`go vet` treats Logf as a special function, even for custom definitions. `go` used to do this, but looks like it was fixed in 1.5.

logf.go

package main

import "fmt"

type Level int

const CODE Level = 1

type Logger struct {
}

func (l *Logger) Logf(code Level, message string, args ...interface{}) {
	fmt.Printf("Code(%d) %s", code, fmt.Sprintf(message, args))
}

func main() {
	logger := Logger{}
	logger.Logf(CODE, "Hello %s", "World")
}

go vet logf.go
logf.go:18: constant 1 not a string in call to Logf
exit status 1