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#include <iostream>
using namespace std;
 
template<class FUNC>
class IndexFunctor
{
public:
    typedef FUNC FUNC_T;
 
	explicit IndexFunctor(const FUNC_T& func) : func(func), index(0) {}
 
	template<typename... Args> 
	auto operator ()(Args&&... args) 
	-> decltype(func(std::forward<Args>(args)..., index++)) //get return type
	{
		return func(std::forward<Args>(args)..., index++);
	}
 
	const FUNC_T& GetFunctor() const
	{
		return func;
	}
 
	int GetIndex() const
	{
		return index;
	}
 
	void SetIndex(int index)
	{
		this->index = index;
	}
 
private:
	FUNC_T func;
	int index;
};
 
template<class FUNC>
IndexFunctor<FUNC> with_index(const FUNC& func)
{
	return IndexFunctor<FUNC>(func);
}
 
int main()
{
    auto f = with_index([](int a, int index){ return a * index; });
    int a = f(5);
    cout << a << endl;
}
 

prog.cpp:14:48: error: ‘index’ was not declared in this scope
  -> decltype(func(std::forward<Args>(args)..., index++)) //get return type
                                                ^
prog.cpp:14:48: error: ‘index’ was not declared in this scope
prog.cpp: In function ‘int main()’:
prog.cpp:48:16: error: no match for call to ‘(IndexFunctor<main()::__lambda0>) (int)’
     int a = f(5);
                ^
prog.cpp:5:7: note: candidate is:
 class IndexFunctor
       ^
prog.cpp:13:7: note: template<class ... Args> decltype (func((forward<Args>)(IndexFunctor::operator()::args)..., <expression error>)) IndexFunctor<FUNC>::operator()(Args&& ...) [with Args = {Args ...}; FUNC = main()::__lambda0]
  auto operator ()(Args&&... args) 
       ^
prog.cpp:13:7: note:   template argument deduction/substitution failed:
prog.cpp: In substitution of ‘template<class ... Args> decltype (func((forward<Args>)(IndexFunctor::operator()::args)..., <expression error>)) IndexFunctor<FUNC>::operator()(Args&& ...) [with Args = {Args ...}; FUNC = main()::__lambda0] [with Args = {int}]’:
prog.cpp:48:16:   required from here
prog.cpp:14:55: error: ‘func’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
  -> decltype(func(std::forward<Args>(args)..., index++)) //get return type
                                                       ^
prog.cpp:14:55: note: declarations in dependent base ‘IndexFunctor<main()::__lambda0>’ are not found by unqualified lookup
prog.cpp:14:55: note: use ‘IndexFunctor::func’ instead

Standard output is empty