Here's the implementation of %S support:

• https://github.com/gofrs/uuid/blob/master/uuid.go#L188-L190

Hmm, I can reproduce the vet warning on go1.11.5, go1.12.1, and go version devel +277609f844 Thu Mar 21 08:41:51 2019 +0000 linux/amd64. They are all using an empty module with just require github.com/gofrs/uuid v3.2.0+incompatible and the file you provided.

I haven't formed an opinion on whether this is a bug or not; I just can't reproduce this being a regression or being fixed in tip.

Seems the same as #30399: cmd/vet shouldn't complain about the use of an unrecognized formatting character with a type that implements fmt.Formatter.

Hmm, I don't think it's an exact duplicate. That was fixed in x/tools master weeks ago, and cmd/vendor contains a recent enough copy of it. I just manually checked the code, and my fix for go/analysis/passes/printf is in there.

Edit: to clarify, the reason I don't think it's an exact duplicate is because I still get the warning with cmd/vet on master, which contains my fix for #30399.

I was the author of the change in gofrs/uuid and I cannot reproduce the go vet warning with v1.12.1. See my comment here -> gofrs/uuid#77 (comment).

I definitely agree that cmd/vet shouldn't be complaining about non-standard formatting characters, though.

Did some more testing and this does seem to be the same issue as #30399. If I change my code to use log.Printf instead of fmt.Printf then go vet does indeed complain about the unknown formatting verb.

This generates the error:

package main

import (
    "log"
    "github.com/gofrs/uuid"
)

func main() {
    u := uuid.Must(uuid.NewV4())
    log.Printf("capitalized UUID: %S", u)
}

This does not:

package main

import (
    "fmt"
    "github.com/gofrs/uuid"
)

func main() {
    u := uuid.Must(uuid.NewV4())
    fmt.Printf("capitalized UUID: %S", u)
}

Ok, I finally get what's going on. This is using github.com/gofrs/uuid@master, not github.com/gofrs/uuid@latest. The %S change was done after the latest tagged release.

This explains why I was getting the warning at every Go version.

Anyway, yes, this is indeed a duplicate of #30399, and a backport is already ready for 1.12.2.

@mvdan I'm not using modules, so I'm using neither.

Well, the original report didn't mention any version, so my assumption was "the latest tagged version". It would have been easier if the post was clear :)