I think it's the other way around. In your package you define an interface and a struct/implementation. The contract to the user is the interface, the struct is merely an implementation detail and ideally shouldn't be public anyway so that are are free to change it.

Then you provide functions like NewCat(...) which provide instances of the interface, and which always returns either a valid implementation (non-nil type,non-nil impl tuple) or nil. The user can then directly use this like var cat Cat = NewTabby().

If you instead would return a struct pointer and the user does var cat Cat = NewTabby() you would actually triggered exactly this error. Instead he would need to check what the function returns before assigning it to an interface.