Flag unnecessary type conversions

[Jacalz]

At the moment, staticcheck does not seem to be flagging any unnecessary type conversions. I have seen one or two lately. I don't know if it is a bug or that there simply isn't a check for it, but I assume the latter.

Incorrect code:

func print(a int64) {
    fmt.Println(a)
}

func main() {
   a := int64(2)
   print(int64(a))
}

Correct code:

func print(a int64) {
    fmt.Println(a)
}

func main() {
   a := int64(2)
   print(a)
}

Probably related to #128

[dominikh]

Non-trivial because of build tags, see https://github.com/mdempsky/unconvert

[Jacalz]

Ah, I see. This was just something that I had expected staticcheck to do.

[dominikh]

I'm currently working on better support for build tags, after which I may look at this again.