cmd/go: incorrect message when replace target fails to resolve to correct module

[myitcv]

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

$ go version
go version devel +396d625330 Thu Sep 12 07:45:53 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build854839250=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Consider the following testscript test:

go mod tidy

-- tools.go --
// +build tools

package tools

import (
        _ "golang.org/x/tools/go/analysis"
        _ "golang.org/x/tools/gopls"
)

-- go.mod --
module example.com/hello

go 1.13

require (
        golang.org/x/tools v0.0.0-20190911230505-6bfd74cf029c
        golang.org/x/tools/gopls v0.1.5
)

replace golang.org/x/tools => github.com/myitcvforks/tools v0.0.0-20190911230505-6bfd74cf029c

replace golang.org/x/tools/gopls => github.com/myitcvforks/tools v0.0.0-20190911230505-6bfd74cf029c

What did you expect to see?

An error telling me the replace for golang.org/x/tools/gopls is incorrect because the target of the replace is the module golang.org/x/tools

What did you see instead?

go: github.com/myitcvforks/tools@v0.0.0-20190911230505-6bfd74cf029c: parsing go.mod:
        module declares its path as: golang.org/x/tools
                but was required as: github.com/myitcvforks/tools

cc @bcmills @jayconrod

[bcmills]

Somewhat related to #34085, in that we are reporting the replacement path in place of what should be the module it replaces.

[myitcv]

Ah thanks. Feel free to close if you think it's a duplicate. I searched for "required as" hence I didn't spot #34085

[bcmills]

I don't think the fix for #34085 will address this case, so it's probably better to keep as a separate issue.

[bcmills]

Seems to be fixed as of Go 1.16. (I'm not sure when exactly, but there have been a lot of fixes in the module loader this cycle.)

hello$ go mod tidy
go: golang.org/x/tools/gopls@v0.1.5 (replaced by github.com/myitcvforks/tools@v0.0.0-20190911230505-6bfd74cf029c): parsing go.mod:
        module declares its path as: golang.org/x/tools
                but was required as: golang.org/x/tools/gopls