[libcalamares] Make RegionZonePair type stronger

- Derive from QPair instead of being QPair
 - Add isValid() for checking
 - Convenience constructors

src/libcalamares/geoip/GeoIP.cpp

@@ -45,10 +45,10 @@ GeoIP::splitTZString( const QString& tz )
         cDebug() << "GeoIP reporting" << timezoneString;
         QString region = tzParts.takeFirst();
         QString zone = tzParts.join( '/' );
-        return qMakePair( region, zone );
+        return RegionZonePair( region, zone );
     }
 
-    return qMakePair( QString(), QString() );
+    return RegionZonePair( QString(), QString() );
 }
 
 }  // namespace

src/libcalamares/geoip/GeoIP.h

@@ -37,7 +37,25 @@ namespace CalamaresUtils
 class GeoIP
 {
 public:
-    using RegionZonePair = QPair<QString, QString>;
+    /** @brief A Region, Zone pair of strings
+     *
+     * A GeoIP lookup returns a timezone, which is represented as a Region,
+     * Zone pair of strings (e.g. "Europe" and "Amsterdam"). Generally,
+     * pasting the strings back together with a "/" is the right thing to
+     * do. The Zone **may** contain a "/" (e.g. "Kentucky/Monticello").
+     */
+    class RegionZonePair : public QPair<QString, QString>
+    {
+    public:
+        /** @brief Construct from an existing pair. */
+        explicit RegionZonePair( const QPair& p ) : QPair(p) { }
+        /** @brief Construct from two strings, like qMakePair(). */
+        RegionZonePair( const QString& region, const QString& zone ) : QPair( region, zone ) { }
+        /** @brief An invalid zone pair (empty strings). */
+        RegionZonePair() : QPair( QString(), QString() ) { }
+
+        bool isValid() const { return !first.isEmpty(); }
+    } ;
 
     virtual ~GeoIP();
 

src/libcalamares/geoip/GeoIPJSON.cpp

@@ -73,7 +73,7 @@ GeoIPJSON::processReply( const QByteArray& data )
         CalamaresUtils::explainYamlException( e, data, "GeoIP data");
     }
 
-    return qMakePair( QString(), QString() );
+    return RegionZonePair( QString(), QString() );
 }
 
 }  // namespace

src/libcalamares/geoip/GeoIPXML.cpp

@@ -52,14 +52,14 @@ GeoIPXML::processReply( const QByteArray& data )
 
         // None of them valid
         cWarning() << "GeopIP XML had no recognizable timezone";
-        return qMakePair( QString(), QString() );
+        return RegionZonePair( QString(), QString() );
     }
     else
     {
         cWarning() << "GeoIP XML data error:" << domError << "(line" << errorLine << errorColumn << ')';
     }
 
-    return qMakePair( QString(), QString() );
+    return RegionZonePair( QString(), QString() );
 }
 
 }  // namespace