near match)
  177 |       operator>>(unsigned short& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:177:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'short unsigned int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(short unsigned int)y1' to 'short unsigned int&'
/usr/include/c++/14/istream:181:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  181 |       operator>>(int& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:181:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(int)y1' to 'int&'
/usr/include/c++/14/istream:184:7: 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>; __istream_type = std::basic_istream<char>]' (near match)
  184 |       operator>>(unsigned int& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:184:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'unsigned int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(unsigned int)y1' to 'unsigned int&'
/usr/include/c++/14/istream:188:7: 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>; __istream_type = std::basic_istream<char>]' (near match)
  188 |       operator>>(long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:188:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'long int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(long int)y1' to 'long int&'
/usr/include/c++/14/istream:192:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  192 |       operator>>(unsigned long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:192:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'long unsigned int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(long unsigned int)y1' to 'long unsigned int&'
/usr/include/c++/14/istream:199:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  199 |       operator>>(long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:199:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'long long int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(long long int)y1' to 'long long int&'
/usr/include/c++/14/istream:203:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  203 |       operator>>(unsigned long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:203:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'long long unsigned int' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(long long unsigned int)y1' to 'long long unsigned int&'
/usr/include/c++/14/istream:328:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  328 |       operator>>(void*& __p)
      |       ^~~~~~~~
/usr/include/c++/14/istream:328:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'void*' [-fpermissive]
   14 |                         cin>>x1>>y1>>z1>>w1>>x2>>y2>>z2>>w2;
      |                                  ^~
      |                                  |
      |                                  double (*)(double) noexcept
a.cc:14:34: error: cannot bind rvalue '(void*)y1' to 'void*&'
/usr/include/c++/14/istream:122:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__istream_type& (*)(__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:122:7: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'double (*)(double) noexcept' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'} [-fpermissive]