go mod tidy often adds dependencies which will never get used, such as indirect test dependencies (test-related dependencies of dependencies), or dependencies via likely-to-be-unused build tags (such as gopherjs dependencies for projects that support the js build tag.

This issue is compounded when followed by go mod vendor, which includes these dependencies as well.

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

$ go version
go version go1.11.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GOARCH="amd64"
GOBIN="/Users/kevingillette/usr/bin"
GOCACHE="/Users/kevingillette/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/kevingillette/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="/Users/kevingillette/sandbox/modules/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/ct/t6rzgvn537bcjcy8n_7g3z2r0000gq/T/go-build062165011=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

// go.mod
module github.com/user/xyz

// main.go
package main

import _ "github.com/afex/hystrix-go/hystrix"

func main() {}

$ go mod tidy

What did you expect to see?

// go.mod
module github.com/user/xyz

require (
	github.com/afex/hystrix-go v0.0.0-20180502004556-fa1af6a1f4f5
)

What did you see instead?

// go.mod
module github.com/user/xyz

require (
	github.com/afex/hystrix-go v0.0.0-20180502004556-fa1af6a1f4f5
	github.com/gopherjs/gopherjs v0.0.0-20181103185306-d547d1d9531e // indirect
	github.com/jtolds/gls v4.2.1+incompatible // indirect
	github.com/smartystreets/assertions v0.0.0-20180927180507-b2de0cb4f26d // indirect
	github.com/smartystreets/goconvey v0.0.0-20181108003508-044398e4856c // indirect
)

hystrix-go has no third-party dependencies except in test code. hystrix-go tests depend on goconvey, which in turn depend on gls, which depends on gopherjs in a file with a js build tag (as an aside: it seems many paths lead back to gopherjs when go modules are involved).

I would expect go.mod to not contain indirect test dependencies (it's not the responsibility of this project to run its dependencies' tests). Further, I would like some level of control in the go.mod file over build tags that are included or excluded. For example, excluding (or not including) the js tag from being considered in module operations, such as via a go.mod "tags" directive, would avoid much of this issue.