math/big: constant 18446744073709551616 overflows int?

[elliotchance]

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

$ go version
go version go1.15 darwin/amd64

Does this issue reproduce with the latest release?

Haven't tried.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/elliot/Library/Caches/go-build"
GOENV="/Users/elliot/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/elliot/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/elliot/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15/libexec/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/c7/q_xt9q_j0_l99wstm08c1_280000gn/T/go-build184301905=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I found some odd looking code in the math/big library and don't understand how it works. I have extracted the constants out into an example:

package main

import (
	"fmt"
	"math/bits"
)

const (
	_S = _W / 8 // word size in bytes

	_W = bits.UintSize // word size in bits
	_B = 1 << _W       // digit base
	_M = _B - 1        // digit mask
)

func main() {
	fmt.Println(_S, _W, _B, _M)
}

What did you expect to see?

Some values for _B and _M.

What did you see instead?

bug/example.go:17:13: constant 18446744073709551616 overflows int
bug/example.go:17:13: constant 18446744073709551615 overflows int

[seankhliao]

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