when go mod init without version.

What do you mean? go mod init does not normally accept a version.

Please post a complete trace of the commands that you ran and the output they produced.

Please excuse my poor English.

The command as follow:

• go mod init github.com/lib/pq
  • run this, cannot download modules even if run go mod download github.com/lib/pq@latest.
• go mod init github.com/lib/pq@latest
  • run this, can download modules if run go mod download github.com/lib/pq@latest.

(I tested on Windows 10.)

• go mod init github.com/lib/pq
  • run this, cannot download modules even if run go mod download github.com/lib/pq@latest.

That may be a bug, but it's a very strange sequence of commands to run.

• go mod init github.com/lib/pq tells the go command, “this module contains the source code for github.com/lib/pq”.
• go mod download github.com/lib/pq@latest tells it “download the latest source code for github.com/lib/pq”.

Since the module itself (by its own definition) contains the source code you requested, in some sense go mod download has no work left to do.

• go mod init github.com/lib/pq@latest

If that command works at all, that's arguably a bug: the @latest is normally a version specification, not part of the module path, but go mod init takes only a module path.

I'll open a separate issue for that.

The lack of a useful error message is closely related to #27783.

Issue seems solved

$GOPATH\src\mymod>go version
go version go1.12.2 windows/amd64

$GOPATH\src\mymod>go mod init github.com/lib/pq@latest
go mod init: module path must not contain '@'

@iwdgo, we could still use a better diagnostic for go mod download with the name of the main module and @latest. It should either download the latest upstream version, or emit some kind of useful diagnostic; today it still does neither:

pq$ gotip mod init github.com/lib/pq
go: creating new go.mod: module github.com/lib/pq

pq$ gotip version
go version devel +a4c82515 Mon Aug 5 23:12:39 2019 +0000 linux/amd64

pq$ gotip mod download github.com/lib/pq@latest

pq$ find $GOPATH/pkg/mod
/tmp/tmp.doY4ctA1kP/_gopath/pkg/mod
/tmp/tmp.doY4ctA1kP/_gopath/pkg/mod/cache
/tmp/tmp.doY4ctA1kP/_gopath/pkg/mod/cache/lock

Change https://golang.org/cl/189797 mentions this issue: cmd/go: print error messages of go mod download without json flag

Following other merges, the CL is updated. go mod download would return an error when the main module path (as per go.mod) is passed as argument. Without arguments, go mod download will do nothing if go.mod does not contain any dependency. This seems in line with the overall plan in #26152.

(edited)