cmd/go: -mod flag doesn't work with module-mode go get

[shmsr]

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

$ go version

go version go1.16 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=""

GOARCH="amd64"

GOBIN=""

GOCACHE="/Users/subhamsarkar/Library/Caches/go-build"

GOENV="/Users/subhamsarkar/Library/Application Support/go/env"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="darwin"

GOINSECURE=""

GOMODCACHE="/Users/subhamsarkar/go/pkg/mod"

GONOPROXY=""

GONOSUMDB=""

GOOS="darwin"

GOPATH="/Users/subhamsarkar/go"

GOPRIVATE=""

GOPROXY="https://proxy.golang.org,direct"

GOROOT="/usr/local/Cellar/go/1.16/libexec"

GOSUMDB="sum.golang.org"

GOTMPDIR=""

GOTOOLDIR="/usr/local/Cellar/go/1.16/libexec/pkg/tool/darwin_amd64"

GOVCS=""

GOVERSION="go1.16"

GCCGO="gccgo"

AR="ar"

CC="clang"

CXX="clang++"

CGO_ENABLED="1"

GOMOD="/Users/subhamsarkar/project/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wz/k505z2bd7rnbbv_611swlj4h0000gp/T/go-build2354586914=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Inside a module, go get didn't accept the mod flag which is against what's mentioned in https://golang.org/ref/mod#build-commands

...
When run in module-aware mode, these commands use go.mod files to interpret import paths listed on the command line or written in Go source files. These commands accept the following flags, common to all module commands.
...

So, I was just playing with go commands based on what's written in the doc and I noticed that -mod works will all module commands except for go get.

$ go get -mod=mod ./...

What did you expect to see?

As per documentation, -mod must work with go get just like it works for go {build,fix,generate,install,list,run,test,vet}

What did you see instead?

$ go get -mod=mod ./...
flag provided but not defined: -mod
usage: go get [-d] [-t] [-u] [-v] [-insecure] [build flags] [packages]
Run 'go help get' for details.

[dmitshur]

CC @bcmills, @jayconrod, @matloob.

[bcmills]

The purpose of go get in module mode is to edit the dependencies of the module. For that purpose, the only sensible -mod behavior is -mod=mod — -mod=readonly or -mod=vendor would be incoherent.

I intentionally removed the -mod flag from go get in CL 198438 so that it could be set in GOFLAGS without causing errors in go get. (See #30345 and #32502.)

[bcmills]

So, probably the thing to fix here is to note the different behavior for go get in https://golang.org/ref/mod#build-commands.

[shmsr]

So, should I send a CL for this?

[bcmills]

Sure!

[tenkoh]

I have happened to meet the same issue.
If @shmsr doesn't mind, I will make a CL on documentation in a few days.