Thank you for PR! I definitely welcome this functionality, but aside from some refinements I'll have to do (make sure nullability annotations added for when optional accepts nullable etc.), I need to check if this would work correctly in general.
To be clear, instead of warning, we introduce a new notion of isOptionalWithDefault. In all behaviors it will be considered as REGULAR attribute having a default (isGenerateDefault), but we use isOptionalWithDefault to add some convenience. In the light of this, I still feel like warning is not that bad as it still would remind of that this will not act as optional, but as regular with default (+some conveniences). It's not that we need to get warning back, but definitely need to make sure the documentation, at least, tries to explain that. We have a paragraph about that I guess, but we might need to revise it (or add if it's not there).
In any case, I would as if you can, for example add/revise a note to the Value.Default Javadoc explaining what will happen if Default is placed on the Optional attribute: i.e. it will lose optional status, but will retain some conveniences like initializing from the unwrapped value (like next PR if this will be merged by then)