QString(View)::mid(int pos, int n): unspecified for pos < 0

While for QString(View)::left() we have

The entire string is returned if n is greater than or equal to size(), or less than zero.

for QString(View)::nid() there is iust one of the out-of-bounds position cases defined:

Returns a null string if the position index exceeds the length of the string.

Given that https://doc.qt.io/qt-5/qstringview.html#mid explicitly states

Until 5.15.1, the behavior was undefined when start < 0 or start > size(). Since 5.15.2, the behavior is compatible with QString::mid().

one is even more puzzled, as one now knows behaviour is compatible, but still undefined?

Wanted:
explicit definition of behaviour, so it could be taken into account for special handling in own code.