cmd/vet: GoVersion=="" on standard library packages

[timothy-king]

Within standard library packages the GoVersion sent to unitchecker is "" for standard library packages. This can be recreated from gotip vet -x ./... from $GOROOT/src (which will be in a module).

This GoVersion is passed along to go/types in types.Config. When this is empty (which is an invalid version), (*types.Info).FileVersions is not populated. The loopclosure vet check uses FileVersions to decide what is the semantics of a given for loop is. So there will now be a divergence in behavior for cmd/vet when running on the standard libraries.

The relevant code to set the field is in buildVetConfig:

	if a.Package.Module != nil {
		v := a.Package.Module.GoVersion
		if v == "" {
			v = gover.DefaultGoModVersion
		}
		vcfg.GoVersion = "go" + v
	}

A hunch for why a.Package.Module is nil is that it comes from PackageModuleInfo which documents that nil is expected:

// PackageModuleInfo returns information about the module that provides
// a given package. If modules are not enabled or if the package is in the
// standard library or if the package was not successfully loaded with
// LoadPackages or ImportFromFiles, nil is returned.
func PackageModuleInfo(ctx context.Context, pkgpath string) *modinfo.ModulePublic {

cc @bcmills

[bcmills]

@timothy-king, if you can write a regression test, I think I see the fix. Probably something like:

	if a.Package.Module != nil {
		v := a.Package.Module.GoVersion
		if v == "" {
			v = gover.DefaultGoModVersion
		}
		vcfg.GoVersion = "go" + v
	} else if a.Package.Standard {
		vcfg.GoVersion = gover.LocalToolchain()
	}

[samthanawalla]

Addressed in #65612
Should be fixed now :)

GoVersion will be set to the local go version in this case.