Deprecate QFont::lastResortFamily() and QFont::lastResortFont()

These functions have not been used by the font selection algorithm
in Qt 5. Calling lastResortFamily() always gave you "helvetica"
and lastResortFont() would actually crash.

Since we cannot remove them before Qt 6, we deprecate them for now
and update the documentation to give a more accurate description
of how the font matching works in Qt 5.

Note that in the non-native QFontDialog, it tries to implement its
own font matching algorithm and therefore was calling
lastResortFamily(). Since fixing QFontDialog is not within the
scope of this patch, I have just replaced the call by "helvetica"
directly, so that it will continue to behave the same.

[ChangeLog][QtGui][Text] Deprecated QFont::lastResortFamily()
and QFont::lastResortFont() which are not in use in Qt 5 and
did not provide any useful information.

Task-number: QTBUG-60813
Change-Id: Id8fe7bfdd427292ef86bc8405de03cb466b9b62a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/text/qfont.cpp

@@ -340,7 +340,7 @@ QFontEngineData::~QFontEngineData()
     \class QFont
     \reentrant
 
-    \brief The QFont class specifies a font used for drawing text.
+    \brief The QFont class specifies a query for a font used for drawing text.
 
     \ingroup painting
     \ingroup appearance
@@ -348,6 +348,7 @@ QFontEngineData::~QFontEngineData()
     \ingroup richtext-processing
     \inmodule QtGui
 
+    QFont can be regarded as a query for one or more fonts on the system.
 
     When you create a QFont object you specify various attributes that
     you want the font to have. Qt will use the font with the specified
@@ -355,9 +356,15 @@ QFontEngineData::~QFontEngineData()
     matching installed font. The attributes of the font that is
     actually used are retrievable from a QFontInfo object. If the
     window system provides an exact match exactMatch() returns \c true.
-    Use QFontMetrics to get measurements, e.g. the pixel length of a
+    Use QFontMetricsF to get measurements, e.g. the pixel length of a
     string using QFontMetrics::width().
 
+    Attributes which are not specifically set will not affect the font
+    selection algorithm, and default values will be preferred instead.
+
+    To load a specific physical font, typically represented by a single file,
+    use QRawFont instead.
+
     Note that a QGuiApplication instance must exist before a QFont can be
     used. You can set the application's default font with
     QGuiApplication::setFont().
@@ -390,8 +397,6 @@ QFontEngineData::~QFontEngineData()
     setStyleHint(). The default family (corresponding to the current
     style hint) is returned by defaultFamily().
 
-    The font-matching algorithm has a lastResortFamily() and
-    lastResortFont() in cases where a suitable match cannot be found.
     You can provide substitutions for font family names using
     insertSubstitution() and insertSubstitutions(). Substitutions can
     be removed with removeSubstitutions(). Use substitute() to retrieve
@@ -419,18 +424,20 @@ QFontEngineData::~QFontEngineData()
     \target fontmatching
     The font matching algorithm works as follows:
     \list 1
-    \li The specified font family is searched for.
-    \li If not found, the styleHint() is used to select a replacement
-       family.
-    \li Each replacement font family is searched for.
-    \li If none of these are found or there was no styleHint(), "helvetica"
-       will be searched for.
-    \li If "helvetica" isn't found Qt will try the lastResortFamily().
-    \li If the lastResortFamily() isn't found Qt will try the
-       lastResortFont() which will always return a name of some kind.
+    \li If the specified font family exists and can be used to represent
+        the writing system in use, it will be selected.
+    \li If not, a replacement font that supports the writing system is
+        selected. The font matching algorithm will try to find the
+        best match for all the properties set in the QFont. How this is
+        done varies from platform to platform.
+    \li If no font exists on the system that can support the text,
+        then special "missing character" boxes will be shown in its place.
     \endlist
 
-    Note that the actual font matching algorithm varies from platform to platform.
+    \note If the selected font, though supporting the writing system in general,
+    is missing glyphs for one or more specific characters, then Qt will try to
+    find a fallback font for this or these particular characters. This feature
+    can be disabled using QFont::NoFontMerging style strategy.
 
     In Windows a request for the "Courier" font is automatically changed to
     "Courier New", an improved version of Courier that allows for smooth scaling.
@@ -2133,14 +2140,14 @@ void QFont::cacheStatistics()
 {
 }
 
+#if QT_DEPRECATED_SINCE(5, 13)
 /*!
     \fn QString QFont::lastResortFamily() const
 
-    Returns the "last resort" font family name.
+    \obsolete
 
-    The current implementation tries a wide variety of common fonts,
-    returning the first one it finds. Is is possible that no family is
-    found in which case an empty string is returned.
+    This function is deprecated and is not in use by the font
+    selection algorithm in Qt 5. It always returns "helvetica".
 
     \sa lastResortFont()
 */
@@ -2148,6 +2155,7 @@ QString QFont::lastResortFamily() const
 {
     return QString::fromLatin1("helvetica");
 }
+#endif
 
 extern QStringList qt_fallbacksForFamily(const QString &family, QFont::Style style,
                                          QFont::StyleHint styleHint, QChar::Script script);
@@ -2169,33 +2177,20 @@ QString QFont::defaultFamily() const
     return QString();
 }
 
+#if QT_DEPRECATED_SINCE(5, 13)
 /*!
     \fn QString QFont::lastResortFont() const
 
-    Returns a "last resort" font name for the font matching algorithm.
-    This is used if the last resort family is not available. It will
-    always return a name, if necessary returning something like
-    "fixed" or "system".
+    \obsolete
 
-    The current implementation tries a wide variety of common fonts,
-    returning the first one it finds. The implementation may change
-    at any time, but this function will always return a string
-    containing something.
-
-    It is theoretically possible that there really isn't a
-    lastResortFont() in which case Qt will abort with an error
-    message. We have not been able to identify a case where this
-    happens. Please \l{bughowto.html}{report it as a bug} if
-    it does, preferably with a list of the fonts you have installed.
-
-    \sa lastResortFamily()
+    Deprecated function. Since Qt 5.0, this is not used by the font selection algorithm. For
+    compatibility it remains in the API, but will always return the same value as lastResortFamily().
 */
 QString QFont::lastResortFont() const
 {
-    qFatal("QFont::lastResortFont: Cannot find any reasonable font");
-    // Shut compiler up
-    return QString();
+    return lastResortFamily();
 }
+#endif
 
 /*****************************************************************************
   QFont stream functions

src/gui/text/qfont.h

@@ -282,8 +282,10 @@ public:
     static void cacheStatistics();
 
     QString defaultFamily() const;
-    QString lastResortFamily() const;
-    QString lastResortFont() const;
+#if QT_DEPRECATED_SINCE(5, 13)
+    QT_DEPRECATED QString lastResortFamily() const;
+    QT_DEPRECATED QString lastResortFont() const;
+#endif
 
     QFont resolve(const QFont &) const;
     inline uint resolve() const { return resolve_mask; }

src/widgets/dialogs/qfontdialog.cpp

@@ -534,7 +534,7 @@ void QFontDialogPrivate::updateFamilies()
 
         //and try some fall backs
         match_t type = MATCH_NONE;
-        if (bestFamilyType <= MATCH_NONE && familyName2 == f.lastResortFamily())
+        if (bestFamilyType <= MATCH_NONE && familyName2 == QStringLiteral("helvetica"))
             type = MATCH_LAST_RESORT;
         if (bestFamilyType <= MATCH_LAST_RESORT && familyName2 == f.family())
             type = MATCH_APP;