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 } {}
^~~~~