go: cmd/go: one new module error is backwards, another seemed backwards

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

go1.13rc1

Does this issue reproduce with the latest release?

Not with Go 1.12, but yes with Go 1.13 rc1.

Issue 1

I think this new Go 1.13 error message is incorrectly swapping the two module paths in the final two lines of the error message:

        github.com/Sirupsen/logrus: github.com/Sirupsen/logrus@v1.4.2: parsing go.mod:
        module declares its path as: github.com/Sirupsen/logrus
                but was required as: github.com/sirupsen/logrus

Given the v1.4.2 version of that module declares its path as github.com/sirupsen/logrus, I think the correct form of the error would be:

        github.com/Sirupsen/logrus: github.com/Sirupsen/logrus@v1.4.2: parsing go.mod:
        module declares its path as: github.com/sirupsen/logrus
                but was required as: github.com/Sirupsen/logrus

Example from scratch:

$ cd $(mktemp -d)
$ go1.13rc1  mod init tempmod
$ cat <<EOF > main.go
package tempmod
import _ "github.com/docker/libcompose/docker"
EOF

$ go1.13rc1 build
[...]
go: tempmod imports
        github.com/docker/libcompose/docker imports
        github.com/Sirupsen/logrus: github.com/Sirupsen/logrus@v1.4.2: parsing go.mod:
        module declares its path as: github.com/Sirupsen/logrus
                but was required as: github.com/sirupsen/logrus

Issue 2

When I first saw this next error message in the wild, at first I thought it was backwards, but later concluded it was just potentially ambiguous (I think?):

$ go1.13rc1 get github.com/iiinsomnia/yiigo@v3.0.0
go: finding github.com v3.0.0
go: finding github.com/iiinsomnia v3.0.0
go: finding github.com/iiinsomnia/yiigo v3.0.0
go: finding github.com/iiinsomnia/yiigo v3.0.0
go get github.com/iiinsomnia/yiigo@v3.0.0: github.com/iiinsomnia/yiigo@v3.0.0: 
 invalid version: module contains a go.mod file, so major version must be compatible: should be v0 or v1, not v3

In particular, it is not immediately clear which major version should be v0 or v1, not v3 – the major version after the @, or the major version in the module path on the module line in the go.mod. Because it leads off with module contains a go.mod file immediately before talking about the problematic major version, it seems like it is suggesting the major version in the go.mod file should be v0 or v1 (which is not the correct resolution, and you can’t even have a v0 or v1 in the module line for a go.mod unless it is a corner case like gopkg.in).

In this example, there is a go.mod at v3.0.0, but that go.mod is missing the required /v3 at the end of the module path on the module line. As it stands, I think the error message is trying to say it would be acceptable to have a @v0.x.x or @v1.x.x in the go get for that go.mod? But that was not the way it read at first read, and moreover, one could at least argue that is not the fundamental error, which is really that the major version after the @ does not agree with the (missing) major version in the module path on the module line (and depending on how you look at it, that the go get is also missing the /v3 before the @).

I have seen other people read this second error message backwards as well. (Side note: I’m not sure why it emits go: finding github.com/iiinsomnia/yiigo v3.0.0 twice).

Finally, sorry – this probably should have been two separate issues.

CC @jayconrod @bcmills