Thanks. I also just used @benchmark to see how many allocations were being made and observed just 1.
Where would the documentation of this feature go? It’s a feature of reduce, but someone looking for advice on how to properly flatten a vector of vectors would be unlikely to look there.
On a completely personal note - I personally find myself not wanting to use reduce because the documentation explicitly states there are no guarantees either now, or in the future, on what it does.