In the first case you are asking for the version of a module within the source tree for that module. To my mind the version of a module in this situation is undefined.

$ cd /tmp
$ git clone https://github.com/hyangah/explist2
Cloning into 'explist2'...
$ cd explist2
$ go list -f '{{.ImportPath}} in {{.Module.Path}}@{{.Module.Version}} [{{.Module.Dir}}]' github.com/hyangah/explist2
github.com/hyangah/explist2 in github.com/hyangah/explist2@ [/tmp/explist2]

In the second case you are querying a module as a dependency (that is present within the readonly-module cache). In this situation the module has been resolved via MVS and hence has a version associated with it.

$ mkdir /tmp/temp
$ cd /tmp/temp
$ go mod init temp
go: creating new go.mod: module temp
$ go get github.com/hyangah/explist2
go: finding github.com/hyangah/explist2 v1.0.0
go: downloading github.com/hyangah/explist2 v1.0.0
$ go list -f '{{.ImportPath}} in {{.Module.Path}}@{{.Module.Version}} [{{.Module.Dir}}]' github.com/hyangah/explist2
github.com/hyangah/explist2 in github.com/hyangah/explist2@v1.0.0 [/root/pkg/mod/github.com/hyangah/explist2@v1.0.0]

What version would you expect to be returned in the first instance out of interest?

Thanks for the explanation @myitcv . That makes sense.
I thought the above was a simple repro case for what I originally wanted to report. But clearly, that is not the case.

$ pwd
/tmp/explist2
$ tree
.
├── f.go
├── go.mod
├── go.sum
└── v2
    ├── f.go
    ├── go.mod
    └── go.sum
$ go list -f '{{.ImportPath}} in {{.Module.Path}}@{{.Module.Version}}' github.com/hyangah/explist2/v2
github.com/hyangah/explist2/v2 in github.com/hyangah/explist2@

The v2 is set up following the major subdirectory model. I expect querying v2 from the repo root directory to return the same result as the query outside the repo. But it does not. Moreover, it doesn't seem to recognize the v2 as the major version mark.

If I query from the v2 directory, at list the go list seemed to recognize v2 as a version mark.

$ cd v2
$ go list -f '{{.ImportPath}} in {{.Module.Path}}@{{.Module.Version}}' github.com/hyangah/explist2/v2
github.com/hyangah/explist2/v2 in github.com/hyangah/explist2/v2@

If I query from the v2 directory, at list the go list seemed to recognize v2 as a version mark.

I don't think the "v2" in "github.com/hyangah/explist2/v2" can be considered a version per se, rather a convention on import paths. Indeed that import path comes entirely from the go.mod, not the fact that the go.mod is in a directory called v2 within github.com/hyangah/explist2.

cc @bcmills who will undoubtedly be tighter on the wording/definitions here than I am.

I should've used better wording.
The confusing part is the go.mod file in the subdirectory is not recognized.
The go list command considers any package under subdirectory to be part of the main module.

Even when I added a dependency on v2 from v1, the result is same.

$ cat f.go
package explist2

import "github.com/hyangah/explist2/v2"

var Name = explist2.Name

$ cat go.mod
module github.com/hyangah/explist2

require github.com/hyangah/explist2/v2 v2.0.1

$ go list -f '{{.ImportPath}} in {{.Module.Path}}@{{.Module.Version}}' github.com/hyangah/explist2/v2
github.com/hyangah/explist2/v2 in github.com/hyangah/explist2@

I don't know what was going on before, but as far as I can tell this is working as designed with go1.12beta2.

The v2/go.mod file prevents go from finding v2 in the main module, so it fetches from the tagged v2.0.1 instead:

explist2$ cat f.go
package explist2

import _ "github.com/hyangah/explist2/v2"

var Name = "explist2"

explist2$ go list -f '{{.ImportPath}} (from {{with .Module}}{{.Path}}@{{.Version}}{{end}}) {{.Dir}}' all
github.com/hyangah/explist2 (from github.com/hyangah/explist2@) /tmp/tmp.dhjf2me7MN/explist2
github.com/hyangah/explist2/v2 (from github.com/hyangah/explist2/v2@v2.0.1) /tmp/tmp.bNoat8kDP6/_gopath/pkg/mod/github.com/hyangah/explist2/v2@v2.0.1