compiler: switch statements report false positive "declared and not used" errors

[zapateo]

package main

func main() {
	var x interface{}
	switch x := x.(type) {
	case float32:
		_ = x
	default:
	}
}

returns error: main:5:9: x declared and not used

The reason is the following: x is declared and marked as used by the float32 case, but it's not used by the default case. A possible solution could be declaring x outside the float32 case, but this is not possibile: x has as different type according to the case where it's used, so must have a different declaration for every case of the switch.

[zapateo]

A workaround for the problem is using the variable in every case.

package main

func main() {
	var y interface{}
	switch x := y.(type) {
	case float32:
		_ = x
	default:
		_ = x
	}
}

[zapateo]

The commit c2b582d marks every identifier declared in a type switch statement as used, so this error is not reported any more.
Now the error is not reported even if the variable is never used: see #474 for more informations.