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#include <iostream>
using namespace std;
 
template <typename T, typename U>
struct BaseS
{
	virtual unsigned int operator()(T t, U u) const;
};
 
template <typename T, typename U>
struct S : BaseS<T,U>
{
	unsigned int operator()(T t, U u) const override
	{
		return 1;
	}
};
 
template <typename T>
struct S<T,nullptr_t> : BaseS<T,nullptr_t>
{
	virtual bool operator()(T t, nullptr_t u) const override
	{
		return 2;
	}
};
 
class A{};
class B{};
 
int main() {
	A a;
	B b;
 
	cout << S<A*,B*>()(&a,&b) << endl;
	cout << S<A*,nullptr_t>()(&a,nullptr) << endl;
 
	return 0;
}
 

prog.cpp: In instantiation of ‘struct S<A*, std::nullptr_t>’:
prog.cpp:36:26:   required from here
prog.cpp:22:15: error: conflicting return type specified for ‘bool S<T, std::nullptr_t>::operator()(T, std::nullptr_t) const [with T = A*; std::nullptr_t = std::nullptr_t]’
  virtual bool operator()(T t, nullptr_t u) const override
               ^~~~~~~~
prog.cpp:7:23: error:   overriding ‘unsigned int BaseS<T, U>::operator()(T, U) const [with T = A*; U = std::nullptr_t]’
  virtual unsigned int operator()(T t, U u) const;
                       ^~~~~~~~

Standard output is empty