Solved

[Graziano Servizi]

I have understood: it has been sufficient to forward declare
friend_inline in global scope.

Thanks,

GS

[Jonathan Wakely]

I have understood: it has been sufficient to forward declare friend_inline
in global scope.

Yes, unless a friend function is also declared outside the class of
which it is a friend then its name is not visible in that scope and it
can only be found by ADL.