case "gc":
		return &gcimports{
			fset:     fset,
			packages: make(map[string]*types.Package),
			lookup:   lookup,
		}

	case "gccgo":
		var inst gccgoimporter.GccgoInstallation
		if err := inst.InitFromDriver("gccgo"); err != nil {
			return nil
		}
		return &gccgoimports{
			packages: make(map[string]*types.Package),
			importer: inst.GetImporter(nil, nil),
			lookup:   lookup,
		}

	case "source":