Having pondered on a bit more.. I think it's the struct that would declare that it's not usable as nil, and that in turn would tell the runtime not to box it if it's nil. That would also help the compiler (and copilot etc) spot calls on nil pointers which will panic.
But that information disappears when you assign to an interface variable/container that is nillable.
It requires an assertion to recover the info about the value inside the interface being not nil.
basically `if v.(nil){...}
creates two branches. In one we know v is not nil (outside the if block) and it can therefore be assigned to non nillable variables so to speak...
