math/big: Int returned by Float.Int(Inf) is not usable

[robpike]

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

$ go version

Does this issue reproduce with the latest release?

Yes

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

GO111MODULE=""
GOARCH="amd64"
GOBIN="/Users/r/bin"
GOCACHE="/Users/r/Library/Caches/go-build"
GOENV="/Users/r/Library/Preferences/go/env"
GOEXE=""
GOFLAGS="-ldflags=-w"
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/r"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/r/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/r/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/r3/cryt3b891yj6rzqr8vzcn2xc0000gn/T/go-build885119120=/tmp/go-build -gno-record-gcc-switches -fno-common"

$ go env

What did you do?

This program crashes, as demonstrated. One is supposed to use the second return value from the call to inf.Int to see what to do (its value in this case is Below) but it's not helpful because the first result is poisoned with an internal nil.

% cat x.go
package main

import (
	"fmt"
	"math/big"
)

func main() {
	var one big.Int
	var inf big.Float
	one.SetInt64(1)
	inf.SetInf(false)
	i, _ := inf.Int(nil)
	i.Add(i, &one)
	fmt.Println(i)
}
% go run x.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10a1fb9]

goroutine 1 [running]:
math/big.(*Int).Add(0x0, 0x0, 0xc00006ef40, 0x10acaff)
	/Users/r/go/src/math/big/int.go:117 +0x29
main.main()
	/Users/r/x.go:14 +0xbb
exit status 2

What did you expect to see?

What did you see instead?

[robpike]

This is clearly a minor case, but it bit me today.

[robpike]

Never mind, this is documented.