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cmd/compile: casting byte to an int8 with a constant value greater than 127 throws an error #27939

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jackwilsdon opened this issue Sep 29, 2018 · 3 comments
Closed

cmd/compile: casting byte to an int8 with a constant value greater than 127 throws an error #27939

jackwilsdon opened this issue Sep 29, 2018 · 3 comments
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@jackwilsdon
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What version of Go are you using (go version)?

go version go1.11 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jack/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jack/Development/Go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
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/67/bwpvphcn1kx4xjjzhvxtt6yw0000gn/T/go-build917588304=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Tried running the following code;

package main

import "fmt"

func main() {
	fmt.Printf("value: %d\n", int8(byte(200)))
}

What did you expect to see?

value: -56

What did you see instead?

./main.go:6:30: constant 200 overflows int8

The following code seems to work fine, so there's something odd going on when using a constant value in byte(...);

package main

import "fmt"

func main() {
	i := 200
	fmt.Printf("value: %d\n", int8(byte(i)))
}
@cznic
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cznic commented Sep 29, 2018

It works correctly. This behavior is defined in the language specification.

@go101
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go101 commented Sep 30, 2018 •
edited

yes, it is well defined. Overflows are allowed at run time, but not at compile time.O

@robpike
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robpike commented Sep 30, 2018

See https://blog.golang.org/constants

@robpike robpike closed this as completed Sep 30, 2018
@golang golang locked and limited conversation to collaborators Sep 30, 2019
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@cznic @jackwilsdon @robpike @gopherbot @go101