cmd/go: test coverpkg panics when defining the same flag in multiple packages

[schoentoon]

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="<HOME>/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="<HOME>/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/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/qk/76c0lh7166nbz10bntlqwqw80000gn/T/go-build524001766=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

In a project consisting of multiple binaries I ran the tests using go test -coverpkg=all ./.... I later managed to reproduce this in a small dummy 'project' with the following file structure

./cmd/test1/main.go
./cmd/test2/main.go
./dummy_test.go

Where both main.go files would look like the following

package main

import (
	"flag"
	"fmt"
)

var t = flag.String("t", "", "Dummy value")

func main() {
	fmt.Printf("t is %s\n", *t)
}

A simple go test ./... works just fine, a go test -coverpkg=all ./... would end up panic'ing.

What did you expect to see?

ok  	_/<HOME>/git/test	0.010s
?   	_/<HOME>/git/test/cmd/test1	[no test files]
?   	_/<HOME>/git/test/cmd/test2	[no test files]

What did you see instead?

/var/folders/qk/76c0lh7166nbz10bntlqwqw80000gn/T/go-build966215784/b001/test.test flag redefined: t
panic: /var/folders/qk/76c0lh7166nbz10bntlqwqw80000gn/T/go-build966215784/b001/test.test flag redefined: t

goroutine 1 [running]:
flag.(*FlagSet).Var(0xc00007a120, 0x1196d20, 0xc000060400, 0x1176a02, 0x1, 0x1177c0f, 0xb)
	/usr/local/go/src/flag/flag.go:805 +0x590
flag.(*FlagSet).StringVar(0xc00007a120, 0xc000060400, 0x1176a02, 0x1, 0x0, 0x0, 0x1177c0f, 0xb)
	/usr/local/go/src/flag/flag.go:708 +0x9e
flag.(*FlagSet).String(0xc00007a120, 0x1176a02, 0x1, 0x0, 0x0, 0x1177c0f, 0xb, 0xc0000603f0)
	/usr/local/go/src/flag/flag.go:721 +0x95
flag.String(0x1176a02, 0x1, 0x0, 0x0, 0x1177c0f, 0xb, 0xc0000603f0)
	/usr/local/go/src/flag/flag.go:728 +0x73
FAIL	_/<HOME>/git/test	0.012s
?   	_/<HOME>/git/test/cmd/test1	[no test files]
?   	_/<HOME>/git/test/cmd/test2	[no test files]

[agnivade]

/cc @bcmills @rsc

[bcmills]

Duplicate of #23910.