cmd/go: Replace a v2 mod where there isn't a v2 directory

[shou1dwe]

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

$ go version
go version go1.17.2 darwin/arm64

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="arm64"
GOBIN=""
GOCACHE="/Users/myusername/Library/Caches/go-build"
GOENV="/Users/myusername/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/myusername/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/myusername/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/opt/go/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/opt/go/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.17.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/myusername/Documents/gitroot/modimport/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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/2_/wb7qpk5n21l9yw2f1kj1_jj00000gn/T/go-build556537930=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I have two repositories, one library: https://github.com/shou1dwe/modoriginal with a go.mod like:

module my.com/modoriginal

go 1.17

Its go.mod contains a name different from its VCS location: my.com/modoriginal

When using this library from another repository https://github.com/shou1dwe/modimport
A replace is required in go.mod like below:

module github.com/shou1dwe/modimport

go 1.17

require my.com/modoriginal v1.0.0

replace my.com/modoriginal => github.com/shou1dwe/modoriginal v1.0.0

With this setup, go run main.go in modimport repository works fine, everything works.

Then my.com/modoriginal author decided to introduce a breaking change and follow this guide: https://go.dev/blog/v2-go-modules, but decided to go with "major versions on separate branches" approach. So now there is https://github.com/shou1dwe/modoriginal/tree/v2 and also tag v2.0.0 created from the latest commit on v2 branch.

And modimport tries to use the same approach to retrieve new v2 version of modoriginal

module github.com/shou1dwe/modimport

go 1.17

require my.com/modoriginal/v2 v2.0.0

replace my.com/modoriginal/v2 => github.com/shou1dwe/modoriginal v2.0.0

What did you expect to see?

go mod tidy in modimport should exit 0 and return
go run main.go in modimport should print prefix test

What did you see instead?

$ go mod tidy                                 
go: errors parsing go.mod:
/Users/myuser/Documents/gitroot/modimport/go.mod:7: replace github.com/shou1dwe/modoriginal: version "v3.0.0" invalid: should be v0 or v1, not v3

$ go run main.go 
go: errors parsing go.mod:
/Users/myuser/Documents/gitroot/modimport/go.mod:7: replace github.com/shou1dwe/modoriginal: version "v2.0.0" invalid: should be v0 or v1, not v2

[bcmills]

The error message here is correct, if somewhat unclear (that's #41512). The v2.0.0 version implies that the module path has a /v2 suffix, and the replacement needs to use that suffix too:

diff --git c/go.mod w/go.mod
index ec2f66b..9fe80b6 100644
--- c/go.mod
+++ w/go.mod
@@ -4,4 +4,4 @@ go 1.17

 require my.com/modoriginal/v2 v2.0.0

-replace my.com/modoriginal/v2 => github.com/shou1dwe/modoriginal v2.0.0
+replace my.com/modoriginal/v2 => github.com/shou1dwe/modoriginal/v2 v2.0.0
diff --git c/go.sum w/go.sum
index 366b3b7..0dbb54a 100644
--- c/go.sum
+++ w/go.sum
@@ -1,2 +1,4 @@
 github.com/shou1dwe/modoriginal v1.0.0 h1:FMM6zxAjkflLwFL1gbHxRTNTBeL0Ud4mUNWIszwg/Og=
 github.com/shou1dwe/modoriginal v1.0.0/go.mod h1:wToed6ZhC0LGFCfmM/5OuDEnsLfHfytDHy0XVW2BXWA=
+github.com/shou1dwe/modoriginal/v2 v2.0.0 h1:5C/g7vxq84qXhHS6YSsubf1yA69MLoWtb/2gssf1p0o=
+github.com/shou1dwe/modoriginal/v2 v2.0.0/go.mod h1:ClKiTOWxsK9gcPPdBI7f0sHmj8aw8+AFOm3/JIeivXg=

/tmp/modimport$ go list -m all
github.com/shou1dwe/modimport
my.com/modoriginal/v2 v2.0.0 => github.com/shou1dwe/modoriginal/v2 v2.0.0

[shou1dwe]

thanks, I can confirm that appending /v2 to right-hand side of the replace directive fixed the problem.
For me the confusing part is that I have been treating github.com/shou1dwe/modoriginal in the above context a VCS location rather than a "module name" or "module path"; and modoriginl's go.mod says the module name is my.com/modoriginal/v2.
Apparently the major version suffix rule applies to both places.