4 files changed, 28 insertions, 2 deletions
diff --git a/src/corelib/tools/qtimezone.cpp b/src/corelib/tools/qtimezone.cpp
index 445ddeb31d..db6be581ec 100644
--- a/src/corelib/tools/qtimezone.cpp
+++ b/src/corelib/tools/qtimezone.cpp
@@ -820,8 +820,8 @@ bool QTimeZone::isTimeZoneIdAvailable(const QByteArray &ianaId)
     // IDs as availableTimeZoneIds() may be slow
     if (!QTimeZonePrivate::isValidId(ianaId))
         return false;
-    const QList<QByteArray> tzIds = availableTimeZoneIds();
-    return std::binary_search(tzIds.begin(), tzIds.end(), ianaId);
+    return QUtcTimeZonePrivate().isTimeZoneIdAvailable(ianaId) ||
+           global_tz->backend->isTimeZoneIdAvailable(ianaId);
 }
 
 static QList<QByteArray> set_union(const QList<QByteArray> &l1, const QList<QByteArray> &l2)
diff --git a/src/corelib/tools/qtimezoneprivate.cpp b/src/corelib/tools/qtimezoneprivate.cpp
index 1a5135f103..2b6c2ea6a5 100644
--- a/src/corelib/tools/qtimezoneprivate.cpp
+++ b/src/corelib/tools/qtimezoneprivate.cpp
@@ -486,6 +486,13 @@ QByteArray QTimeZonePrivate::systemTimeZoneId() const
     return QByteArray();
 }
 
+bool QTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray& ianaId) const
+{
+    // Fall-back implementation, can be made faster in subclasses
+    const QList<QByteArray> tzIds = availableTimeZoneIds();
+    return std::binary_search(tzIds.begin(), tzIds.end(), ianaId);
+}
+
 QList<QByteArray> QTimeZonePrivate::availableTimeZoneIds() const
 {
     return QList<QByteArray>();
@@ -864,6 +871,17 @@ QByteArray QUtcTimeZonePrivate::systemTimeZoneId() const
     return utcQByteArray();
 }
 
+bool QUtcTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
+{
+    for (int i = 0; i < utcDataTableSize; ++i) {
+        const QUtcData *data = utcData(i);
+        if (utcId(data) == ianaId) {
+            return true;
+        }
+    }
+    return false;
+}
+
 QList<QByteArray> QUtcTimeZonePrivate::availableTimeZoneIds() const
 {
     QList<QByteArray> result;
diff --git a/src/corelib/tools/qtimezoneprivate_p.h b/src/corelib/tools/qtimezoneprivate_p.h
index 4d357111f2..c9a5726216 100644
--- a/src/corelib/tools/qtimezoneprivate_p.h
+++ b/src/corelib/tools/qtimezoneprivate_p.h
@@ -128,6 +128,7 @@ public:
 
     virtual QByteArray systemTimeZoneId() const;
 
+    virtual bool isTimeZoneIdAvailable(const QByteArray &ianaId) const;
     virtual QList<QByteArray> availableTimeZoneIds() const;
     virtual QList<QByteArray> availableTimeZoneIds(QLocale::Country country) const;
     virtual QList<QByteArray> availableTimeZoneIds(int utcOffset) const;
@@ -204,6 +205,7 @@ public:
 
     QByteArray systemTimeZoneId() const override;
 
+    bool isTimeZoneIdAvailable(const QByteArray &ianaId) const override;
     QList<QByteArray> availableTimeZoneIds() const override;
     QList<QByteArray> availableTimeZoneIds(QLocale::Country country) const override;
     QList<QByteArray> availableTimeZoneIds(int utcOffset) const override;
@@ -323,6 +325,7 @@ public:
 
     QByteArray systemTimeZoneId() const override;
 
+    bool isTimeZoneIdAvailable(const QByteArray &ianaId) const override;
     QList<QByteArray> availableTimeZoneIds() const override;
     QList<QByteArray> availableTimeZoneIds(QLocale::Country country) const override;
 
diff --git a/src/corelib/tools/qtimezoneprivate_tz.cpp b/src/corelib/tools/qtimezoneprivate_tz.cpp
index 6a5df6272a..f1735a80e7 100644
--- a/src/corelib/tools/qtimezoneprivate_tz.cpp
+++ b/src/corelib/tools/qtimezoneprivate_tz.cpp
@@ -1098,6 +1098,11 @@ QByteArray QTzTimeZonePrivate::systemTimeZoneId() const
     return ianaId;
 }
 
+bool QTzTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
+{
+    return tzZones->contains(ianaId);
+}
+
 QList<QByteArray> QTzTimeZonePrivate::availableTimeZoneIds() const
 {
     QList<QByteArray> result = tzZones->keys();