cmd/go: go mod download doesn't do anything if you give it a module without a version

[natefinch]

So, I didn't realize that go mod download required an @version at the end... I presumed it would default to @latest just like it would if you import a library without specifying a version in your go.mod. However, instead, go mod download just silently fails.

What did you do?

(in a directory outside GOPATH with a go.mod that did not include github.com/lib/pq)
go mod download github.com/lib/pq

What did you expect to see?

some version of lib/pq downloaded, or an error

What did you see instead?

nothing happened and no error

System details

go version go1.11 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/finchnat/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/finchnat"
GOPROXY=""
GORACE=""
GOROOT="/Users/finchnat/sdk/go1.11"
GOTMPDIR=""
GOTOOLDIR="/Users/finchnat/sdk/go1.11/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/finchnat/dev/test/go.mod"
GOROOT/bin/go version: go version go1.11 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.11
uname -v: Darwin Kernel Version 17.7.0: Thu Jun 21 22:53:14 PDT 2018; root:xnu-4570.71.2~1/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.13.6
BuildVersion:	17G65
lldb --version: lldb-902.0.79.7
  Swift-4.1

[natefinch]

Notably, I'd prefer "no version" to default to @latest ... but it at the very least should tell me I gave it something invalid.

[hyangah]

'go mod download' should report errors and failure if any.

$ go mod download somethingveryfake.com/fake
$

[gopherbot]

Change https://golang.org/cl/144106 mentions this issue: cmd/go/internal/modcmd: warn when no version supplied to go mod download

[gopherbot]

Change https://golang.org/cl/151559 mentions this issue: cmd/go/internal/modcmd: check for errors in Download

[gopherbot]

Change https://golang.org/cl/174697 mentions this issue: cmd/go: fix error message for go mod in GOPATH mode

[iwdgo]

Issue is related to #31237, an error message displays with the submitted fix which seems to solve the issue:

$GOPATH\src\mymod>set GO111MODULE=on

$GOPATH\src\mymod>goissue mod download rsc.io/doesnotexist
go: cannot find main module; see 'go help modules'

[gopherbot]

Change https://golang.org/cl/179998 mentions this issue: cmd/go: fix error messages for go mod download in GOPATH mode