reflect: StructOf of does not allow use of "_" field name more than once

[kortschak]

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

$ go version
go1.17.1

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build3606891520=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/mvNenqW5RdZ

What did you expect to see?

Reflect safely construct a struct equivalent to the struct created by the compiler.

What did you see instead?

panic: reflect.StructOf: duplicate field _

Additional information

I am using this to pad structs to match C structs dynamically, so I can work around this by providing the padding fields with an iota suffix, but it would be nice if this just worked. Looking at reflect.StructOf, making the duplication check here conditional on the Name not being "_" fixes the issue.

[gopherbot]

Change https://golang.org/cl/357959 mentions this issue: reflect: skip duplicate check in StructOf when the name of a filed is "_"