I see sample code around there, but I really want a dumbed down step by step explanation of how an index_sequence is coded and the meta programming principal in question for each stage.
What you ask isn't exactly trivial to explain...
Well... std::index_sequence itself is very simple: is defined as follows
template<std::size_t... Ints>
using index_sequence = std::integer_sequence<std::size_t, Ints...>;
that, substantially, is a template container for unsigned integer.
The tricky part is the implementation of std::make_index_sequence. That is: the tricky part is pass from std::make_index_sequence<N> to std::index_sequence<0, 1, 2, ..., N-1>.
I propose you a possible implementation (not a great implementation but simple (I hope) to understand) and I'll try to explain how it works.
Non exactly the standard index sequence, that pass from std::integer_sequence, but fixing the std::size_t type you can get a reasonable indexSequence/makeIndexSequence pair with the following code.
// index sequence only
template <std::size_t ...>
struct indexSequence
{ };
template <std::size_t N, std::size_t ... Next>
struct indexSequenceHelper : public indexSequenceHelper<N-1U, N-1U, Next...>
{ };
template <std::size_t ... Next>
struct indexSequenceHelper<0U, Next ... >
{ using type = indexSequence<Next ... >; };
template <std::size_t N>
using makeIndexSequence = typename indexSequenceHelper<N>::type;
I suppose that a good way to understand how it works is follows a practical example.
We can see, point to point, how makeIndexSequence<3> become index_sequenxe<0, 1, 2>.
We have that makeIndexSequence<3> is defined as typename indexSequenceHelper<3>::type [N is 3]
indexSequenceHelper<3> match only the general case so inherit from indexSequenceHelper<2, 2> [N is 3 and Next... is empty]
indexSequenceHelper<2, 2> match only the general case so inherit from indexSequenceHelper<1, 1, 2> [N is 2 and Next... is 2]
indexSequenceHelper<1, 1, 2> match only the general case so inherit from indexSequenceHelper<0, 0, 1, 2> [N is 1 and Next... is 1, 2]
indexSequenceHelper<0, 0, 1, 2> match both cases (general an partial specialization) so the partial specialization is applied and define type = indexSequence<0, 1, 2> [Next... is 0, 1, 2]
Conclusion: makeIndexSequence<3> is indexSequence<0, 1, 2>.
Hope this helps.
--- EDIT ---
Some clarifications:
std::index_sequence and std::make_index_sequence are available starting from C++14
my example is simple (I hope) to understand but (as pointed by aschepler) has the great limit that is a linear implementation; I mean: if you need index_sequence<0, 1, ... 999>, using makeIndexSequence<1000> you implement, in a recursive way, 1000 different indexSequenceHelper; but there is a recursion limit (compiler form compiler different) that can be less than 1000; there are other algorithms that limits the number of recursions but are more complicated to explain.
