Did you mean to reverse the “inequalities” here:
abstract type MyTypeB >: MyTypeA end
abstract type MyTypeC >: MyTypeA end
In other words, are you trying to define a struct and “later” (in another module that may not be used in all calls to the struct) make it a subtype of something else? I don’t think this is possible.
My conjecture is that you are doing this so that dispatch can be written based on the abstract type? Then you should not use a type hierarchy but traits. BinaryTraits.jl is a nice implementation, but there are others.