Deprecate QDateTime(const QDate &) in favor of QDate::startOfDay()

It needed re-implemented in terms of the new API (in case QTime(0, 0)
was skipped, on the date in question, by a spring-forwrd), which makes
it redundant (and supports choice of spec and zone or offset, which it
did not).

Change-Id: I1e3c3e794632c234f254be754ed6e4ebdaaaa6bc
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

4 files changed, 17 insertions, 12 deletions

diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index b417683d2a..3be9a02248 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2019 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Copyright (C) 2018 Intel Corporation.
 ** Copyright (C) 2015 Olivier Goffart <ogoffart@woboq.com>
 ** Contact: https://www.qt.io/licensing/
@@ -662,7 +662,7 @@ static bool convert(const QVariant::Private *d, int t, void *result, bool *ok)
 #  endif
 #endif
         case QMetaType::QDate:
-            *dt = QDateTime(*v_cast<QDate>(d));
+            *dt = v_cast<QDate>(d)->startOfDay();
             break;
         default:
             return false;
@@ -1229,7 +1229,7 @@ static bool convert(const QVariant::Private *d, int t, void *result, bool *ok)
             *static_cast<QCborValue *>(result) = *v_cast<QByteArray>(d);
             break;
         case QMetaType::QDate:
-            *static_cast<QCborValue *>(result) = QCborValue(QDateTime(*v_cast<QDate>(d)));
+            *static_cast<QCborValue *>(result) = QCborValue(v_cast<QDate>(d)->startOfDay());
             break;
         case QMetaType::QDateTime:
             *static_cast<QCborValue *>(result) = QCborValue(*v_cast<QDateTime>(d));
diff --git a/src/corelib/time/qdatetime.cpp b/src/corelib/time/qdatetime.cpp
index 80751e60a0..7cc8568bd5 100644
--- a/src/corelib/time/qdatetime.cpp
+++ b/src/corelib/time/qdatetime.cpp
@@ -3631,15 +3631,18 @@ QDateTime::QDateTime() noexcept(Data::CanBeSmall)
 }
 
 
+#if QT_DEPRECATED_SINCE(5, 17) // ### Qt 6: remove
 /*!
-    Constructs a datetime with the given \a date, a valid
-    time(00:00:00.000), and sets the timeSpec() to Qt::LocalTime.
-*/
+    Constructs a datetime with the given \a date, using Qt::LocalTime as the
+    timeSpec() and the time at the start of that date.
 
+    \sa QDate::startOfDay()
+*/
 QDateTime::QDateTime(const QDate &date)
-    : d(QDateTimePrivate::create(date, QTime(0, 0), Qt::LocalTime, 0))
+    : QDateTime(date.startOfDay(Qt::LocalTime, 0))
 {
 }
+#endif
 
 /*!
     Constructs a datetime with the given \a date and \a time, using
diff --git a/src/corelib/time/qdatetime.h b/src/corelib/time/qdatetime.h
index 3eae8ebf64..2c2b52b1c2 100644
--- a/src/corelib/time/qdatetime.h
+++ b/src/corelib/time/qdatetime.h
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2019 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Copyright (C) 2016 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
@@ -287,7 +287,9 @@ class Q_CORE_EXPORT QDateTime
 
 public:
     QDateTime() noexcept(Data::CanBeSmall);
-    explicit QDateTime(const QDate &); // ### Qt 6: plain QDate, QTime
+#if QT_DEPRECATED_SINCE(5, 15) // ### Qt 6: remove
+    QT_DEPRECATED_X("Use QDate::startOfDay()") explicit QDateTime(const QDate &);
+#endif
     QDateTime(const QDate &, const QTime &, Qt::TimeSpec spec = Qt::LocalTime);
     // ### Qt 6: Merge with above with default offsetSeconds = 0
     QDateTime(const QDate &date, const QTime &time, Qt::TimeSpec spec, int offsetSeconds);
diff --git a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
index 6aae91f62f..ec5f65ee56 100644
--- a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
@@ -2941,19 +2941,19 @@ void tst_QDateTime::fewDigitsInYear() const
 void tst_QDateTime::printNegativeYear() const
 {
     {
-        QDateTime date(QDate(-20, 10, 11));
+        QDateTime date(QDate(-20, 10, 11).startOfDay());
         QVERIFY(date.isValid());
         QCOMPARE(date.toString(QLatin1String("yyyy")), QString::fromLatin1("-0020"));
     }
 
     {
-        QDateTime date(QDate(-3, 10, 11));
+        QDateTime date(QDate(-3, 10, 11).startOfDay());
         QVERIFY(date.isValid());
         QCOMPARE(date.toString(QLatin1String("yyyy")), QString::fromLatin1("-0003"));
     }
 
     {
-        QDateTime date(QDate(-400, 10, 11));
+        QDateTime date(QDate(-400, 10, 11).startOfDay());
         QVERIFY(date.isValid());
         QCOMPARE(date.toString(QLatin1String("yyyy")), QString::fromLatin1("-0400"));
     }