I was thinking that some people would want to figure it out for themselves so I didn't want to spoil the challenge for them... I kind of hoped someone else would post the solution.

Anyway, here is a solution: http://play.golang.org/p/Mg8x7sJ-hR

In other words, just insert the following line:

    type int string

The thing is, "int" is not a reserved word in Go. It's just a predeclared binding. You can rebind it -- even use "int" as a local variable name, like so: http://play.golang.org/p/0_iTzkg-5m

There are other interesting cases. For example,

    var true = false // Perfectly valid Go code!

I think of it as a prank because you could sneak such a line into somebody's code and the compiler would not help them realize what had happened, it would just complain about a lot of nonsense like 1 not being an int. In the "var true = false" example, there would not even be a compiler error, you'd just get baffling runtime errors.

For a list of all the predeclared identifiers, see here: http://golang.org/ref/spec#Predeclared_identifiers