etl::nth_type not implemented correctly

[bitwizeshift]

I was just browsing through some of the utilities in ETL, and I noticed that nth_type appears to be implemented incorrectly as of 6780a56f52e38d4d7c095165694905afa202aaa9.

nth_type_helper does not seem to check if it's at the Nth type -- it always falls through to the last type in the variadic set due to what I believe to be an incorrect partial-specialization:

etl/include/etl/nth_type.h

Lines 42 to 52 in 6780a56

	template <size_t N, typename T1, typename... TRest>
	struct nth_type_helper
	{
	using type = typename nth_type_helper<N - 1U, TRest...>::type;
	};
	template <size_t N, typename T1>
	struct nth_type_helper<N, T1>
	{
	using type = T1;
	};

The size_t N parameter is always decremented, but never checked for a specific value -- so once N = 0 it will just roll over to static_cast<size_t>(-1).

This can be tested with:

static_assert(std::is_same<etl::nth_type_t<0,int,long>, int>::value, "");
static_assert(std::is_same<etl::nth_type_t<1,int,long>, long>::value, "");

The first test fails because it yields long instead of int (Live Example)

From inspection, it appears that the partial specialization for nth_type_helper should actually be:

    template <typename T1, typename... TRest>
    struct nth_type_helper<0, T1, TRest...>

which I believe is the desired behavior (Live Example)

---

^{(I don't have time to clone/make the change/open a PR, but this was quick enough to type up, and should be easy enough to fix)}

[jwellbelove]

Thanks 👍

[jwellbelove]

Fixed 20.27.2