Prefer ICU over system info on MS-Win

The ICU data is more complete; and its APIs allow it to tell the whole
story, which MS-Win's APIs can't (e.g. Morocco's otherwise-normal DST
has a hole in it for Ramadan; when this makes four transitions in a
year, the MS-APIs can't report more than two, so ignore the Ramadan
gap); and their design obliges us to use heuristics to kludge round
mis-description of simple changes to standard time in non-DST zones,
which can't be distinguished from certain (more) obscure cases of
changes to DST coinciding with a change to standard offset (causing a
DST transition to hide, leaving its other end *looking like* a non-DST
change to standard offset).  Using ICU, when available, reduces how
many end users shall be mislead by such kludges.

[ChangeLog][Windows][QTimeZone] Changed MS-Win to use ICU for
time-zone data, when available, in preference to MS's TZ APIs.  The
choice is made when building Qt.  This will give reliable results when
non-DST transitions arise, or when a zone's DST is not simple
(e.g. interrupted by Ramadan).

Task-number: QTBUG-42021
Change-Id: I9cdd65713ecdaf8cce52dd924fbc7371630977c8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 13 insertions, 12 deletions

diff --git a/src/corelib/tools/qtimezone.cpp b/src/corelib/tools/qtimezone.cpp
index 6a514abbfe..a9a9b4f25e 100644
--- a/src/corelib/tools/qtimezone.cpp
+++ b/src/corelib/tools/qtimezone.cpp
@@ -66,11 +66,10 @@ static QTimeZonePrivate *newBackendTimeZone()
     return new QAndroidTimeZonePrivate();
 #elif defined Q_OS_UNIX
     return new QTzTimeZonePrivate();
-    // Registry based timezone backend not available on WinRT
-#elif defined Q_OS_WIN
-    return new QWinTimeZonePrivate();
 #elif QT_CONFIG(icu)
     return new QIcuTimeZonePrivate();
+#elif defined Q_OS_WIN
+    return new QWinTimeZonePrivate();
 #else
     return new QUtcTimeZonePrivate();
 #endif // System Locales
@@ -93,11 +92,10 @@ static QTimeZonePrivate *newBackendTimeZone(const QByteArray &ianaId)
     return new QAndroidTimeZonePrivate(ianaId);
 #elif defined Q_OS_UNIX
     return new QTzTimeZonePrivate(ianaId);
-    // Registry based timezone backend not available on WinRT
-#elif defined Q_OS_WIN
-    return new QWinTimeZonePrivate(ianaId);
 #elif QT_CONFIG(icu)
     return new QIcuTimeZonePrivate(ianaId);
+#elif defined Q_OS_WIN
+    return new QWinTimeZonePrivate(ianaId);
 #else
     return new QUtcTimeZonePrivate(ianaId);
 #endif // System Locales
diff --git a/src/corelib/tools/qtimezoneprivate_win.cpp b/src/corelib/tools/qtimezoneprivate_win.cpp
index 8bde07c710..ec91b7e8a8 100644
--- a/src/corelib/tools/qtimezoneprivate_win.cpp
+++ b/src/corelib/tools/qtimezoneprivate_win.cpp
@@ -49,6 +49,7 @@
 QT_BEGIN_NAMESPACE
 
 #ifndef Q_OS_WINRT
+// The registry-based timezone backend is not available on WinRT, which falls back to equivalent APIs.
 #define QT_USE_REGISTRY_TIMEZONE 1
 #endif
 
diff --git a/src/corelib/tools/tools.pri b/src/corelib/tools/tools.pri
index 56b30279bd..86afb46b8a 100644
--- a/src/corelib/tools/tools.pri
+++ b/src/corelib/tools/tools.pri
@@ -159,16 +159,18 @@ qtConfig(timezone) {
     SOURCES += \
         tools/qtimezone.cpp \
         tools/qtimezoneprivate.cpp
-    !nacl:darwin: \
+    !nacl:darwin: {
         SOURCES += tools/qtimezoneprivate_mac.mm
-    else: android: \
+    } else: android: {
         SOURCES += tools/qtimezoneprivate_android.cpp
-    else: unix: \
+    } else: unix: {
         SOURCES += tools/qtimezoneprivate_tz.cpp
-    else: win32: \
-        SOURCES += tools/qtimezoneprivate_win.cpp
-    qtConfig(icu): \
+        qtConfig(icu): SOURCES += tools/qtimezoneprivate_icu.cpp
+    } else: qtConfig(icu): {
         SOURCES += tools/qtimezoneprivate_icu.cpp
+    } else: win32: {
+        SOURCES += tools/qtimezoneprivate_win.cpp
+    }
 }
 
 qtConfig(datetimeparser) {