[assbuttbuttass]

The fix is pretty simple, just declare err ahead of time:

    func doThing() string {    
        result := "OK"
        var err error
        if result, err = somethingelse(); err != nil {
            return "ERROR"  
        }

        return result
    }

[rocqua]

Part of the reason OP liked the `if assignment` was to avoid polluting the higher-level scope with a variable that is only needed during the if statement.

Your solution fixes the error, but at the cost of losing the upside OP saw.

[jaggederest]

Is there no equivalent to a lexical scope let definition?

let {

  var err := error
  
  scoped code

}

[usrbinbash]

Of course there is:

    {
        var err error
        scoped code
    }

[ptomato]

yes, you can wrap code in {} to scope it.

[Gwypaas]

The issue is now you have a potential nil value hanging around in the code with no real reason for its existence. Three refactors and some moving around later and you manage to hit a runtime panic in production.