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fmt: Regression in fmt.Printf("%d", Error) where Error is an integer type satisfying error #30472

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ncw opened this issue Feb 28, 2019 · 3 comments
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@ncw
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@ncw ncw commented Feb 28, 2019

What version of Go are you using (go version)?

$ go version
go version devel +c2b707bcf1 Thu Feb 28 11:24:52 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ncw/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/ncw/go"
GOPROXY=""
GORACE=""
GOROOT="/opt/go/go1.12"
GOTMPDIR=""
GOTOOLDIR="/opt/go/go1.12/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build543488653=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Compile and run this program

package main

import (
	"fmt"
)

type Error byte

func (e Error) Error() string {
	return fmt.Sprintf("Error number %d", e)
}

func main() {
	fmt.Printf("Error %d\n", Error(99))
}

What did you expect to see?

$ go version
go version go1.12 linux/amd64
$ go run err-bug.go
Error 99

What did you see instead?

$ go version
go version devel +c2b707bcf1 Thu Feb 28 11:24:52 2019 +0000 linux/amd64
$ go run err-bug.go
Error %!d(main.Error=99)

I used git-bisect to discover that it is caused by 6be6f11 on CL https://go-review.googlesource.com/c/go/+/163559 from #29934

I'm not sure if this is intended?

Cc: @mpvl

@bradfitz
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@bradfitz bradfitz commented Feb 28, 2019

This also broke x/net/idna's tests.

I sent off https://go-review.googlesource.com/c/net/+/164518 but it's a hacky workaround.

@neild
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@neild neild commented Feb 28, 2019

I've got a fix, will send it off when I finish testing it.

@gopherbot
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@gopherbot gopherbot commented Feb 28, 2019

Change https://golang.org/cl/164557 mentions this issue: fmt: fix %d et al. on errors

@gopherbot gopherbot closed this in 444039e Feb 28, 2019
@golang golang locked and limited conversation to collaborators Feb 28, 2020
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