added a comment - - edited
I can't help but feel that the validation should be implemented outside of the metadata drivers. To me the driver shouldn't require the discriminator map to be present because it limits the ability to add discriminators in other ways.
I use annotations for my models, but I don't set the discriminator maps using the annotations driver, instead I use the loadClassMetadata event to fill in the discriminatorMap from configs or a database. Perhaps this is a job for a custom metadata driver, but I still can't help but feel that the metadata driver shouldn't be the one making these validations, it should be when you try to use the inheritance-mapped model.
Edit: To clarify the situation: I now get mappingException::missingDiscriminatorMap exception because my model annotations don't have a @DiscriminatorMap, but they would be loaded with the loadClassMetadata function.