cmd/go: 'go mod download $(go list -m)' fails silently

[JohnTitor]

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

go1.11 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Windows 10, amd64

What did you do?

go mod download doesn't work well when go mod init without version.
(If you run go mod download something.com/something@latest, doesn't work well too.)

What did you expect to see?

Display message (e.g. go mod init: must be set version)

What did you see instead?

Nothing.
Displaying no message isn't user-friendly I think.

[bcmills]

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.

[JohnTitor]

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.)

[bcmills]

• 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.

[bcmills]

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