go mod: simultaneous imports of different casings for a dependency fails

[voutasaurus]

Please answer these questions before submitting your issue. Thanks!

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

See readme: https://github.com/voutasaurus/modtest

Using different cased characters in an import path for the same dependency.

What did you expect to see?

Resolution to the same dependency.

What did you see instead?

upper/upper.go:4:2: case-insensitive import collision: "github.com/Sirupsen/logrus" and "github.com/sirupsen/logrus"

(See https://github.com/voutasaurus/modtest README for more info)

Does this issue reproduce with the latest release (go1.10.3)?

Applies to release candidate go1.11rc1, not go1.10.3.

System details

(from golang:rc docker image)

go version go1.11rc1 linux/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/go/src/github.com/voutasaurus/modtest/go.mod"
GOROOT/bin/go version: go version go1.11rc1 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.11rc1
uname -sr: Linux 4.9.93-linuxkit-aufs
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Debian GLIBC 2.24-11+deb9u3) stable release version 2.24, by Roland McGrath et al.

[voutasaurus]

Note that this particular package "github.com/sirupsen/logrus" was imported in the docker project as "github.com/Sirupsen/logrus" as recently as last July and earlier versions of docker are still vendored in various other projects, including "github.com/hyperledger/fabric".

Here's the commit to docker (now at github.com/moby/moby) that updated the import path:

commit 4f3616fb1c112e206b88cb7a9922bf49067a7756
Author: Derek McGowan <[email protected]>
Date:   Wed Jul 26 15:03:47 2017 -0700

    Update imports for logrus version
    
    Add forks for changes which only make logrus change without functional
    change.
    
    Signed-off-by: Derek McGowan <[email protected]>

[thepudds]

@gopherbot, please add label modules

[voutasaurus]

Looks like a duplicate of #26208 my bad