Answer by Schwern for Pointer receiver methods are getting called on interfaces holding values

Your pointer String method is not being called. That is expected behavior, pointer methods cannot be called on values for the reasons you gave.

{old 12} is the default output for a struct. See fmt Printing...

struct:             {field0 field1 ...}

Perhaps confusingly, fmt.Println(u1.String()) will work. This is because Go has quietly changed that to fmt.Println(&u1.String()) for your convenience. From Effective Go: Methods, Pointers vs. Values...

There is a handy exception, though. When the value is addressable, the language takes care of the common case of invoking a pointer method on a value by inserting the address operator automatically.