[Development] Can we pass QTextDocument arbitary data?

[Thiago Macieira]

On Wednesday, 21 January 2026 15:17:50 Pacific Standard Time Albert Astals Cid 
wrote:
> We have found a garbage string that when passed to QTextDocument makes it
> assert

Code in question is this:

    const bool isSurrogate = c.isHighSurrogate() && i < length - 1;
    const char32_t ucs4 = isSurrogate
                            ? QChar::surrogateToUcs4(c, string[++i])
                            : c.unicode();
    const QUnicodeTables::Properties *p = QUnicodeTables::properties(ucs4);

It doesn't verify that the code unit after the high surrogate is a low 
surrogate. That must be why it ended up with a UTF-32 code unit outside of the 
Unicode range.

This could be fixed in QTextEngine. But I would argue that passing corrupted 
UTF-16 to any Qt API outside of QString itself is the bug.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCG - Platform & Sys. Eng.
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