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.

How is this an issue though? (Its just a miswording not an issue, the problem tells you what you need to do, so just do it or find other problems if you aren't satisfied with that detail).

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

I wanted to demonstrate how many different ways this particular task could be solved, I suppose there are still more than what I found...

Also, I learn from playing with code and experimenting in this way. For example, in this case I learned about ast and literal_eval.

Why the random choice of 3 methods performing globally the same task (although eval is not recommended as usual)?

This code won't work.

1. The lambda expression should be assigned to a variable.
2. This code is converting something to int, not int to str.

Great job!