i've never seen decltype before. looks cool

True

I didn't see that

But I did improve on the one i forked.

The check for size == 0 is redundant. If the size is zero std::accumulate returns 0 anyway.
I think returning -1 isn't a good practice anymore. In general use std::optional for that.
In this case returning -1 is simply wrong! the sum of an empty vector just isn't -1, also suppose the vector was { 1, 1, -3 } then it's sum would also be -1.

Code for checking test cases isn't written in c++? Tests will always fail no matter what.

Very nice puzzle Kata. Had a lot of fun solving it! :) Thanks

You can just use a gcd implementation from num (num::integer::gcd)

You don't need this check here:

if not arr:
  return 0

Thanks!

Ok, but ranking will not be changed.

1. Wrong comment - here I use not manhattan distance, equlidian
2. Not very stable solution - some times it passes all the test, some times - hits execution timeout (12 sec)
3. Honestly this solution shuold not pass :)