cc @matloob @bcmills

Module paths without a dot in the first path element are allowed, but they cannot be downloaded using that name. That means they cannot appear in a require directive unless there's also a replace directive that provides an alternative location. So module paths like this really only useful for top-level modules not required by anything.

I don't think this documented anywhere. CL 214377 covers this.

I think this will cause the term module path to be no more consistent.

What is the definition of the term module path?
Some commands say that is a file path, with the first path segment having a dot.
Other commands say that is just a file path.

A module path is an import-path suffix of the packages contained within a module.

It may or may not contain a dot. We should update the “malformed module path” error message to be clearer about what, exactly, is wrong. (The problem is not that the module path is malformed per se, but rather that paths of that form cannot be fetched to the module cache.)

The problem is that it is not entirely true that a module path without a dot in the first path segment can not be fetched. It depends on the resolver.

Consider an import path like localhost/test. I suspect that the go tool would be able to fetch it, if not for the explicit check for the dot.

No, it would not. Dotless import paths are in general reserved for the standard library: if they are not in the standard library, and not supplied via a replace directive, then they will not be fetched even if the hostname can be resolved.

(Otherwise the meaning of an import path could become nondeterministic based on the set of hostnames on the network, which seems way too subtle.)

I tried with a module path localhost.localdomain/a.git but go is not able to fetch it:

build localhost.localdomain/b.git: cannot load localhost.localdomain/a.git: git ls-remote -q git://localhost.localdomain/a in /home/manlio/.local/lib/go/pkg/mod/cache/vcs/05929cb418a01ea28941e79c19cd8cf77a461495799d14636e9011e6b0d72a4a: exit status 128:
	fatal: unable to connect to localhost.localdomain:
	localhost.localdomain[0: ::1]: errno=Connection refused
	localhost.localdomain[1: 127.0.0.1]: errno=Connection refused

So I guess the term remote module path, similar to https://golang.org/pkg/cmd/go/#hdr-Remote_import_paths, is more easy to understand.