readonly checks assignment to package variables.
package a
var (
V1 int = 10 // OK - first assignment
V2 int
)
func init() {
V1 = 20 // OK - in init
V2 = 100 // OK - in init
}
func main() {
V1 = 30 // NG
V2 = 200 // NG
// assign
V2 = 300 // OK - explicit assign
}