diff --git a/src/libsync/nextcloudtheme.h b/src/libsync/nextcloudtheme.h
index f7c215b8f..6ad138615 100644
--- a/src/libsync/nextcloudtheme.h
+++ b/src/libsync/nextcloudtheme.h
@@ -29,7 +29,7 @@ class NextcloudTheme : public Theme
 public:
     NextcloudTheme();
 
-    QString wizardUrlHint() const override;
+    [[nodiscard]] QString wizardUrlHint() const override;
 };
 }
 #endif // NEXTCLOUD_THEME_H
diff --git a/src/libsync/theme.h b/src/libsync/theme.h
index 5c805fd8f..a936ebc93 100644
--- a/src/libsync/theme.h
+++ b/src/libsync/theme.h
@@ -90,7 +90,7 @@ public:
      *
      * @return true if branded, false otherwise
      */
-    virtual bool isBranded() const;
+    [[nodiscard]] bool isBranded() const;
 
     /**
      * @brief appNameGUI - Human readable application name.
@@ -103,7 +103,7 @@ public:
      *
      * @return QString with human readable app name.
      */
-    virtual QString appNameGUI() const;
+    [[nodiscard]] QString appNameGUI() const;
 
     /**
      * @brief appName - Application name (short)
@@ -121,61 +121,61 @@ public:
      *
      * @return QString with app name.
      */
-    virtual QString appName() const;
+    [[nodiscard]] QString appName() const;
 
     /**
      * @brief Returns full path to an online state icon
      * @return QUrl full path to an icon
      */
-    QUrl stateOnlineImageSource() const;
+    [[nodiscard]] QUrl stateOnlineImageSource() const;
 
     /**
      * @brief Returns full path to an offline state icon
      * @return QUrl full path to an icon
      */
-    QUrl stateOfflineImageSource() const;
+    [[nodiscard]] QUrl stateOfflineImageSource() const;
     
     /**
      * @brief Returns full path to an online user status icon
      * @return QUrl full path to an icon
      */
-    QUrl statusOnlineImageSource() const;
+    [[nodiscard]] QUrl statusOnlineImageSource() const;
     
     /**
      * @brief Returns full path to an do not disturb user status icon
      * @return QUrl full path to an icon
      */
-    QUrl statusDoNotDisturbImageSource() const;
+    [[nodiscard]] QUrl statusDoNotDisturbImageSource() const;
     
     /**
      * @brief Returns full path to an away user status icon
      * @return QUrl full path to an icon
      */
-    QUrl statusAwayImageSource() const;
+    [[nodiscard]] QUrl statusAwayImageSource() const;
     
     /**
      * @brief Returns full path to an invisible user status icon
      * @return QUrl full path to an icon
      */
-    QUrl statusInvisibleImageSource() const;
+    [[nodiscard]] QUrl statusInvisibleImageSource() const;
 
-    QUrl syncStatusOk() const;
+    [[nodiscard]] QUrl syncStatusOk() const;
 
-    QUrl syncStatusError() const;
+    [[nodiscard]] QUrl syncStatusError() const;
 
-    QUrl syncStatusRunning() const;
+    [[nodiscard]] QUrl syncStatusRunning() const;
 
-    QUrl syncStatusPause() const;
+    [[nodiscard]] QUrl syncStatusPause() const;
 
-    QUrl syncStatusWarning() const;
+    [[nodiscard]] QUrl syncStatusWarning() const;
 
-    QUrl folderOffline() const;
+    [[nodiscard]] QUrl folderOffline() const;
 
     /**
      * @brief configFileName
      * @return the name of the config file.
      */
-    virtual QString configFileName() const;
+    [[nodiscard]] QString configFileName() const;
 
 #ifndef TOKEN_AUTH_ONLY
     static QString hidpiFileName(const QString &fileName, QPaintDevice *dev = nullptr);
@@ -187,25 +187,25 @@ public:
     /**
       * get an sync state icon
       */
-    virtual QIcon syncStateIcon(SyncResult::Status, bool sysTray = false) const;
+    [[nodiscard]] QIcon syncStateIcon(SyncResult::Status, bool sysTray = false) const;
 
-    virtual QIcon folderDisabledIcon() const;
-    virtual QIcon folderOfflineIcon(bool sysTray = false) const;
-    virtual QIcon applicationIcon() const;
+    [[nodiscard]] QIcon folderDisabledIcon() const;
+    [[nodiscard]] QIcon folderOfflineIcon(bool sysTray = false) const;
+    [[nodiscard]] QIcon applicationIcon() const;
 #endif
 
-    virtual QString statusHeaderText(SyncResult::Status) const;
-    virtual QString version() const;
+    [[nodiscard]] QString statusHeaderText(SyncResult::Status) const;
+    [[nodiscard]] QString version() const;
 
     /**
      * Characteristics: bool if more than one sync folder is allowed
      */
-    virtual bool singleSyncFolder() const;
+    [[nodiscard]] bool singleSyncFolder() const;
 
     /**
      * When true, client works with multiple accounts.
      */
-    virtual bool multiAccount() const;
+    [[nodiscard]] bool multiAccount() const;
 
     /**
     * URL to documentation.
@@ -217,7 +217,7 @@ public:
     *
     * Defaults to Nextclouds client documentation website.
     */
-    virtual QString helpUrl() const;
+    [[nodiscard]] QString helpUrl() const;
 
     /**
      * The url to use for showing help on conflicts.
@@ -228,76 +228,79 @@ public:
      * documentation website. If helpUrl() is empty, this function will also return the
      * empty string.
      */
-    virtual QString conflictHelpUrl() const;
+    [[nodiscard]] QString conflictHelpUrl() const;
 
     /**
      * Setting a value here will pre-define the server url.
      *
      * The respective UI controls will be disabled only if forceOverrideServerUrl() is true
      */
-    virtual QString overrideServerUrl() const;
+    [[nodiscard]] QString overrideServerUrl() const;
 
     /**
      * Enforce a pre-defined server url.
      *
      * When true, the respective UI controls will be disabled
      */
-    virtual bool forceOverrideServerUrl() const;
+    [[nodiscard]] bool forceOverrideServerUrl() const;
 
-    virtual bool isVfsEnabled() const;
+    /**
+     * Enforce use of virtual files whenever possible.
+     */
+    [[nodiscard]] bool isVfsEnabled() const;
 
     /**
      * Automatically start login flow
      *
      * When true, the browser will get opened automatically
      */
-    virtual bool startLoginFlowAutomatically() const;
+    [[nodiscard]] bool startLoginFlowAutomatically() const;
     
     /**
      * Enable OCSP stapling for SSL handshakes
      *
      * When true, peer will be requested for Online Certificate Status Protocol response
      */
-    virtual bool enableStaplingOCSP() const;
+    [[nodiscard]] bool enableStaplingOCSP() const;
 
     /**
      * Enforce SSL validity
      *
      * When true, trusting the untrusted certificate is not allowed
      */
-    virtual bool forbidBadSSL() const;
+    [[nodiscard]] bool forbidBadSSL() const;
 
     /**
      * Forbid use of proxy
      *
      * When true, the app always connects to the server directly
      */
-    virtual bool doNotUseProxy() const;
+    [[nodiscard]] bool doNotUseProxy() const;
 
     /**
      * This is only usefull when previous version had a different overrideServerUrl
      * with a different auth type in that case You should then specify "http" or "shibboleth".
      * Normaly this should be left empty.
      */
-    virtual QString forceConfigAuthType() const;
+    [[nodiscard]] QString forceConfigAuthType() const;
 
     /**
      * The default folder name without path on the server at setup time.
      */
-    virtual QString defaultServerFolder() const;
+    [[nodiscard]] QString defaultServerFolder() const;
 
     /**
      * The default folder name without path on the client side at setup time.
      */
-    virtual QString defaultClientFolder() const;
+    [[nodiscard]] QString defaultClientFolder() const;
 
     /**
      * Override to encforce a particular locale, i.e. "de" or "pt_BR"
      */
-    virtual QString enforcedLocale() const { return QString(); }
+    [[nodiscard]] QString enforcedLocale() const { return QString(); }
 
     /** colored, white or black */
-    QString systrayIconFlavor(bool mono) const;
+    [[nodiscard]] QString systrayIconFlavor(bool mono) const;
 
 #ifndef TOKEN_AUTH_ONLY
     /**
@@ -305,18 +308,18 @@ public:
      * The default implementation will try to look up
      * :/client/theme/<type>.png
      */
-    virtual QVariant customMedia(CustomMediaType type);
+    [[nodiscard]] QVariant customMedia(CustomMediaType type);
 
     /** @return color for the setup wizard */
-    virtual QColor wizardHeaderTitleColor() const;
+    [[nodiscard]] QColor wizardHeaderTitleColor() const;
 
     /** @return color for the setup wizard. */
-    virtual QColor wizardHeaderBackgroundColor() const;
+    [[nodiscard]] QColor wizardHeaderBackgroundColor() const;
 
-    virtual QPixmap wizardApplicationLogo() const;
+    [[nodiscard]] QPixmap wizardApplicationLogo() const;
 
     /** @return logo for the setup wizard. */
-    virtual QPixmap wizardHeaderLogo() const;
+    [[nodiscard]] QPixmap wizardHeaderLogo() const;
 
     /**
      * The default implementation creates a
@@ -325,23 +328,23 @@ public:
      *
      * @return banner for the setup wizard.
      */
-    virtual QPixmap wizardHeaderBanner() const;
+    [[nodiscard]] QPixmap wizardHeaderBanner() const;
 #endif
 
     /**
      * The SHA sum of the released git commit
      */
-    QString gitSHA1() const;
+    [[nodiscard]] QString gitSHA1() const;
 
     /**
      * About dialog contents
      */
-    virtual QString about() const;
+    [[nodiscard]] QString about() const;
 
     /**
      * Legal notice dialog version detail contents
      */
-    virtual QString aboutDetails() const;
+    [[nodiscard]] QString aboutDetails() const;
 
     /**
      * Define if the systray icons should be using mono design
@@ -351,56 +354,56 @@ public:
     /**
      * Retrieve wether to use mono icons for systray
      */
-    bool systrayUseMonoIcons() const;
+    [[nodiscard]] bool systrayUseMonoIcons() const;
 
     /**
      * Check if mono icons are available
      */
-    bool monoIconsAvailable() const;
+    [[nodiscard]] bool monoIconsAvailable() const;
 
     /**
      * @brief Where to check for new Updates.
      */
-    virtual QString updateCheckUrl() const;
+    [[nodiscard]] QString updateCheckUrl() const;
 
     /**
      * When true, the setup wizard will show the selective sync dialog by default and default
      * to nothing selected
      */
-    virtual bool wizardSelectiveSyncDefaultNothing() const;
+    [[nodiscard]] bool wizardSelectiveSyncDefaultNothing() const;
 
     /**
      * Default option for the newBigFolderSizeLimit.
      * Size in MB of the maximum size of folder before we ask the confirmation.
      * Set -1 to never ask confirmation.  0 to ask confirmation for every folder.
      **/
-    virtual qint64 newBigFolderSizeLimit() const;
+    [[nodiscard]] qint64 newBigFolderSizeLimit() const;
 
     /**
      * Hide the checkbox that says "Ask for confirmation before synchronizing folders larger than X MB"
      * in the account wizard
      */
-    virtual bool wizardHideFolderSizeLimitCheckbox() const;
+    [[nodiscard]] bool wizardHideFolderSizeLimitCheckbox() const;
     /**
      * Hide the checkbox that says "Ask for confirmation before synchronizing external storages"
      * in the account wizard
      */
-    virtual bool wizardHideExternalStorageConfirmationCheckbox() const;
+    [[nodiscard]] bool wizardHideExternalStorageConfirmationCheckbox() const;
 
     /**
      * @brief Sharing options
      *
      * Allow link sharing and or user/group sharing
      */
-    virtual bool linkSharing() const;
-    virtual bool userGroupSharing() const;
+    [[nodiscard]] bool linkSharing() const;
+    [[nodiscard]] bool userGroupSharing() const;
 
     /**
      * If this returns true, the user cannot configure the proxy in the network settings.
      * The proxy settings will be disabled in the configuration dialog.
      * Default returns false.
      */
-    virtual bool forceSystemNetworkProxy() const;
+    [[nodiscard]] bool forceSystemNetworkProxy() const;
 
     /**
      * @brief How to handle the userID
@@ -417,7 +420,7 @@ public:
      *
      *  @return UserIDType::UserIDUserName, unless reimplemented
      */
-    virtual UserIDType userIDType() const;
+    [[nodiscard]] UserIDType userIDType() const;
 
     /**
      * @brief Allows to customize the type of user ID (e.g. user name, email)
@@ -427,7 +430,7 @@ public:
      *
      * @return An empty string, unless reimplemented
      */
-    virtual QString customUserID() const;
+    [[nodiscard]] QString customUserID() const;
 
     /**
      * @brief Demo string to be displayed when no text has been
@@ -435,7 +438,7 @@ public:
      *
      * @return An empty string, unless reimplemented
      */
-    virtual QString userIDHint() const;
+    [[nodiscard]] QString userIDHint() const;
 
     /**
      * @brief Postfix that will be enforced in a URL. e.g.
@@ -443,14 +446,14 @@ public:
      *
      * @return An empty string, unless reimplemented
      */
-    virtual QString wizardUrlPostfix() const;
+    [[nodiscard]] QString wizardUrlPostfix() const;
 
     /**
      * @brief String that will be shown as long as no text has been entered by the user.
      *
      * @return An empty string, unless reimplemented
      */
-    virtual QString wizardUrlHint() const;
+    [[nodiscard]] virtual QString wizardUrlHint() const;
 
     /**
      * @brief the server folder that should be queried for the quota information
@@ -461,14 +464,14 @@ public:
      *
      * Defaults: "/"
      */
-    virtual QString quotaBaseFolder() const;
+    [[nodiscard]] QString quotaBaseFolder() const;
 
     /**
      * The OAuth client_id, secret pair.
      * Note that client that change these value cannot connect to un-branded owncloud servers.
      */
-    virtual QString oauthClientId() const;
-    virtual QString oauthClientSecret() const;
+    [[nodiscard]] QString oauthClientId() const;
+    [[nodiscard]] QString oauthClientSecret() const;
 
     /**
      * @brief What should be output for the --version command line switch.
@@ -476,7 +479,7 @@ public:
      * By default, it's a combination of appName(), version(), the GIT SHA1 and some
      * important dependency versions.
      */
-    virtual QString versionSwitchOutput() const;
+    [[nodiscard]] QString versionSwitchOutput() const;
 	
 	/**
     * @brief Request suitable QIcon resource depending on the background colour of the parent widget.
@@ -484,7 +487,7 @@ public:
     * This should be replaced (TODO) by a real theming implementation for the client UI 
     * (actually 2019/09/13 only systray theming).
     */
-	virtual QIcon uiThemeIcon(const QString &iconName, bool uiHasDarkBg) const;
+    [[nodiscard]] QIcon uiThemeIcon(const QString &iconName, bool uiHasDarkBg) const;
 
     Q_INVOKABLE static double getColorDarkness(const QColor &color);
 
@@ -584,9 +587,9 @@ public:
      * By default, the options are not shown unless experimental options are
      * manually enabled in the configuration file.
      */
-    virtual bool showVirtualFilesOption() const;
+    [[nodiscard]] bool showVirtualFilesOption() const;
 
-    virtual bool enforceVirtualFilesSyncFolder() const;
+    [[nodiscard]] bool enforceVirtualFilesSyncFolder() const;
 
     static QColor defaultColor();
 
@@ -596,10 +599,10 @@ public:
     bool darkMode();
 
 public slots:
-    virtual void setOverrideServerUrl(const QString &overrideServerUrl);
-    virtual void setForceOverrideServerUrl(bool forceOverride);
-    virtual void setVfsEnabled(bool enabled);
-    virtual void setStartLoginFlowAutomatically(bool startLoginFlowAuto);
+    void setOverrideServerUrl(const QString &overrideServerUrl);
+    void setForceOverrideServerUrl(bool forceOverride);
+    void setVfsEnabled(bool enabled);
+    void setStartLoginFlowAutomatically(bool startLoginFlowAuto);
 
 protected:
 #ifndef TOKEN_AUTH_ONLY