./logger/vendor/*
./logger/go.mod
./logger/logger.go
./logger/go.sum
./main.go
./main

In main.go:

import "./logger"

It works with go build main.go, but does not work with go build -mod vendor main.go because it says:

build flag -mod=vendor only valid when using modules

Of course it tells me this because go.mod doesn't exist for main.go, so upon creating it through go mod init main, it creates the file with only module main and no dependencies. This doesn't help though, because then it tells me this:

$ go build -mod vendor main.go
build main: cannot find module for path _/home/bmeh/foo/logger

even though logger/go.mod exists, and is correct.

The thing is, go build main.go works, but go build -mod vendor main.go doesn't, so my question is, how do I make it work with go build -mod vendor main.go? Ideally main.go's dependency on ./logger should be in main's go.mod, but it might not support "local" modules for all I know. I'm not exactly sure what's the proper way to use internal, library packages that are used only by this project. The reason I need it for is modularization. Some projects import github.com/user/project/foo/utils, github.com/user/project/foo/logger and so on, but using import "./logger" seems to do fine for me, apart from not working well with vendoring. I also have read something about naming internal packages internal, but I have not came across any usable, working examples (to be honest, I wasn't trying to find any hard enough :P). Any input and suggestions from "experts" would be greatly appreciated.

Worth noting that go build alone will give me back this error:

go build _/home/bmeh/foo: build output "logger" already exists and is a directory

Not sure if this behavior is intended or not either.

Thank you for your help in advance!

Edit:

Changing the import path to the absolute path ("./logger" -> "/home/bmeh/foo/logger") makes go hung up and eat 100% CPU when using go build -mod vendor main.go. No longer gives the aforementioned errors. I believe this is a bug. Please do correct me if I am wrong.

Passing -x:

WORK=/tmp/go-build518771439

The directory is empty.

Partial strace:

futex(0xd618c0, FUTEX_WAIT_PRIVATE, 0, {tv_sec=0, tv_nsec=100000}) = -1 EAGAIN (Resource temporarily unavailable)
futex(0xc000046840, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0xd617f0, FUTEX_WAKE_PRIVATE, 1)  = 1
futex(0xd627c0, FUTEX_WAIT_PRIVATE, 0, NULL

and it just hangs indefinitely.