[CodeCompletion] Avoid creating circles in the USRBasedType supertype hierarchy #58522
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… hierarchy
You would think that superclass + conformances form a DAG. You are wrong!
We can achieve a circular supertype hierarcy with
```swift
protocol Proto : Class {}
class Class : Proto {}
```
USRBasedType is not set up for this. Serialization of code completion results from global modules can't handle cycles in the supertype hierarchy
because it writes the DAG leaf to root(s) and needs to know the type offsets. To get consistent results independent of where we start
constructing USRBasedTypes, ignore superclasses of protocols. If we kept track of already visited types, we would get different results depending on
whether we start constructing the USRBasedType hierarchy from Proto or Class.
Ignoring superclasses of protocols is safe to do because USRBasedType is an under-approximation anyway.
rdar://91765262
|
@swift-ci Please smoke test |
bnbarham
approved these changes
May 2, 2022
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It's weird that we specify Class as a superclass of Proto here. As far as I can tell, it's actually just a requirement that conforming types must either be Class or inherit from Class. So the fix is fine, but it'd be nice if it was modeled differently in the AST.
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You would think that superclass + conformances form a DAG. You are wrong!
We can achieve a circular supertype hierarcy with
USRBasedType is not set up for this. Serialization of code completion results from global modules can't handle cycles in the supertype hierarchy
because it writes the DAG leaf to root(s) and needs to know the type offsets. To get consistent results independent of where we start
constructing USRBasedTypes, ignore superclasses of protocols. If we kept track of already visited types, we would get different results depending on
whether we start constructing the USRBasedType hierarchy from Proto or Class.
Ignoring superclasses of protocols is safe to do because USRBasedType is an under-approximation anyway.
rdar://91765262