Seems like a Clang thing for C++17 and up (which is what Codewars uses). On Compiler Explorer, both GCC and MSVC compile without issue (and Clang C++14 ironically). For whatever reason, Clang seems unable to see the size of your array at compile time(?) No idea why that is. But if you are explicit with it, that should solve it.

Is there a point of the last two lines?

you don't need integer_sequence at all, also, lambdas might be useful for this...

I haven't tried this yet, and I hardly even use C++, but this series is such a cool concept I think I'd really like to try it.

And more to the point, this is exactly what doing "kata" should be about, at least in the traditional Japanese sense—repetitive application of difficult procedures with the aim of achieving mastery. Bravo!

Sorry, I should read the problem description more carefully。=_=||

what's the difference?

What do you think is the difference between a recursive function with a correct base case, and an incorrect base case?

And this

Take this!