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cmd/compile: "cannot use _ as value" compile error for blank identifier variable declaration with non-empty composite literal #20484

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dmitshur opened this issue May 24, 2017 · 2 comments

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@dmitshur
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commented May 24, 2017

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

$ go version
go version go1.8.3 darwin/amd64

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

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/Dmitri/Dropbox/Work/2013/GoLanding:/Users/Dmitri/Dropbox/Work/2013/GoLand"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/tw/kgz4v2kn4n7d7ryg5k_z3dk40000gn/T/go-build536199591=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
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?

I was writing some new Go code, and wanted to assign an unused struct to the blank identifier to prevent a compiler error.

package main

func main() {
	_ = int(5)               // ok
	_ = struct{ int }{}      // ok
	_ = struct{ int }{5}     // ok
	var _ = int(5)           // ok
	var _ = struct{ int }{}  // ok
	var _ = struct{ int }{5} // cannot use _ as value
}

https://play.golang.org/p/KZlFqfhE1z

What did you expect to see?

No compile errors. (Probably? I'm not sure if this is intended behavior or not, but it doesn't seem to be.)

What did you see instead?

I got "cannot use _ as value" compile error.

Additional Notes

I tried gc compiler from Go 1.4.3, and got the same error.

go/types doesn't seem to give an error for that code.

@odeke-em

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commented May 24, 2017

Looks like a duplicate of #15481 and in particular this permutation #15481 (comment).

@dmitshur

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commented May 24, 2017

Thanks, that does look the same. Closing as duplicate.

I'll copy my snippet to that issue, since I don't see a case of struct literals mentioned there.

@dmitshur dmitshur closed this May 24, 2017

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