QVariant: add fromMetaType

The QVariant(QMetaType) constructor is a major anti-pattern: unlike
*every* other QVariant's constructor, it doesn't build a QVariant
holding the QMetaType object, but a QVariant of the specified type.

Introduce a named constructor for this use case instead.

In principle, this should lead to a deprecation of the QMetaType
constructor... except that it's used everywhere, so I can't do it at
this time.

Drive-by, improve the documentation of the QVariant(QMetaType)
constructor (since it's basically c&p for the new fromMetaType
function).

[ChangeLog][QtCore][QVariant] Added the QVariant::fromMetaType named
constructor, that builds a QVariant of a given QMetaType.

Change-Id: I4a499526bd0fe98eed0c1a3e91bcfc21efa9e352
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 18 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index 3a621b91d6..a1f9531093 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -442,11 +442,21 @@ void tst_QVariant::constructor()
     QVERIFY(var3.isNull());
     QVERIFY(var3.isValid());
 
+    QVariant var3a = QVariant::fromMetaType(QMetaType::fromType<QString>());
+    QCOMPARE(var3a.typeName(), "QString");
+    QVERIFY(var3a.isNull());
+    QVERIFY(var3a.isValid());
+
     QVariant var4 {QMetaType()};
     QCOMPARE(var4.typeId(), QMetaType::UnknownType);
     QVERIFY(var4.isNull());
     QVERIFY(!var4.isValid());
 
+    QVariant var4a = QVariant::fromMetaType(QMetaType());
+    QCOMPARE(var4a.typeId(), QMetaType::UnknownType);
+    QVERIFY(var4a.isNull());
+    QVERIFY(!var4a.isValid());
+
     QVariant var5(QLatin1String("hallo"));
     QCOMPARE(var5.typeId(), QMetaType::QString);
     QCOMPARE(var5.typeName(), "QString");
@@ -489,6 +499,14 @@ void tst_QVariant::constructor_invalid()
     }
     {
         QTest::ignoreMessage(QtWarningMsg, QRegularExpression("^Trying to construct an instance of an invalid type"));
+        QVariant variant = QVariant::fromMetaType(QMetaType(typeId));
+        QVERIFY(!variant.isValid());
+        QVERIFY(variant.isNull());
+        QCOMPARE(variant.typeId(), int(QMetaType::UnknownType));
+        QCOMPARE(variant.userType(), int(QMetaType::UnknownType));
+    }
+    {
+        QTest::ignoreMessage(QtWarningMsg, QRegularExpression("^Trying to construct an instance of an invalid type"));
         QVariant variant(QMetaType(typeId), /* copy */ nullptr);
         QVERIFY(!variant.isValid());
         QVERIFY(variant.isNull());