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cmd/compile: error if trying to print constant > int #20108

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karalabe opened this issue Apr 25, 2017 · 4 comments
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cmd/compile: error if trying to print constant > int #20108

karalabe opened this issue Apr 25, 2017 · 4 comments
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@karalabe
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This is mostly a funky error that got me surprised when I saw it. It's of course trivial to work around but I though it could be worth a question whether it's working as intended or not.

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

go version go1.8.1 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/work:/opt/google/cloud-sdk/platform/google_appengine/goroot/src"
GORACE=""
GOROOT="/opt/google/go"
GOTOOLDIR="/opt/google/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build623255510=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

https://play.golang.org/p/5GARX4Q2Tc

package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Println(math.MaxUint64)
}

What did you expect to see?

18446744073709551615

What did you see instead?

tmp/sandbox131021564/main.go:9: constant 18446744073709551615 overflows int
@davecheney
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I think this is working as intended. It's a side effect of coercing an untyped integer constant math.MaxUint64 to an int while being boxed into the ...interface{} for fmt.Println.

@karalabe
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I guess the compiler needs to make some reproducible decision as to what to box the constant to and can't just pick whatever is the best based on value. That being said, boxing to int would still have an interesting corner case:

package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Println(math.MaxUint32)
}
$ GOARCH=amd64 go run main.go 
4294967295

$ GOARCH=386 go run main.go 
main.go:9: constant 4294967295 overflows int

The reason is again obvious, but I'd expect a bit more "portability" than to choke so easily between 32/64 bits.

@karalabe
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Anyway, if you think it works as intended feel free to close :) At least you have a new slide for your quiz collection @davecheney ;)

@davecheney
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The spec is the spec is the spec, https://golang.org/ref/spec#Constants

@bradfitz bradfitz mentioned this issue May 12, 2017
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@golang golang locked and limited conversation to collaborators Apr 25, 2018
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@davecheney @karalabe @gopherbot