typeswitch finds a type which implement an interfaces which are used in type-switch but the type does not appear in any cases of the type-switch.
package main
type I interface{ F() }
type A struct{I} // implements I
type B struct{I} // implements I
func main() {
var i I = A{}
switch i.(type) {
case A:
}
}$go vet -vettool=`which typeswitch` main.go
./main.go:9:2: type B does not appear in any cases