[swift-evolution] Invariant checks on strong typedefs?

[Daryle Walker]

In my retype idea, you can bring back symbols from the implementation type with an "export" command. There's an "export default" command to bring back all the numbered fields of a tuple or all the cases of an enum. But what would it mean to NOT include all enum cases?

One option is to ban this; a retype of an enumeration gets all cases automatically, no need to be explicit nor any way to exclude. But I think leaving it in is cooler; we can make a retype be a restricted type relative to its implementation type. Now there would be a new problem. What happens when we have a retyped object pose as its original type, but assign it a case value banned in the retype?  How would we let the object check its invariant, if at all (i.e. leave it quasi-undefined behavior)? How would we implement any rejection, a throw?

I was thinking of a special method. And it would apply to any retype; not just enumerations, but tuples and structs too. 

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