Work round macOS's omission of en_DE from its own uiLanguages()

When the system locale is en_DE, macOS seems to think we should use
en_GB as the right translation. While that probably is a sensible
choice in the absence of an en_DE translation, we should definitely
use the en_DE translation if available, especially if en_GB isn't
available (which lead to a fall-back to de_DE, given later entries in
macOS's list). So prepend the system locale's own pcp47Name() if it
(isn't the C locale and) is missing from what we would otherwise have
used for uiLanguages(), after likely sub-tag perturbations.

Add a test simulating (some approximation to) what macOS was doing
that would have caught this case; and add a scope-guard reporter to
the test to report what shows up when lists don't match.

Fixes: QTBUG-104930
Change-Id: I116234708067e1717d9157aebc84da76e04a9f38
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 753bfdf6a1336cc296e5fc8175aedd000c2cc013)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qlocale.cpp

@@ -4419,6 +4419,24 @@ QStringList QLocale::uiLanguages() const
             locales.append(QLocale(entry));
         if (locales.isEmpty())
             locales.append(systemLocale()->fallbackLocale());
+        // If the system locale (isn't C and) didn't include itself in the list,
+        // or as fallback, presume to know better than it and put its name
+        // first. (Known issue, QTBUG-104930, on some macOS versions when in
+        // locale en_DE.) Our translation system might have a translation for a
+        // locale the platform doesn't believe in.
+        const QString name = bcp47Name();
+        if (!name.isEmpty() && language() != C && !uiLanguages.contains(name)) {
+            // That uses contains(name) as a cheap pre-test, but there may be an
+            // entry that matches this on purging likely subtags.
+            const QLocaleId mine = d->m_data->id().withLikelySubtagsRemoved();
+            const auto isMine = [mine](const QString &entry) {
+                return QLocaleId::fromName(entry).withLikelySubtagsRemoved() == mine;
+            };
+            if (std::none_of(uiLanguages.constBegin(), uiLanguages.constEnd(), isMine)) {
+                locales.prepend(*this);
+                uiLanguages.prepend(name);
+            }
+        }
     } else
 #endif
     {

tests/auto/corelib/text/qlocale/tst_qlocale.cpp

@@ -3238,7 +3238,12 @@ public:
 
     QVariant query(QueryType type, QVariant /*in*/) const override
     {
-        return type == UILanguages ? QVariant(QStringList{m_name}) : QVariant();
+        if (type == UILanguages) {
+            if (m_name == u"en-DE") // QTBUG-104930: simulate macOS's list not including m_name.
+                return QVariant(QStringList{QStringLiteral("en-GB"), QStringLiteral("de-DE")});
+            return QVariant(QStringList{m_name});
+        }
+        return QVariant();
     }
 
     QLocale fallbackLocale() const override
@@ -3267,6 +3272,12 @@ void tst_QLocale::systemLocale_data()
     QTest::addRow("ukrainian")
         << QString("uk") << QLocale::Ukrainian
         << QStringList{QStringLiteral("uk"), QStringLiteral("uk-Cyrl-UA"), QStringLiteral("uk-UA")};
+    QTest::addRow("english-germany")
+        << QString("en-DE") << QLocale::English
+        // First two were missed out before fix to QTBUG-104930:
+        << QStringList{QStringLiteral("en-DE"), QStringLiteral("en-Latn-DE"),
+                       QStringLiteral("en-GB"), QStringLiteral("en-Latn-GB"),
+                       QStringLiteral("de-DE"), QStringLiteral("de-Latn-DE"), QStringLiteral("de")};
     QTest::addRow("german")
         << QString("de") << QLocale::German
         << QStringList{QStringLiteral("de"), QStringLiteral("de-Latn-DE"), QStringLiteral("de-DE")};
@@ -3291,7 +3302,11 @@ void tst_QLocale::systemLocale()
         MySystemLocale sLocale(name);
         QCOMPARE(QLocale().language(), language);
         QCOMPARE(QLocale::system().language(), language);
+        auto reporter = qScopeGuard([]() {
+            qDebug("\n\t%s", qPrintable(QLocale::system().uiLanguages().join(u"\n\t")));
+        });
         QCOMPARE(QLocale::system().uiLanguages(), uiLanguages);
+        reporter.dismiss();
     }
 
     QCOMPARE(QLocale(), originalLocale);