cmd/go: no required module provides package main.go; to add it

[korzhao]

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

$ go version
go1.17

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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/kezhao/Library/Caches/go-build"
GOENV="/Users/kezhao/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/kezhao/go/pkg/mod"
GONOPROXY="git.code.oa.com,git.woa.com"
GONOSUMDB="git.code.oa.com,git.woa.com"
GOOS="darwin"
GOPATH="/Users/kezhao/go"
GOPRIVATE="git.code.oa.com,git.woa.com"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/kezhao/google/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/kezhao/google/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="devel go1.18-8dd1dd5b7f Sun Aug 29 06:19:02 2021 +0800"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/kezhao/go/src/apnstest/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/2g/yv3ln7wj5vbdtwfj0js10krh0000gn/T/go-build1388352891=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

rm main.go
go build main.go

What did you expect to see?

stat main.go: no such file or directory

What did you see instead?

$ go build main.go
no required module provides package main.go; to add it:
go get main.go

$ go get main.go
go get main.go: arguments must be package or module paths

[jayconrod]

cc @bcmills @matloob

This is a consequence of #34653, so maybe it's working as intended. We treat an argument ending with .go as a package path, not a source file, if the source file with that name doesn't exist. That's necessary for github.com/nats/nats.go.

[bcmills]

The error message from go build is not wrong: main.go could be either a file, or a package (perhaps one provided by a local proxy, workspace, or replace directive).

That said, we should probably not suggest go get on a path ending in .go (since the cause is more likely to be a missing file than a missing package), and we probably should mention no such file: there are two possible interpretations of the argument, and we should explain why both interpretations fail.

[bcmills]

The error message from go get main.go does look outright wrong: go get should not assume that paths ending in .go are file paths, and AFAICT main.go is also valid module path. So it's fine for go get main.go to report that main.go does not exist as a file, but it also ought to see if a module main.go exists.

[jayconrod]

PackagesAndErrors only calls GoFilesPackage if main.go exists; otherwise it treats it like a package path, which I think is correct.

Maybe ImportMissingError just needs a special case for paths ending with .go?