Is that actually returning err as the deferred return value of test? In other words, to the caller of test it appears test returns the result of the deferred?
Deferred functions don't return anything. You can modify the values that are returned by the parent function, and those will be returned as the deferred function modified them.
Yep, and one cannot simulate that with local variables. In Go "return v" copies v into the return location before calling the deferred code. If that location is not named, the deferred function has no way to change it, see https://play.golang.org/p/Opg4XI08P7
I got it. That's a neat example. Do you have to define that deferred function inside the caller, in order to reference the name? Or can you factor out deferred functions to be used by various callers and pass in the return names for them to modify?