[Sema] Construct OpenedArchetypeType with a canonical existential type.
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slavapestov
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Jan 20, 2022
Opened archetypes can be created in the constraint system, and the existential type it wraps can contain type variables. This can happen when the existential type is inferred through a typealias inside a generic type, and a member reference whose base is the opened existential gets bound before binding the generic arguments of the parent type. However, simplifying opened archetypes to replace type variables is not yet supported, which leads to type variables escaping the constraint system. We can support cases where the underlying existential type doesn't depend on the type variables by canonicalizing it when opening the existential. Cases where the underlying type requires resolved generic arguments are still unsupported for now.
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See887777
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Jan 20, 2022
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Opened archetypes can be created in the constraint system, and the existential type it wraps can contain type variables. This can happen when the existential type is inferred through a typealias inside a generic type, and a member reference whose base is the opened existential gets bound before binding the generic arguments of the parent type.
However, simplifying opened archetypes to replace type variables is not yet supported, which leads to type variables escaping the constraint system. This went unnoticed until my recent
ExistentialTyperefactoring, where this issue started manifesting as a use-after-free viagetCanonicalType()and other code that attempted to touch the type variables after they were freed with the constraint system.Cases where the underlying existential type doesn't depend on the type variables previously appeared to work, despite the invalid AST, because nothing ever touched the type variable types. We can still support these cases by canonicalizing the existential type when opening it. Cases where the underlying type requires resolved generic arguments are still unsupported for now.
Resolves: SR-15742 / rdar://87684281