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

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

template <class... Types, class... Args>
list<Types...> __f(Args... ); // constructor

template <class... Args>
list<Args...>  __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<int, double, char>.

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

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