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

go version go1.12.1 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="/Users/egreer200/go/bin"
GOCACHE="/Users/egreer200/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/egreer200/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.1/libexec/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/7h/qd9xrbq10lb03g4lkqcl_cdr0000gp/T/go-build194066640=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I have a module that has a couple layers of packages under it. The module is here. Specifically notice that the checks directory has other packages within it.

I can go mod tidy from the pkg/ directory linked above.

What did you expect to see?

A few packages that were not used possibly being removed.

What did you see instead?

Nearly all packages were removed, but some were kept. It looked like this:

and so on.

It appears that tidy is targeting only one (or a few) of the packages. Perhaps only the packages at the first level of directory depth under the pkg/ dir were analyzed?

In short, it looks like go mod tidy will not work when a module is used for multiple packages.