[kenshaw]

This is because nil is not the same as SomeInterface. They are different types. In the above example, the correct code would be this:

    fmt.Println(bar.(SomeType) == nil) // this will print true

You need to explicitly convert the interface to a specific type (as above) before you can compare whether or not it points to a concrete value. Since SomeInterface could point to any type (by implementing all the members of SomeInterface, you can make any type be of SomeInterface). This is because variables that have a interface type are never nil.

I wouldn't call this a design issue, I believe this was done very purposefully and it makes perfect sense to me and other long time Go developers. The biggest problem I've seen with learning Go is for others coming from Ruby / Java / PHP / etc languages, who have this expectation of what "null" is (or nil as the case may be) and what "interfaces" are. The key to learning Go is to come at it with the thinking of C/C++.