Improve the default selection of locale for QCollator

QCollator was using the default-constructed QLocale() as its default;
this locale's default was the system locale; however, that didn't pay
any attention to system settings for collation such as Unix's
environment variable LC_COLLATE or the MS-Win LOCALE_SSORTLOCALE
configuration option.

Teach the system locale back-ends to look up their relevant settings,
add QLocale::collation() as a channel via which to access that and
change no-parameter construction of QCollator to a separate
implementation (rather than the constructor taking QLocale having a
default) using it.

[ChangeLog][QtCore][QLocale] The system locale now knows what to use
for collation, QLocale::system().collation().

[ChangeLog][QtCore][QCollator] The default QCollator now uses the
system's collation locale, rather than the system locale itself.

Fixes: QTBUG-58621
Change-Id: I90f86621541385330315d1f9d6a4b982bdcea9d5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcollator.cpp

@@ -46,7 +46,6 @@
 
 QT_BEGIN_NAMESPACE
 
-
 /*!
     \class QCollator
     \inmodule QtCore
@@ -71,9 +70,20 @@ QT_BEGIN_NAMESPACE
 */
 
 /*!
-    Constructs a QCollator for \a locale.
+  \since 5.13
 
-    If \a locale is not specified, the system's default locale is used.
+  Constructs a QCollator using the system's default collation locale.
+
+  \sa setLocale(), QLocale::collation()
+*/
+QCollator::QCollator()
+    : d(new QCollatorPrivate(QLocale::system().collation()))
+{
+    d->init();
+}
+
+/*!
+    Constructs a QCollator from \a locale.
 
     \sa setLocale()
  */

src/corelib/tools/qcollator.h

@@ -81,7 +81,8 @@ inline bool operator<(const QCollatorSortKey &lhs, const QCollatorSortKey &rhs)
 class Q_CORE_EXPORT QCollator
 {
 public:
-    explicit QCollator(const QLocale &locale = QLocale());
+    QCollator();
+    explicit QCollator(const QLocale &locale);
     QCollator(const QCollator &);
     ~QCollator();
     QCollator &operator=(const QCollator &);

src/corelib/tools/qlocale.cpp

@@ -4131,6 +4131,29 @@ QStringList QLocale::uiLanguages() const
     return uiLanguages;
 }
 
+/*!
+  \since 5.13
+
+  Returns the locale to use for collation.
+
+  The result is usually this locale; however, the system locale (which is
+  commonly the default locale) will return the system collation locale.
+  The result is suitable for passing to QCollator's constructor.
+
+  \sa QCollator
+*/
+QLocale QLocale::collation() const
+{
+#ifndef QT_NO_SYSTEMLOCALE
+    if (d->m_data == systemData()) {
+        QString res = systemLocale()->query(QSystemLocale::Collation, QVariant()).toString();
+        if (!res.isEmpty())
+            return QLocale(res);
+    }
+#endif
+    return *this;
+}
+
 /*!
     \since 4.8
 

src/corelib/tools/qlocale.h

@@ -1048,7 +1048,7 @@ public:
     QString pmText() const;
 
     MeasurementSystem measurementSystem() const;
-
+    QLocale collation() const;
     Qt::LayoutDirection textDirection() const;
 
     QString toUpper(const QString &str) const;

src/corelib/tools/qlocale_p.h

@@ -113,6 +113,7 @@ public:
         Weekdays, // QList<Qt::DayOfWeek>
         CurrencySymbol, // QString in: CurrencyToStringArgument
         CurrencyToString, // QString in: qlonglong, qulonglong or double
+        Collation, // QString
         UILanguages, // QStringList
         StringToStandardQuotation, // QString in: QStringRef to quote
         StringToAlternateQuotation, // QString in: QStringRef to quote

src/corelib/tools/qlocale_unix.cpp

@@ -69,6 +69,7 @@ struct QSystemLocaleData
     QLocale lc_messages;
     QByteArray lc_messages_var;
     QByteArray lc_measurement_var;
+    QByteArray lc_collate_var;
     QStringList uiLanguages;
 };
 
@@ -82,6 +83,7 @@ void QSystemLocaleData::readEnvironment()
     QByteArray monetary = all.isEmpty() ? qgetenv("LC_MONETARY") : all;
     lc_messages_var     = all.isEmpty() ? qgetenv("LC_MESSAGES") : all;
     lc_measurement_var  = all.isEmpty() ? qgetenv("LC_MEASUREMENT") : all;
+    lc_collate_var      = all.isEmpty() ? qgetenv("LC_COLLATE") : all;
     QByteArray lang = qgetenv("LANG");
     if (lang.isEmpty())
         lang = QByteArray("C");
@@ -95,6 +97,8 @@ void QSystemLocaleData::readEnvironment()
         lc_messages_var = lang;
     if (lc_measurement_var.isEmpty())
         lc_measurement_var = lang;
+    if (lc_collate_var.isEmpty())
+        lc_collate_var = lang;
     lc_numeric = QLocale(QString::fromLatin1(numeric));
     lc_time = QLocale(QString::fromLatin1(time));
     lc_monetary = QLocale(QString::fromLatin1(monetary));
@@ -247,13 +251,15 @@ QVariant QSystemLocale::query(QueryType type, QVariant in) const
         return QString();
     }
     case MeasurementSystem: {
-        const QString meas_locale = QString::fromLatin1(d->lc_measurement_var.constData(), d->lc_measurement_var.size());
+        const QString meas_locale = QString::fromLatin1(d->lc_measurement_var);
         if (meas_locale.compare(QLatin1String("Metric"), Qt::CaseInsensitive) == 0)
             return QLocale::MetricSystem;
         if (meas_locale.compare(QLatin1String("Other"), Qt::CaseInsensitive) == 0)
             return QLocale::MetricSystem;
         return QVariant((int)QLocale(meas_locale).measurementSystem());
     }
+    case Collation:
+        return QString::fromLatin1(d->lc_collate_var);
     case UILanguages: {
         if (!d->uiLanguages.isEmpty())
             return d->uiLanguages;

src/corelib/tools/qlocale_win.cpp

@@ -120,6 +120,7 @@ struct QSystemLocalePrivate
     QVariant toString(const QTime &, QLocale::FormatType);
     QVariant toString(const QDateTime &, QLocale::FormatType);
     QVariant measurementSystem();
+    QVariant collation();
     QVariant amText();
     QVariant pmText();
     QVariant firstDayOfWeek();
@@ -455,6 +456,11 @@ QVariant QSystemLocalePrivate::measurementSystem()
     return QLocale::MetricSystem;
 }
 
+QVariant QSystemLocalePrivate::collation()
+{
+    return getLocaleInfo(LOCALE_SSORTLOCALE);
+}
+
 QVariant QSystemLocalePrivate::amText()
 {
     wchar_t output[15]; // maximum length including  terminating zero character for Win2003+
@@ -808,6 +814,8 @@ QVariant QSystemLocale::query(QueryType type, QVariant in = QVariant()) const
         return QVariant(QLocale::AnyScript);
     case MeasurementSystem:
         return d->measurementSystem();
+    case Collation:
+        return d->collation();
     case AMText:
         return d->amText();
     case PMText: