text/template: 0 divided by a negative number prints -0 instead of 0

[cmarkh]

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

$ go version
go version go1.18 windows/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
set GO111MODULE=
set GOARCH=amd64
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.18
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOWORK=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -build1173738419=/tmp/go-build -gno-record-gcc-switches

What did you do?

https://go.dev/play/p/l196xxd7doS

package main

import (
	"bytes"
	"fmt"
	"log"
	"text/template"
)

func main() {
	t := template.New("e")
	t.Funcs(template.FuncMap{
		"divide": func(part, total float64) string {
			return fmt.Sprint(part / total)
		},
	})

	t, err := t.Parse(`0 / -2 using template generates: {{divide 0 -2}}`)
	if err != nil {
		log.Fatal(err)
	}

	var output bytes.Buffer
	err = t.Execute(&output, nil)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("0 / -2 prints: %v\n", 0/-2)
	fmt.Printf("0 / -2 prints: %v\n", 0.0/-2.0)
	fmt.Println(output.String())

	return
}

What did you expect to see?

Normally dividing 0 by a negative number (here I used 0 / -2) will print 0.

What did you see instead?

fmt.Println(0/-2) prints 0 as expected.
When I use a custom division function inside a template, 0/-2 prints -0.
0 == -0 but I don't typically see the negative version printed.

[robpike]

The same happens in Go (and C, for that matter, and probably Fortran and many other languages). It's a property of floating-point arithmetic.

https://go.dev/play/p/2-qmOoq7mgX

Search for "what every computer scientist should know about floating point arithmetic".