math/big: Unexpected behavior when get a value of *big.Int, call .Add func and return de initial value.

[vinigracindo]

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

$ go version
go version go1.18.1 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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/vgracindo/Library/Caches/go-build"
GOENV="/Users/vgracindo/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/vgracindo/go/pkg/mod"
GOOS="darwin"
GOPATH="/Users/vgracindo/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/opt/go/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/opt/go/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/vgracindo/workspace/poc/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/75/9hndtrr15f9b85q3qmwwvywrwn62nq/T/go-build2606683508=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. Get a copy of a big.int value.
2. Increment the value attr big.Int of the struct by 1.
3. Return de copy (1).

For example: https://go.dev/play/p/t0Tw1oDgxIh

What did you expect to see?

package main

import (
	"fmt"
	"math/big"
)

type Anything struct {
	Current *big.Int
}

func (a *Anything) getCurrentAndInc() big.Int {
	current := *a.Current
	a.Current.Add(a.Current, big.NewInt(1))
	return current
}

func main() {
	a := Anything{
		Current: big.NewInt(10),
	}
	current := a.getCurrentAndInc()
	fmt.Println(a.Current.String())
	fmt.Println(current.String())

	current2 := a.getCurrentAndInc()
	fmt.Println(a.Current.String())
	fmt.Println(current2.String())
}

I expect the copy result to be one less than Current after increment.

11 // Current value after increment.
10 // Copy of current value before increment.
12 // Current value after increment.
11 // Copy of current value before increment.

Program exited.

OR

I expect the value of the returned value to always be equal to current (even after increment).

11 // Current value after increment.
11 // Copy of current value before increment.
12 // Current value after increment.
12 // Copy of current value before increment.

Program exited.

What did you see instead?

In the first execution it has a behavior and from the first execution the behavior changes.

11 // Current value after increment.
10 // Copy of current value before increment.
12 // Current value after increment.
12 // Copy of current value before increment.

Program exited.

[randall77]

A big.Int is not meant to be copied shallowly like that. See https://pkg.go.dev/math/big#Int , second paragraph.
Generally you never want a big.Int, only *big.Int.

current := new(big.Int).Set(a) // copies a

Or something like that.