Is this nested template base class ambiguous?

Question

This code used to compile in Clang 3.8, and still compiles in VS 2017, but started emitting an error in Clang 3.9.

template <typename D, typename ... I>
struct impl : I ...
{};

template <typename D, typename ... I>
void f(impl<D, I...> const&)
{}

struct A : impl<A> {};
struct B : impl<B, A> {};

int main()
{
    f(A());
    f(B()); // Error
}

Clang 3.9 says

<source>:15:5: error: no matching function for call to 'f'
    f(B());
    ^
<source>:6:6: note: candidate template ignored: failed template argument deduction
void f(impl<D, I...> const&)
     ^

All three compilers in action: https://godbolt.org/g/OKFpPl

I want f(A()) to deduce D = A, I... = <> and f(B()) to deduce D = B, I... = A, which is then deducible as an instance of impl. My end goal is to detect type in impl's parameter pack that are themselves instances of impl. Am I going about this the wrong way?

Answer 1

Not sure but...

Your B class inherit from impl<B, A> that inherit from impl<A>.

So B inherit from a couples of different impl<D, I...> base classes, so the type deduction is ambigous (D == B and I... == <A> or D == A and I... == <>).

So I think that clang 3.8 is wrong and that 3.9 is right.

My end goal is to detect type in impl's parameter pack that are themselves instances of impl. Am I going about this the wrong way?

Yes, you're doing it in the wrong way (I think).