Thanks, all. BTW, I realize the topic has been discussed often but I was missing the right keywords to search for. Now that I know what to look for, these old threads were very relevant and interesting.
The solutions are the same – either stick to OOP way of thinking about inheritance and use macros to work around Julia’s “limitations”, or adopt the composition approach suggested by @lmiq and @Sukera with the Common struct (perhaps renamed PetState or PetData) containing all the info that functions dispatching on abstract Pet need to be aware of.
There is still the slightly annoying book-keeping issue of accessing fields through nested getproperty() calls e.g.
struct Dog <: Pet
data::PetData
end
d = Dog(PetData("Fido"))
d.name # Error
d.data.name # returns "Fido"
But looks like Lazy.@forward can help save time implementing a bunch of pass-through methods like get_name(pet::Pet) = pet.data.name. Can probably even overload getproperty() but that could come with some performance issues.