cmd/go: 'get -d' failing on modules with only a build-ignored Go file in their root directory

[mvdan]

#!/bin/bash
  
docker run -i golang:1.15beta1 <<-SCRIPT

        set -ex

        go version
        go env

        mkdir /tmp/foo
        cd /tmp/foo
        go mod init test

        go get -d google.golang.org/protobuf || cat go.mod

SCRIPT

This fails:

+ go mod init test
go: creating new go.mod: module test
+ go get -d google.golang.org/protobuf
go: downloading google.golang.org/protobuf v1.25.0
go: google.golang.org/protobuf upgrade => v1.25.0
go get google.golang.org/protobuf: no Go source files
+ cat go.mod
module test

go 1.15

Happens on my local go version devel +4f2a2d7e26 Wed Jul 8 22:16:24 2020 +0000 linux/amd64 too.

Note the build-ignored test file at the root of the module: https://github.com/protocolbuffers/protobuf-go/blob/e14d6b3cdce27a8743907161e84fa6d07e30266d/integration_test.go

The script above succeeds if I use a module that has no Go files at its root, such as mvdan.cc/sh/v3. For the time being, I've worked around the issue by manually editing go.mod, and then go mod tidy ensures it's OK and updates go.sum.

This seems like a pretty straightforward bug, and I'm surprised it hasn't surfaced before, but I couldn't find a previous issue. Happy to attempt a fix if the bug can be confirmed - in that case, pointers for the fix would be appreciated. CC @jayconrod @bcmills @matloob

[jayconrod]

I think #33526 is a duplicate of this issue, though the situation you describe seems more serious. I left a comment on that issue and milestoned it for 1.16.

[mvdan]

Thanks, that makes sense! I tried to find that existing bug, but failed :)