Json writer, support larger signed integers upto 2^53

Previously only 32bit signed integers were ensured to be supported by
JsonValue type via API but it is expected that a larger integer range
should be supported by a JSON implementation.

This commit brings the Qt implementation into parity with NodeJS
JSON.stringify() in respect of the JavaScript Number type.

Change-Id: If91153cb3b13ecc14c50da97055b35ce42f341e7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/json/qjsonvalue.cpp

@@ -155,6 +155,18 @@ QJsonValue::QJsonValue(int n)
     this->dbl = n;
 }
 
+/*!
+    \overload
+    Creates a value of type Double, with value \a n.
+    NOTE: the integer limits for IEEE 754 double precision data is 2^53 (-9007199254740992 to +9007199254740992).
+    If you pass in values outside this range expect a loss of precision to occur.
+ */
+QJsonValue::QJsonValue(qint64 n)
+    : d(0), t(Double)
+{
+    this->dbl = n;
+}
+
 /*!
     Creates a value of type String, with value \a s.
  */

src/corelib/json/qjsonvalue.h

@@ -79,6 +79,7 @@ public:
     QJsonValue(bool b);
     QJsonValue(double n);
     QJsonValue(int n);
+    QJsonValue(qint64 n);
     QJsonValue(const QString &s);
     QJsonValue(QLatin1String s);
     QJsonValue(const QJsonArray &a);

tests/auto/corelib/json/tst_qtjson.cpp

@@ -1128,6 +1128,8 @@ void tst_QtJson::toJsonLargeNumericValues()
     array.append(QJsonValue(-std::numeric_limits<double>::epsilon()));
     array.append(QJsonValue(-std::numeric_limits<double>::denorm_min()));
     array.append(QJsonValue(-0.0));
+    array.append(QJsonValue(9007199254740992LL));  // JS Number max integer
+    array.append(QJsonValue(-9007199254740992LL)); // JS Number min integer
     object.insert("Array", array);
 
     QByteArray json = QJsonDocument(object).toJson();
@@ -1150,7 +1152,9 @@ void tst_QtJson::toJsonLargeNumericValues()
             "        -1.7976931348623157e+308,\n"
             "        -2.2204460492503131e-16,\n"
             "        -4.9406564584124654e-324,\n"
-            "        0\n"
+            "        0,\n"
+            "        9007199254740992,\n"
+            "        -9007199254740992\n"
             "    ]\n"
             "}\n";