QIcuTimeZonePrivate constructor: save iteration over all zone IDs

ICU returns a "valid" representation of GMT when given an unrecognised
ID, so QTZ's constructor has been checking the ID is available before
passing it to the backend constructor. That availability check was
done by generating the list of available IDs to see if the given ID
was in it; this is very inefficient. Furthermore, the QTZ constructor
was also checking availability, to work round the same issue in only
this one backend, making the check redundant.

So overide isTimeZoneIdAvailable() in the ICU backend, calling
ucal_getCanonicalTimeZoneID(), which answers the question directly;
and drop the duplicate check in the QTZ constructor. Expand a test to
verify an invalid name is rejected.

Fixes: QTBUG-121807
Pick-to: 6.7 6.6 6.5
Change-Id: I34f996b607b958d12607a94eb273bb1b406cca1a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
index 75c8e16fa5..a350ffeb04 100644
--- a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
@@ -1289,7 +1289,9 @@ void tst_QTimeZone::icuTest()
     QIcuTimeZonePrivate tzpd;
     QVERIFY(tzpd.isValid());
 
-    // Test invalid constructor
+    // Test invalid is not available:
+    QVERIFY(!tzpd.isTimeZoneIdAvailable("Gondwana/Erewhon"));
+    // and construction gives an invalid result:
     QIcuTimeZonePrivate tzpi("Gondwana/Erewhon");
     QCOMPARE(tzpi.isValid(), false);