Don't access moved-from object

Clang's static analyzer tooling warns about this access-after-move [1].
While the comment above the function indicated that this was deliberate
and relying on a moved-from QString being valid, it is still bad
practice.

Since 'str' is empty in moved-from state if - and only if - it was a
non-const reference, make a compile-time check of the constness of type
T instead.

[1] https://testresults.qt.io/codechecker/daily_analyses/qtbase/dev/qtbase-dev-20210301-e14ccf0553/qstring.cpp_clang-tidy_b0545db57a2cc5dac67a56f76322ffd0.plist.html#reportHash=209ee3db0b17d21919326a1ad6635318

Pick-to: 6.1
Change-Id: Iac1813b61b6a3c2ef4053b911a4043c5382f85e4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

1 files changed, 6 insertions, 7 deletions

diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index a6f3cd2b5d..390c10e1fc 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -6342,9 +6342,9 @@ namespace QUnicodeTables {
     \endlist
 
     In copy-convert mode, the local variable \c{s} is detached from the input
-    \a str. In the in-place convert mode, \a str is in moved-from state (which
-    this function requires to be a valid, empty string) and \c{s} contains the
-    only copy of the string, without reallocation (thus, \a it is still valid).
+    \a str. In the in-place convert mode, \a str is in moved-from state and
+    \c{s} contains the only copy of the string, without reallocation (thus,
+    \a it is still valid).
 
     There is one pathological case left: when the in-place conversion needs to
     reallocate memory to grow the buffer. In that case, we need to adjust the \a
@@ -6355,7 +6355,7 @@ Q_NEVER_INLINE
 static QString detachAndConvertCase(T &str, QStringIterator it, QUnicodeTables::Case which)
 {
     Q_ASSERT(!str.isEmpty());
-    QString s = std::move(str);             // will copy if T is const QString
+    QString s = std::move(str);         // will copy if T is const QString
     QChar *pp = s.begin() + it.index(); // will detach if necessary
 
     do {
@@ -6374,9 +6374,8 @@ static QString detachAndConvertCase(T &str, QStringIterator it, QUnicodeTables::
                 s.replace(outpos, 1, reinterpret_cast<const QChar *>(folded.data()), folded.size());
                 pp = const_cast<QChar *>(s.constBegin()) + outpos + folded.size();
 
-                // do we need to adjust the input iterator too?
-                // if it is pointing to s's data, str is empty
-                if (str.isEmpty())
+                // Adjust the input iterator if we are performing an in-place conversion
+                if constexpr (!std::is_const<T>::value)
                     it = QStringIterator(s.constBegin(), inpos + folded.size(), s.constEnd());
             }
         } else {