QChar cannot represent all Unicode characters (it's only 16 bit), but it unfortunately has a ctor from 32-bit values. The QFontEngine APIs for single characters take QChar, so they cannot be used to query non-BMP characters. I didn't check, but it would be fatal if the QChar overload silently accepted char32_t arguments.

We should port the single-character functions from QChar to char32_t, and deprecate the QChar overloads. I found these, there may be more:

• inFont() (can then be merged with inFontUcs4())
• left/rightBearing()
• horizontalAdvance(QChar)
• boundingRect(QChar)

See also QTBUG-125871 for an overarching theme.

Found while fixing Clang 21 -Wcharacter-conversion warnings.