What is difference between decltype(auto) and decltype(returning expr) as return type?

Question

What is the difference between decltype(auto) and decltype(returning expression) as return type of a function (template) if expr used with

Answer 1

decltype(auto) is used to

• Forward return type in generic code, you don't want to type a lot of duplicate thing

  template
  decltype(auto) foo(Func f, Args&&... args) 
  { 
      return f(std::forward(args)...); 
  }
  

• Delay type deduction, as you can see in this question, compilers have some problem with out decltype(auto)

  • This doesn't work well as you can see with g++ and clang++

    template struct Int {};
    constexpr auto iter(Int<0>) -> Int<0>;
    
    template constexpr auto iter(Int) -> decltype(iter(Int{}));
    
    int main(){
      decltype(iter(Int<10>{})) a;
    }
    

  • This works well as you can see here:

    template struct Int {};
    
    constexpr auto iter(Int<0>) -> Int<0>;
    
    template
    constexpr auto iter(Int) -> decltype(auto) {
      return iter(Int{});
    }
    
    int main(){
      decltype(iter(Int<10>{})) a;
    }
    

decltype(expr):

• applies SFINAE while decltype(auto) is not