Variadic deduction guide not taken by g++, taken by clang++ - who is correct?

Question

Consider the following code:

template 
struct list
{
    template 
    list(Args...) 
    {
        static_a         


        

Answer 1

This is gcc bug 80871. The issue is, we end up with this set of candidates for deduction:

template 
list __f(Args... ); // constructor

template 
list  __f(Args... ); // deduction-guide

Both are valid (Types... can deduce as empty in the first case), but the call here should be ambiguous - neither is more specialized than the other. Types... does not participate in ordering here (similar to the example in [temp.deduct.partial]/12). So the correct behavior is to proceed to the next tiebreaker, which favors deduction-guides. Hence, this should be a list.

However, gcc's behavior is to favor the constructor, hence the static_assert triggers becuase Types... would indeed be empty in that situation.