cmd/go: allow local replacements without a corresponding explicit requirement?

[bcmills]

In CL 153157, I added code so that a module path that has a replace directive resolves to a pseudo-version of the form vX.0.0-00010101000000-000000000000 rather than checking for upstream dependencies.

That unblocks builds, but means that the module graph may indicate a commit that doesn't actually exist. There are use-cases for such a configuration, such as testing against unpublished commits of upstream modules (see #26241 (comment) and #32776), but since the commit does not actually exist, the requirement graph of the resulting module cannot be used without a corresponding replace directive in every downstream consumer.

Since this pseudo-version has an unambiguous and very unique commit hash (all zeroes!), I propose that we special-case it (if unreplaced) and treat it as equivalent to an empty commit.

CC @jayconrod @thepudds @rogpeppe

[bcmills]

Thinking about this some more, I think using the zero-version was a mistake.

Instead, we shouldn't be writing the require directive out at all: we don't know any valid version to require, and if a downstream consumer requires our module they're better off resolving latest instead.

That seems to support the second case considered in #26344 (comment): this sort of wildcard replace directive effectively means “select the latest version (none), and then select this replacement as if it were a slightly-higher version.”

[gopherbot]

Change https://golang.org/cl/220080 mentions this issue: design/36460: add design for lazy module loading