qtbase/3dcd6b7ec98b2edf9654bcefdb83134c4c3d2a38 changed AlphaCode from a char[4] to an uint16_t c1:5, c2:5, c3:5, reserved:1, halving the structure size from four to 2 bytes, but also adding the precondition that the AlphaCode(char, char, char) ctor no longer accepts arbitrary char values, but only those inside the set {'\0', 'a'...'z'} (plus a few theoretical characters above 'z' that are unused, but would still fit into the 5-bit bit-fields).

The QLocale::codeToLanguage() function, by way of QLocalePrivate::codeToLanguage(), only checks input characters for being <= 0x7F, but that is no longer sufficient. It has to check for <= 'z' && (== 0 || >= 'a') instead.