I have prepared minimal test case at https://github.com/jmlich/testEnum/

It seems to be regression, because it compiles with Qt 6.5.3, but not with Qt 6.6.1 (compiled with gcc 13.2.1 in both cases)

The LLONG_MAX in combination with Q_ENUM generates some code which cannot be compiled:

    enum Constants {
        InvalidValue64 = LLONG_MAX
    };  

Removal of LLONG_MAX at https://github.com/jmlich/testEnum/blob/main/testclass.h#L16 or removal of Q_ENUM macro workarounds the issue.

I was trying to find out specification and it seems enum should be able to store long long int.

Code originally comes from https://github.com/sailfishos/libqofonoext/blob/1bc9c91172c998e06547f862ddaeef18e8b435d6/src/qofonoextcell.h#L71