Slightly simpler:
struct SomeType(K, V) {}
alias X(V) = V;
alias X(V, K...) = SomeType!(K[0], X!(V, K[1 .. $]));
That's a recurring pattern to get used to: aliasing away to one
of the parameters in a terminal and/or degenerate case. Also:
that an empty tuple matches no parameter "more exactly" than a
tuple parameter. I know that's not at all obvious, but it's what
I've found to be the case; so when K[1 ..$] above ends up as the
empty tuple (that is, when all K have been exhausted), the
template for X(V) will be the "more exact match."
