MOC cannot parse nested inline namespace (Parse error at "::")

Attempting to build the following:

#include <QObject>

namespace A::inline B {}

class Foo : public QObject { 
    Q_OBJECT 
};

 will fail at the auto MOC step with the message:

foo.h:3:1 error: Parse error at "::"

 Using:

#ifndef Q_MOC_RUN
namespace A::inline B {}
#endif

around the offending namespace line allows the auto MOC step to complete.

It would appear the MOC is not able to handle c++20 nested inline namespace. (See syntax (9) here:  https://en.cppreference.com/w/cpp/language/namespace)

Use case of interest

The problem was discovered while trying out the mp-units library (https://github.com/mpusz/units) which as of its 0.7.0 release does use the nested inline namespace feature. (For instance, here: https://github.com/mpusz/units/blob/v0.7.0/src/systems/isq-natural/include/units/isq/natural/acceleration.h)

While this can be deactivated with a macro in this case (i.e. UNITS_NO_ALIASES), nested inline namespace are still something that should probably be supported by the MOC.