cmd/go: go get forces upgrade without `-u`

[ucirello]

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

$ go version
go version go1.13.3 darwin/amd64

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=""
GOCACHE="/Users/user/Library/Caches/go-build"
GOENV="/Users/user/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/user/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/user/gobug/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/_l/9yb78fmx0jzb8khycvrzw3qw0000gn/T/go-build968937557=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

With this environment: https://play.golang.org/p/bUF9KgVU1ow
I execute either

go get -tags tools github.com/gogo/protobuf/protoc-gen-gofast

or

go get github.com/gogo/protobuf/protoc-gen-gofast

What did you expect to see?

Silent output - and protoc-gen-gofast v1.2.1 installed

What did you see instead?

It upgrades to v1.3.1

$ go get github.com/gogo/protobuf/protoc-gen-gofast
go: finding github.com/gogo/protobuf v1.3.1
go: downloading github.com/gogo/protobuf v1.3.1
go: extracting github.com/gogo/protobuf v1.3.1

$ go get -tags tools github.com/gogo/protobuf/protoc-gen-gofast
go: finding github.com/gogo/protobuf v1.3.1
go: downloading github.com/gogo/protobuf v1.3.1
go: extracting github.com/gogo/protobuf v1.3.1

[bcmills]

This is working as documented. Per https://golang.org/cmd/go/#hdr-Add_dependencies_to_current_module_and_install_them:

For each named package or package pattern, get must decide which version of the corresponding module to use. By default, get looks up the latest tagged release version, such as v0.4.5 or v1.2.3.

If you just want to install the current version of a binary already in your module's dependencies, use go install rather than go get.