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.