Reported by dsymonds about vet, but apparently a go/types issue.

package pkg

import (
        "fmt"
        "xpb" // not a standard package
)

func f() {
        var h xpb.SomeEnum
        fmt.Printf("%s", xpb.SomeEnum_name[int32(h)])
}

$ hg id
326a3ee842a2 tip
$ go tool vet -v x.go
vet: x.go:5:2: could not import xpb (can't find import: xpb)
Checking file x.go
x.go:10: arg xpb.SomeEnum_name[int32(h)] for printf verb %s of wrong type: uint8

I would expect the type reported here by go/types to be nil, but it's uint8. If it were
nil, vet wouldn't check it against the %s printf verb.