Change documentation of some toDouble()s to reflect reality

They actually return infinity if conversion overflows, while still
setting ok to false; they were documented to return 0 on failure, with
no mention of this special handling of overflow.  Documented reality
rather than changing the behavior.  Gave underflow as an example of
failure other than overflow (toDouble()s do indeed fail on it).
Added some tests of out-of-range values, infinities and NaNs.

[ChangeLog][QtCore][toDouble] QString, QByteArray and QLocale return
an infinity on overflow (since 5.7), while setting ok to false; this
was at odds with their documented behavior of returning 0 on failure.
The documentation now reflects the actual behavior.

Fixes: QTBUG-71256
Change-Id: I8d7e80ba1f06091cf0f1480c341553381103703b
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/tools/qbytearray.cpp

@@ -3871,7 +3871,8 @@ ushort QByteArray::toUShort(bool *ok, int base) const
 /*!
     Returns the byte array converted to a \c double value.
 
-    Returns 0.0 if the conversion fails.
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for other reasons (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.

src/corelib/tools/qlocale.cpp

@@ -1373,8 +1373,10 @@ float QLocale::toFloat(const QString &s, bool *ok) const
 }
 
 /*!
-    Returns the double represented by the localized string \a s, or
+    Returns the double represented by the localized string \a s.
-    0.0 if the conversion failed.
+
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for any other reason (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.
@@ -1542,8 +1544,10 @@ float QLocale::toFloat(const QStringRef &s, bool *ok) const
 }
 
 /*!
-    Returns the double represented by the localized string \a s, or
+    Returns the double represented by the localized string \a s.
-    0.0 if the conversion failed.
+
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for any other reason (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.
@@ -1711,8 +1715,10 @@ float QLocale::toFloat(QStringView s, bool *ok) const
 }
 
 /*!
-    Returns the double represented by the localized string \a s, or
+    Returns the double represented by the localized string \a s.
-    0.0 if the conversion failed.
+
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for any other reason (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.

src/corelib/tools/qstring.cpp

@@ -7169,7 +7169,8 @@ ushort QString::toUShort(bool *ok, int base) const
 /*!
     Returns the string converted to a \c double value.
 
-    Returns 0.0 if the conversion fails.
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for other reasons (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.
@@ -11794,7 +11795,8 @@ ushort QStringRef::toUShort(bool *ok, int base) const
 /*!
     Returns the string converted to a \c double value.
 
-    Returns 0.0 if the conversion fails.
+    Returns an infinity if the conversion overflows or 0.0 if the
+    conversion fails for other reasons (e.g. underflow).
 
     If \a ok is not \c nullptr, failure is reported by setting *\a{ok}
     to \c false, and success by setting *\a{ok} to \c true.

tests/auto/corelib/tools/qlocale/tst_qlocale.cpp

@@ -83,7 +83,7 @@ private slots:
     void matchingLocales();
     void stringToDouble_data();
     void stringToDouble();
-    void stringToFloat_data() { stringToDouble_data(); }
+    void stringToFloat_data();
     void stringToFloat();
     void doubleToString_data();
     void doubleToString();
@@ -155,6 +155,7 @@ private:
     QString m_decimal, m_thousand, m_sdate, m_ldate, m_time;
     QString m_sysapp;
     bool europeanTimeZone;
+    void toReal_data();
 
     class TransientLocale
     {
@@ -679,7 +680,7 @@ void tst_QLocale::unixLocaleName()
 #undef TEST_NAME
 }
 
-void tst_QLocale::stringToDouble_data()
+void tst_QLocale::toReal_data()
 {
     QTest::addColumn<QString>("locale_name");
     QTest::addColumn<QString>("num_str");
@@ -801,6 +802,32 @@ void tst_QLocale::stringToDouble_data()
     QTest::newRow("de_DE 9.876543,0e--2") << QString("de_DE") << QString("9.876543,0e")+QChar(8722)+QString("2")   << false << 0.0;
 }
 
+void tst_QLocale::stringToDouble_data()
+{
+    toReal_data();
+    if (std::numeric_limits<double>::has_infinity) {
+        double huge = std::numeric_limits<double>::infinity();
+        QTest::newRow("C inf") << QString("C") << QString("inf") << true << huge;
+        QTest::newRow("C +inf") << QString("C") << QString("+inf") << true << +huge;
+        QTest::newRow("C -inf") << QString("C") << QString("-inf") << true << -huge;
+        // Overflow:
+        QTest::newRow("C huge") << QString("C") << QString("2e308") << false << huge;
+        QTest::newRow("C -huge") << QString("C") << QString("-2e308") << false << -huge;
+    }
+    if (std::numeric_limits<double>::has_quiet_NaN)
+        QTest::newRow("C qnan") << QString("C") << QString("NaN") << true << std::numeric_limits<double>::quiet_NaN();
+
+    // In range (but outside float's range):
+    QTest::newRow("C big") << QString("C") << QString("3.5e38") << true << 3.5e38;
+    QTest::newRow("C -big") << QString("C") << QString("-3.5e38") << true << -3.5e38;
+    QTest::newRow("C small") << QString("C") << QString("1e-45") << true << 1e-45;
+    QTest::newRow("C -small") << QString("C") << QString("-1e-45") << true << -1e-45;
+
+    // Underflow:
+    QTest::newRow("C tiny") << QString("C") << QString("2e-324") << false << 0.;
+    QTest::newRow("C -tiny") << QString("C") << QString("-2e-324") << false << 0.;
+}
+
 void tst_QLocale::stringToDouble()
 {
 #define MY_DOUBLE_EPSILON (2.22045e-16) // 1/2^{52}; double has a 53-bit mantissa
@@ -825,28 +852,33 @@ void tst_QLocale::stringToDouble()
         QCOMPARE(ok, good);
     }
 
-    if (ok) {
+    if (ok || std::isinf(num)) {
         // First use fuzzy-compare, then a more precise check:
         QCOMPARE(d, num);
-        double diff = d - num;
+        if (std::isfinite(num)) {
-        if (diff < 0)
+            double diff = d > num ? d - num : num - d;
-            diff = -diff;
+            QVERIFY(diff <= MY_DOUBLE_EPSILON);
-        QVERIFY(diff <= MY_DOUBLE_EPSILON);
+        }
     }
 
     d = locale.toDouble(num_strRef, &ok);
     QCOMPARE(ok, good);
 
-    if (ok) {
+    if (ok || std::isinf(num)) {
         QCOMPARE(d, num);
-        double diff = d - num;
+        if (std::isfinite(num)) {
-        if (diff < 0)
+            double diff = d > num ? d - num : num - d;
-            diff = -diff;
+            QVERIFY(diff <= MY_DOUBLE_EPSILON);
-        QVERIFY(diff <= MY_DOUBLE_EPSILON);
+        }
     }
 #undef MY_DOUBLE_EPSILON
 }
 
+void tst_QLocale::stringToFloat_data()
+{
+    toReal_data();
+}
+
 void tst_QLocale::stringToFloat()
 {
 #define MY_FLOAT_EPSILON (2.384e-7) // 1/2^{22}; float has a 23-bit mantissa