QString: fix regression comparing null QString with null const char *

Commit 4a40c717f3cf1ae181df49c91261a12d5e33e5a4 optimized
QString::compare_helper(QChar*, int, char*, int), but got
the case wrong where the rhs is null, but the lhs is empty,
not null (which is the case even with a null QString, as
QString().constData() != nullptr). The correct result in
this case is 0, since in Qt empty and null strings compare
equal.

Fix by checking the length of lhs, not its pointer.

Task-number: QTBUG-55154
Change-Id: I3ec2cd25d9bdca90cf3f5568a875b1e52c779979
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qstring/tst_qstring.cpp b/tests/auto/corelib/tools/qstring/tst_qstring.cpp
index 6df9ad7cdf..36784435b8 100644
--- a/tests/auto/corelib/tools/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/tools/qstring/tst_qstring.cpp
@@ -5012,6 +5012,12 @@ void tst_QString::operator_eqeq_nullstring()
     QVERIFY( QString("") == "" );
     QVERIFY( "" == QString("") );
 
+    QVERIFY(QString() == nullptr);
+    QVERIFY(nullptr == QString());
+
+    QVERIFY(QString("") == nullptr);
+    QVERIFY(nullptr == QString(""));
+
     QVERIFY( QString().size() == 0 );
 
     QVERIFY( QString("").size() == 0 );
@@ -5025,6 +5031,8 @@ void tst_QString::operator_smaller()
     QString null;
     QString empty("");
     QString foo("foo");
+    const char *nullC = nullptr;
+    const char *emptyC = "";
 
     QVERIFY( !(null < QString()) );
     QVERIFY( !(null > QString()) );
@@ -5035,6 +5043,12 @@ void tst_QString::operator_smaller()
     QVERIFY( !(null < empty) );
     QVERIFY( !(null > empty) );
 
+    QVERIFY( !(nullC < empty) );
+    QVERIFY( !(nullC > empty) );
+
+    QVERIFY( !(null < emptyC) );
+    QVERIFY( !(null > emptyC) );
+
     QVERIFY( null < foo );
     QVERIFY( !(null > foo) );
     QVERIFY( foo > null );