cmd/go: app.test -test.coverprofile fails silently

[networkimprov]

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

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

y

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/root/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/mnm-hammer/go.mod"
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-build417943255=/tmp/go-build -gno-record-gcc-switches"

What did you do?

file main.go:

package main
import "flag"
func main() { flag.Parse() }
func m2() int { return t2() }

file test.go:

package main
import "flag"
var sT = "b"
func init() { flag.StringVar(&sT, "b", sT, "B") }
func t2() int { return 0 }

file main_test.go:

package main
import "testing"
func TestCoverage(i *testing.T) {
   if m2() != 0 { i.Fail() }
}

build:

$ cd app
$ go test -c -covermode=count -coverpkg ./...
$ ./app.test -b a:b -test.coverprofile z.cov #fails silently
$ ./app.test        -test.coverprofile z.cov #works

What did you expect to see?

z.cov in directory

What did you see instead?

z.cov missing

[networkimprov]

Problem is unrelated to -test.coverprofile; never mind!