if err == nil {
		repo = modfetch.Lookup(ctx, proxy, path)
	} else {
		repo = emptyRepo{path: path, err: err}
	}

	if MainModules == nil {
		return repo, err
	} else if _, ok := MainModules.HighestReplaced()[path]; ok {
		return &replacementRepo{repo: repo}, nil
	}

	return repo, err
}

// An emptyRepo is a versionRepo that contains no versions.
type emptyRepo struct {
	path string

	// module (golang.org/issue/26241).
	var mods []module.Version
	if MainModules != nil { // TODO(#48912): Ensure MainModules exists at this point, and remove the check.
		for mp, mv := range MainModules.HighestReplaced() {
			if !maybeInModule(path, mp) {
				continue
			}
			if mv == "" {
				// The only replacement is a wildcard that doesn't specify a version, so