I'm classifying this as a bug, because there's no reason it shouldn't be.

Background:
C++20 allows class types as non-type template arguments and I found it useful to use a QFlags<Enum> there, but that fails because QFlags isn't "structural". Says GCC 11:

error: ‘QFlags<QIODevice::OpenModeFlag>’ is not a valid type for a template non-type parameter because it is not structural
[...]
qflags.h:165:9: note: ‘QFlags<QIODevice::OpenModeFlag>::i’ is not public
  165 |     Int i;
      |         ^

I'm not sure this is fixable in Qt 6, but it should be fixed for Qt 7, at least (but see epic for why we may want to wait).