more precise error message

more precise error message

package main

func main() {
       var x interface {} = 42;
       switch x := x.(type) {
       case int:
       case foo:
       }
}

before:
x.go:7: non-type case in type switch
x.go:7: inappropriate case for a type switch

now:
x.go:7: foo: undefined

R=ken
OCL=31221
CL=31221