Make QString's formatting of doubles be consistent with other places

QString::sprintf(), like the C printf-family, always includes two
digits in any exponent it outputs.  Up to 5.6, number() and arg()
taking a double did the same; but changes at 5.7 to enable opting out
of the leading zero this implies for a single-digit exponent
accidentally opted out of it in args() and number().  This commit
fixes number() and arg() to include QLocaleData::ZeroPadExponent in
the flags they pass down to the C locale's doubleToString(), restoring
the prior behavior, including consistency with sprintf().

[ChangeLog][QtCore][QString] Formatting of doubles with single-digit
exponent, by number() or args(), now includes a leading zero in that
exponent, consistently with sprintf(), as it did up to 5.6.

Task-number: QTBUG-63620
Change-Id: I10c491902b8556e9f19e605177ead8d9fd32abd9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 23 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qstring/tst_qstring.cpp b/tests/auto/corelib/tools/qstring/tst_qstring.cpp
index 28014840a3..2074c9789a 100644
--- a/tests/auto/corelib/tools/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/tools/qstring/tst_qstring.cpp
@@ -516,6 +516,7 @@ private slots:
     void toUcs4();
     void arg();
     void number();
+    void doubleOut();
     void arg_fillChar_data();
     void arg_fillChar();
     void capacity_data();
@@ -4882,6 +4883,28 @@ void tst_QString::number()
 #endif
 }
 
+void tst_QString::doubleOut()
+{
+    // Regression test for QTBUG-63620; the first two paths lost the exponent's
+    // leading 0 at 5.7; C's printf() family guarantee a two-digit exponent (in
+    // contrast with ECMAScript, which forbids leading zeros).
+    const QString expect(QStringLiteral("1e-06"));
+    const double micro = 1e-6;
+    QCOMPARE(QString::number(micro), expect);
+    QCOMPARE(QString("%1").arg(micro), expect);
+    {
+        QString text;
+        text.sprintf("%g", micro);
+        QCOMPARE(text, expect);
+    }
+    {
+        QString text;
+        QTextStream stream(&text);
+        stream << micro;
+        QCOMPARE(text, expect);
+    }
+}
+
 void tst_QString::capacity_data()
 {
     length_data();