QBA(V)/QS(V)::lastIndexOf: fix the search of 1-char needles

When a needle has length 1 (because it's a QChar/char16_t, or because
it's a string-like of length 1) then an ad-hoc search algorithm is
used. This algorithm had a off-by-one, by not allowing to match at
the last position of a haystack (in case `from` was `haystack.size()`).

That is inconsistent with the general search of substring needles
(and what QByteArray does). Fix that case and amend wrong tests.
This in turn unveiled the fact that the algorithm was unable to cope
with 0-length haystacks (whops), so fix that as well. Drive-by, add a
similar fix for QByteArray.

Amends 6cee204d56205e250b0675c9c6d4dd8a2367f3c4.

Change-Id: I6b3effc4ecd74bcbcd33dd2e550da2df7bf05ae3
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit f0c2c987e3e7eb046303892b1eee4f848759923a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 74ea0cfc7d..c2601b0b0b 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2506,6 +2506,8 @@ static qsizetype lastIndexOfHelper(const char *haystack, qsizetype l, const char
 
 static inline qsizetype lastIndexOfCharHelper(QByteArrayView haystack, qsizetype from, char needle) noexcept
 {
+    if (haystack.size() == 0)
+        return -1;
     if (from < 0)
         from += haystack.size();
     else if (from > haystack.size())
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 3c795e2a4b..d74ec6b132 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -10358,10 +10358,12 @@ template <typename Haystack>
 static inline qsizetype qLastIndexOf(Haystack haystack, QChar needle,
                                      qsizetype from, Qt::CaseSensitivity cs) noexcept
 {
+    if (haystack.size() == 0)
+        return -1;
     if (from < 0)
         from += haystack.size();
-    if (std::size_t(from) >= std::size_t(haystack.size()))
-        return -1;
+    else if (std::size_t(from) > std::size_t(haystack.size()))
+        from = haystack.size() - 1;
     if (from >= 0) {
         char16_t c = needle.unicode();
         const auto b = haystack.data();