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#include <vector>
 
template <typename R>
class Proxy {
public:
	Proxy(R& r) :range{ r } {}
	auto begin() const { return range.begin(); }
	auto end() const { return range.end(); }
private:
	R& range;
};
 
class C {
	using Type = std::vector<int>;
 
public:
	auto data() { return Proxy<Type>(data_); }
	auto data() const {
		return Proxy<Type>(data_);
	}
private:
	Type data_;
};
 
int main()
{
	C c;
}

prog.cpp: In member function ‘auto C::data() const’:
prog.cpp:19:22: error: binding reference of type ‘std::vector<int>&’ to ‘const Type’ {aka ‘const std::vector<int>’} discards qualifiers
   return Proxy<Type>(data_);
                      ^~~~~
prog.cpp:6:2: note:   initializing argument 1 of ‘Proxy<R>::Proxy(R&) [with R = std::vector<int>]’
  Proxy(R& r) :range{ r } {}
  ^~~~~

Standard output is empty