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cmd/compile: expected "not used" error #35021

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serhatgiydiren opened this issue Oct 20, 2019 · 1 comment

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@serhatgiydiren
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@serhatgiydiren serhatgiydiren commented Oct 20, 2019

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

$ go version
go version go1.13.1 linux/amd64

Does this issue reproduce with the latest release?

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

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/me/.cache/go-build"
GOENV="/home/me/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/me/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build399130454=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.13.1 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.13.1
uname -sr: Linux 5.0.0-31-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.3 LTS
Release:	18.04
Codename:	bionic
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.27-3ubuntu1) stable release version 2.27.
gdb --version: GNU gdb (Ubuntu 8.1-0ubuntu3.1) 8.1.0.20180409-git

What did you do?

https://play.golang.org/p/O_76qweGQ0g

package main

func main() {
	var x int
	x = 7 - x
}

What did you expect to see?

Although x is used in assignment over itself, there is no usage of x, it is nice to get "not used" error for preventing unnecessary computations.

What did you see instead?

Nothing, run successfully

@ianlancetaylor ianlancetaylor changed the title Expected "not used" error cmd/compile: expected "not used" error Oct 20, 2019
@ianlancetaylor

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@ianlancetaylor ianlancetaylor commented Oct 20, 2019

At this date I would worry that making this change would cause many correct Go programs to stop compiling.

See #20802 for a related issue.

@ianlancetaylor ianlancetaylor added this to the Unplanned milestone Oct 20, 2019
@golang golang deleted a comment from laothuong Oct 21, 2019
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