This may be working as intended. But it would be good if someone who is more familiar with modules could confirm or correct me. cc @bcmills

If it is intended, perhaps the error could be made clearer.

@bhcleek, is the source code for the command actually present in /Users/bhcleek/src/$MODULE/vendor/some/vendored/command?

(Can you provide specific steps to reproduce the problem, including the steps used to populate the vendor directory?)

Yes, it's actually present.

Here's a script to duplicate this issue. Notice that it does go install in both modes to demonstrate the differences:

#!/bin/bash

set -eu -o pipefail

MODULE="github.com/golang/issue-45115"
cd $(mktemp -d)
export GOPATH=$PWD

mkdir -p src/$MODULE
cd src/$MODULE
printf "=====> $PWD\n" >&2

go mod init github.com/golang/issue-45115

cat <<EOF > anchor.go
// +build tools

package foo

import (
	_ "github.com/digitalocean/gta/cmd/gta"
)
EOF

go get github.com/digitalocean/gta/cmd/gta@latest
go mod tidy
go mod vendor

printf "=====> installing in GOPATH mode\n" >&2

GO111MODULE=off go install github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta

printf "=====> install in module aware mode\n" >&2

GO111MODULE=on go install github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta

Thanks.

github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta is a package import path, not a filesystem path.

In GOPATH mode, each vendored copy of a package has its own unique identity, in order to distinguish it from all of the other (distinct) copies of github.com/digitalocean/gta/cmd/gta that might be vendored into the tree.

In contrast, in module mode with -mod=vendor, the vendored dependencies of the main module are authoritative: there is only one copy of github.com/digitalocean/gta/cmd/gta and it is the one in ./vendor/github.com/digitalocean/gta/cmd/gta. So it does not receive a mangled or expanded import path; it is just github.com/digitalocean/gta/cmd/gta, full stop, and it is the only such package that will be used during a build.

So in module mode, the command you're looking for should either use the actual canonical import path:

$ GO111MODULE=on go install github.com/digitalocean/gta/cmd/gta

or, if you want to be agnostic to whether you are in module mode, you can use the absolute filesystem path (which is recognizably not a package import path):

$ GO111MODULE=on go install $(pwd)/vendor/github.com/digitalocean/gta/cmd/gta

go list confirms that those two paths are identical in module mode, but distinct in GOPATH mode:

$ GO111MODULE=on go list -f '{{.ImportPath}}: {{.Dir}}'  github.com/digitalocean/gta/cmd/gta
github.com/digitalocean/gta/cmd/gta: /tmp/tmp.1Kc7aaRkbU/.gopath/src/github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta

$ GO111MODULE=on go list -f '{{.ImportPath}}: {{.Dir}}' $(pwd)/vendor/github.com/digitalocean/gta/cmd/gta
github.com/digitalocean/gta/cmd/gta: /tmp/tmp.1Kc7aaRkbU/.gopath/src/github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta

$ GO111MODULE=off go list -f '{{.ImportPath}}: {{.Dir}}' $(pwd)/vendor/github.com/digitalocean/gta/cmd/gta
github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta: /tmp/tmp.1Kc7aaRkbU/.gopath/src/github.com/golang/issue-45115/vendor/github.com/digitalocean/gta/cmd/gta

I think if anything this is a documentation issue, but I'm not sure which documentation we would need to update.

@stevetraut: do our new modules user-docs have a section on how to create and use vendored dependencies (go mod vendor and -mod=vendor)?

@bcmills I think this may be more than a documentation issue. Today I found some inconsistencies in how vendored commands can be built in module aware mode.

A vendored golang.org/x/tools/cmd/stringer will install with or without the vendor directory in the import path path without error in module aware mode. Both of these seem to work when stringer is vendored:

GO111MODULE=on go install do/vendor/golang.org/x/tools/cmd/stringer

GO111MODULE=on go install golang.org/x/tools/cmd/stringer

Stringer is not in a module, so it would seem that there are differences in go install behavior when the vendored tool is in a module vs not.

@bcmills this (go install module/vendor/module/path) seems to work consistently now, maybe after all the go.mod files were dropped from vendor?.

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)