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.

No check for -ve?

.

Your solution has a very, very serious performance problem. Testing a single value of 2_000_000_000 makes it time out. Your for loop needs fixing, because the way it is now, it works terribly slowly.

I would consider a solution which loops a billion times buggy, and not a kata issue.

Why is this a kata issue? Maybe your solution is too slow or falls into some infinite loop?

execution always times out, please help!

(objective-C).