cmd/go: does not verify existence of replace directory

[komuw]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myname/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/myname/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="/home/myname/Downloads/test/go.mod"
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-build816763754=/tmp/go-build -gno-record-gcc-switches"

What did you do?

create this go.mod file;

module aha

require (
	github.com/davecgh/go-spew v1.1.1 // indirect
	github.com/gin-contrib/sse v0.0.0-20170109093832-22d885f9ecc7 // indirect
	github.com/gin-gonic/gin v1.3.0 // indirect
	github.com/golang/protobuf v1.2.0 // indirect
	github.com/json-iterator/go v1.1.5 // indirect
	github.com/mattn/go-isatty v0.0.4 // indirect
	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
	github.com/modern-go/reflect2 v1.0.1 // indirect
	github.com/pmezard/go-difflib v1.0.0 // indirect
	github.com/rs/cors v1.5.0
	github.com/stretchr/testify v1.2.2 // indirect
	github.com/ugorji/go/codec v0.0.0-20180927125128-99ea80c8b19a // indirect
	golang.org/x/net v0.0.0-20181005035420-146acd28ed58 // indirect
	golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f // indirect
	golang.org/x/sys v0.0.0-20181004145325-8469e314837c // indirect
	gopkg.in/go-playground/assert.v1 v1.2.1 // indirect
	gopkg.in/go-playground/validator.v8 v8.18.2 // indirect
	gopkg.in/yaml.v2 v2.2.1 // indirect
)

replace github.com/rs/cors/wrapper/gin => ../nonexistentDirectory/AntherDirectoryThatDoesNotExist

and this main.go file;

package main

import (
	"fmt"

	"github.com/rs/cors/wrapper/gin"
)

func main() {
	fmt.Println(gin.Options{})
}

then run;

go mod tidy && \
go mod verify && \
go run main.go

What did you expect to see?

a failure with a nice error message pointing out that the path ../nonexistentDirectory/AntherDirectoryThatDoesNotExist does not exist.

What did you see instead?

all modules verified
{[] <nil> [] [] [] 0 false false false}

[komuw]

very odd; if I however have the following;
go.mod

module aha
require github.com/pkg/errors v0.8.0
replace github.com/pkg/errors => ../nonexistentDirectory/AntherDirectoryThatDoesNotExist

and main.go

package main

import (
	"fmt"

	"github.com/pkg/errors"
)

func main() {
	fmt.Println(errors.New("Hey ya"))
}

I get a nice error message;

go mod tidy && \
go mod verify && \
go run main.go

go: parsing ../nonexistentDirectory/AntherDirectoryThatDoesNotExist/go.mod: open /home/myname/Downloads/nonexistentDirectory/AntherDirectoryThatDoesNotExist/go.mod: no such file or directory
go: error loading module requirements

[komuw]

@gopherbot please add modules

[bcmills]

go mod tidy probably does not report the error because go mod tidy has trouble reporting errors in general (#27063).

go mod verify correctly does not report the error, because it only checks downloaded modules and in this case the replacement is not a downloaded module.

go run main.go correctly does not report the error, because it has no reason to examine the module github.com/rs/cors/wrapper/gin at all: the module at that path is not in the set of active modules, and the import in main.go can be satisfied from a different module that is active (specifically, github.com/rs/cors v1.5.0).

[bcmills]

I'm not sure that go mod tidy should report an error here either. It does not generally change replace or exclude directives, and the require directives here are consistent and satisfy all of the imports of package main.

[bcmills]

There are three possible directions we could go here:

1. We could make go mod tidy verify the consistency of replace directives even if they are not used.
2. We could make go mod tidy remove replace directives that have no effect.
3. We could leave the behavior of go mod tidy as is.

I'm not sure which of those makes the most sense. I'll discuss with @rsc.