Go compiler spec explanation for "Compile error: val declared and not used"

I you get this compiler error, it's because you have declared a variable that isn't used anywhere in your code. Just use this variable by doing something like

fmt.Println(yourVal)

The analogon for imported and not (yet) used packages is (in this case for the fmt package)

var _ = fmt.Printf // delete before submitting 

But much more interesting is Rob's explanation for this Go spec decision to let the compiler report "val declared and not used" as an error and not a warning.