Variadic template recursive return type deduction compilation error

Question

Why the code below does not compile?

template 
T sum(T t){
    return t;
}

template 
auto sum(T t, U... u         


        

Answer 1

Quoted from [basic.scope.pdecl]:

The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer (if any)

The declaration of the second function template is complete after the trailing return type decltype(t + sum(u...)). So, when parsing decltype(t + sum(u...)), the second template is not in scope yet, and the compiler can only see the first template which does not match the call.

One possible fix:

template 
struct ReturnType;

template 
struct ReturnType {
  typedef T Type;
};

template 
struct ReturnType {
  typedef typename ReturnType::Type Type_;
  typedef decltype(std::declval() + std::declval()) Type;
};

template 
T sum(T t){
    return t;
}

template 
typename ReturnType::Type sum(T t, U... u) {
    return t + sum(u...);
}