What does 'incompatible' in go.mod mean, will it cause harm?

Accepted answer is correct, but really not friendly for me who just get in touch with go module. I made some investigation base on the answer & make a conclusion base on this as next, in case anyone needed:

Standard commands like go build or go test will automatically add new dependencies as needed to satisfy imports (updating go.mod and downloading the new dependencies). But there are several different situations which will result in the different version selections:

1. If a repository has not opted in to modules but has been tagged with valid semver tags, meanwhile, it's v0/v1 module, see this:

   not opted in to modules: means no go.mod in source tree

   valid semver tags: means the repo use git tag to tagged as something like vX.Y.Z

   v0/v1 module: means the value of major version(that is X) is 0 or 1, e.g. v0.1.0, v1.2.3

   Then, it will use a pseudo-version, something like github.com/kolo/xmlrpc v0.0.0-20190717152603-07c4ee3fd181

2. If a repository has not opted in to modules but has been tagged with valid semver tags, meanwhile, it's a v2+ module, see this:

   v2+ module: means the value of major version(that is X) is >=2,e g. v4.1.0

   Then, it will show as incompatible, something like github.com/zeromq/goczmq v4.1.0+incompatible

3. If a repository has already opted in to modules, but not have been tagged with valid semver tags:

   Then, it will behave as 1, use pseudo-version.

4. If a repository has already opted in to modules, and has been tagged with valid semver tags, meanwhile, it's a v0/v1 module:

   Then, it will behave normally like github.com/stretchr/testify v1.3.0

5. If a repository has already opted in to modules, and has been tagged with valid semver tags, meanwhile, it's a v2+ module:

   Then, when import in sourcecode, we need add /vN at the end, e.g., import "github.com/my/mod/v4", and in go.mod it will behave like github.com/my/mod/v4 v4.1.0

+incompatible means the dependency has a semver major version of 2 or higher and is not a Go module yet (it doesn't have go.mod in its source code).

The module name should have been github.com/zeromq/goczmq/v4 instead of github.com/zeromq/goczmq for versions v4 and above (v4.1.0, v4.2.0, etc).

Since github.com/zeromq/goczmq has not adopted Go modules correctly, the go get will fail if Go 1.13 is used and the GOPROXY is set to direct or to some other server that does not host this file -

    go get github.com/zeromq/[email protected]+incompatible
    go: finding github.com v4.2.0+incompatible
    go: finding github.com/zeromq v4.2.0+incompatible
    go: finding github.com/zeromq/goczmq v4.2.0+incompatible
    go: finding github.com/zeromq/goczmq v4.2.0+incompatible
    go get github.com/zeromq/[email protected]+incompatible: github.com/zeromq/[email protected]+incompatible: invalid version: +incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required

More details mentioned under the 'Version validation' section here - https://golang.org/doc/go1.13#modules

Note - GoSUMDB also won't have such entries so even if you set the GOPROXY to a server that hosts this file and if GOSumDB is enabled, then you will get something like this -

    ➜  ~ export GOPROXY=https://gocenter.io
    ➜  ~ go get github.com/zeromq/[email protected]+incompatible
    go: finding github.com/zeromq/goczmq v4.2.0+incompatible
    go: finding github.com/zeromq v4.2.0+incompatible
    go: finding github.com v4.2.0+incompatible
    go: downloading github.com/zeromq/goczmq v4.2.0+incompatible
    verifying github.com/zeromq/[email protected]+incompatible: github.com/zeromq/[email protected]+incompatible: reading https://gocenter.io/sumdb/sum.golang.org/lookup/github.com/zeromq/[email protected]+incompatible: 404 Not Found

The correct solution will be to follow up with the module author to make sure that they are adopting Go modules correctly by adding a suffix to the module name.

There is a workaround but have to check if it's working by design i.e. point GOPROXY to a server that hosts this file and then use GOPRIVATE to exclude this specific module version from GoSumDB validation -

    root@715c3b39bb12:/go# export GOPROXY=https://gocenter.io 
    root@715c3b39bb12:/go# unset GOPRIVATE

    root@715c3b39bb12:/go# go get github.com/zeromq/[email protected]+incompatible
    go: finding github.com v4.2.0+incompatible
    go: finding github.com/zeromq/goczmq v4.2.0+incompatible
    go: finding github.com/zeromq v4.2.0+incompatible
    go: downloading github.com/zeromq/goczmq v4.2.0+incompatible
    verifying github.com/zeromq/[email protected]+incompatible: github.com/zeromq/[email protected]+incompatible: reading https://gocenter.io/sumdb/sum.golang.org/lookup/github.com/zeromq/[email protected]+incompatible: 404 Not Found

    root@715c3b39bb12:/go# export GOPRIVATE=github.com/zeromq/goczmq

    root@715c3b39bb12:/go# go get github.com/zeromq/[email protected]+incompatible

    go: downloading github.com/zeromq/goczmq v4.2.0+incompatible
    go: extracting github.com/zeromq/goczmq v4.2.0+incompatible
    # pkg-config --cflags  -- libczmq libzmq libsodium
    Package libczmq was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libczmq.pc'

However, will still recommend reaching out to the module author to fix the module name in their go.mod file.