fmt: state that "%q" verb should not be used to print int value

[leonklingele]

Go version

go version go1.26.1 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='0'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN='…'
GOCACHE='…'
GOCACHEPROG=''
GODEBUG=''
GOENV='…'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/sl/yg232rm16b74lh69dhhscfz40000gn/T/go-build3024692398=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='…'
GOMODCACHE='…'
GONOPROXY='…'
GONOSUMDB='…'
GOOS='darwin'
GOPATH='…'
GOPRIVATE='…'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/nix/store/kh43nhaz1qcpwws2xq805lrmwpmn9i3k-go-1.26.1/share/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='off'
GOTELEMETRYDIR='…'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/nix/store/kh43nhaz1qcpwws2xq805lrmwpmn9i3k-go-1.26.1/share/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.26.1'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

package main

import "fmt"

func main() {
	var i int = 42
	fmt.Printf("%q\n", i)
}

https://go.dev/play/p/19ECZqyEHs_A

What did you see happen?

prog.go:7:14: fmt.Printf format %q has arg i of wrong type int

Go vet failed.

'*'

Program exited.

What did you expect to see?

The fmt docs state that the %q verb can be used to print integers:
%q a single-quoted character literal safely escaped with Go syntax.:
https://pkg.go.dev/fmt#:~:text=Integer%3A

As of Go 1.26, this however no longer passes go vet:

$ go vet prog.go
prog.go:7:14: fmt.Printf format %q has arg i of wrong type int

The docs on fmt should reflect that.

That vet error was introduced as part of #72850.

[gabyhelp]

Related Issues

• cmd/vet: add check for using %q with non-{rune,byte} integer types #72850 (closed)
• fmt: accepts non strings/chars with verb "%q" #2851 (closed)
• math/big: big.Int `fmt.Formatter` implementation should support `%q` #42022
• fmt: `%q` quotes out-of-range codepoints to strings containing invalid runes #51526 (closed)
• cmd/vet: printf %q with pointer argument not detected #25233 (closed)
• fmt: Precision for strings formatting matches documented neither for %x nor %q #8151 (closed)
• fmt: "%q", "%x", "%X" doesn't work for integer-ish fmt.Stringer #22032 (closed)
• fmt: error in the documentation for fmt.Formatter? #60358 (closed)
• fmt.Sprintf: verb %q returns a surjective function #40175 (closed)
• %q formatting verb ought to respect width and precision modifiers #1652 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

I agree this is a documentation bug. Also, "character literal" in the fmt documentation is a misnomer for rune literal.

(The actual logic, of fmt and the printf vet check, are correct: fmt.Printf("%q", 'a') prints 'a', but using int('a') would elicit a diagnotsic.)

[gopherbot]

Change https://go.dev/cl/762580 mentions this issue: fmt: fix %q verb documentation to say "rune literal"