Can a stable variable of type A or B be safely upgraded to the type A or B or C?
You can safely upgrade to a supertype, and X or Y, if legal, always is a supertype of both X and Y.
@claudio will this be supported going forwards if Motoko is moving away from candid serialization?
(Pretty sure it won’t be, but I just wanted to post here for posterity’s sake).
Yes, it will be. Anything that passed the previous dfx compatibility check should be accepted.
In particular, if a stable variable is retained it can only evolve to a Motoko supertype (so the new version can consume the current value of the old version). For example, if the field has a record type, it can lose fields or weaken the type of retained fields. Similary, if the field has a variant type, it can add a new variant, or weaken the type of an existing variant.
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