go1.13 errors package

[divinerapier]

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

$ go version                    
go version go1.13 darwin/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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/myuser/Library/Caches/go-build"
GOENV="/Users/myuser/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH=""
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cn/1_54kmt57s135ygwjyg64p_00000gn/T/go-build709008938=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import (
	"errors"
	"fmt"
)

type Error struct {
	Code    int
	Message string
}

func NewError(code int, message string) *Error {
	return &Error{
		Code:    code,
		Message: message,
	}
}

func (e *Error) Error() string {
	return fmt.Sprintf("code: %d, message: %s", e.Code, e.Message)
}

func main() {
	testVW()
	testWrapErrors()
}

func foo() error {
	return NewError(32, "make an error")
}

func bar() error {
	return NewError(83, "make another error")
}

func testVW() {
	fmt.Println("format with %w")
	err1 := foo()
	err2 := bar()
	err3 := fmt.Errorf("err1: (%w), err2: (%w)", err1, err2)
	err4 := fmt.Errorf("err1: (%v), err2: (%v)", err1, err2)
	fmt.Printf("format with %%v: %v\n", err3)
	fmt.Printf("format with %%w: %v\n", err4)
	fmt.Println()
}

func testWrapErrors() {
	fmt.Println("wrap errors")
	err1 := foo()
	err2 := bar()
	err3 := fmt.Errorf("err1: (%w), err2: (%w)", err1, err2)
	fmt.Printf("%v\n", errors.Unwrap(err3))
}

https://play.golang.org/p/NLhAo_KxfFO

What did you expect to see?

1. format with %w should same as %v
2. unwrap some errors

What did you see instead?

format with %w
format with %v: err1: (code: 32, message: make an error), err2: (%!w(*main.Error=&{83 make another error}))
format with %w: err1: (code: 32, message: make an error), err2: (code: 83, message: make another error)

wrap errors
<nil>

[ainar-g]

The fmt docs say:

If the format specifier includes a %w verb with an error operand, the returned error will implement an Unwrap method returning the operand. It is invalid to include more than one %w verb or to supply it with an operand that does not implement the error interface. The %w verb is otherwise a synonym for %v.

(Emphasis added.)

[divinerapier]

thanks @ainar-g