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#include <iostream>
#include <tuple>
#include <iostream>
 
template<class T>
struct X {};
 
template<class T>
bool operator==( X<T>, X<T> ) { return true; }
 
template<class T>
struct Y {
	friend bool operator==( Y, Y ) { return true; }
};
 
struct A {
	template<class T>
	operator X<T>() const { return {}; }
};
 
struct B {
	template<class T>
	operator Y<T>() const { return {}; }
};
 
 
int main() {
	A a;
	X<int> x;
	B b;
	Y<int> y;
	b == y;
	a == x;
}
 

prog.cpp: In function ‘int main()’:
prog.cpp:33:4: error: no match for ‘operator==’ (operand types are ‘A’ and ‘X<int>’)
  a == x;
    ^
prog.cpp:33:4: note: candidate is:
prog.cpp:9:6: note: template<class T> bool operator==(X<T>, X<T>)
 bool operator==( X<T>, X<T> ) { return true; }
      ^
prog.cpp:9:6: note:   template argument deduction/substitution failed:
prog.cpp:33:7: note:   ‘A’ is not derived from ‘X<T>’
  a == x;
       ^

Standard output is empty