QLocal8Bit::convertFromUnicode[win]: fix code unit pairing

When we restore a high surrogate from the state, we need to make sure
that the next code unit is a low surrogate. And if it is not then we
should at least not throw it away.

Amends d8d5922f16f1710b66caf718c302b633d2f78b0b

Pick-to: 6.6 6.5
Task-number: QTBUG-118185
Task-number: QTBUG-105105
Change-Id: I64afa0d323d73422128e24e16755e648a8811523
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/src/corelib/text/qstringconverter.cpp b/src/corelib/text/qstringconverter.cpp
index f27283a06d..6bac9ade7d 100644
--- a/src/corelib/text/qstringconverter.cpp
+++ b/src/corelib/text/qstringconverter.cpp
@@ -1420,14 +1420,20 @@ QByteArray QLocal8Bit::convertFromUnicode_sys(QStringView in, quint32 codePage,
         Q_ASSERT(state->remainingChars == 1);
         // Let's try to decode the pending character
         wchar_t wc[2] = { wchar_t(state->state_data[0]), ch[0] };
-        int len = WideCharToMultiByte(codePage, 0, wc, int(std::size(wc)), out, outlen, nullptr,
+        // Check if the second character is a valid low surrogate,
+        // otherwise we'll just decode the first character, for which windows
+        // will output a replacement character.
+        const bool validCodePoint = QChar::isLowSurrogate(wc[1]);
+        int len = WideCharToMultiByte(codePage, 0, wc, validCodePoint ? 2 : 1, out, outlen, nullptr,
                                       nullptr);
         if (!len)
             return {}; // Cannot recover, and I refuse to believe it was a size limitation
         out += len;
         outlen -= len;
-        ++ch;
-        --uclen;
+        if (validCodePoint) {
+            ++ch;
+            --uclen;
+        }
         state->remainingChars = 0;
         state->state_data[0] = 0;
         if (uclen == 0)