While I agree nils are a problem with this: as long as they have access to the type at all, they can create zero values. E.g.

  var c color.Color

That's a valid color whether it's a nil interface value or an empty typed string.

You can return a private type to prevent this, but that also means they can't refer to it as an argument or return value anywhere outside the implementing package, which is a rather severe limit in many cases.

Sometimes* I really miss constructors.

*: all the time