When constructing the metadata for a type Gen<T : Super>
where Super is a superclass constraint, the generic argument K at which
the metadata for Gen is being instantiated is verified to be a subclass
of Super via _checkGenericRequirements.
Previously, that check was done using swift_dynamicCastMetatype. That
worked for the most part but provided an incorrect answer if the
metadata for K was not yet complete. These classes are incomplete more
often thanks to __swift_instantiateConcreteTypeFromMangledNameAbstract.
That issue occurred concretely in the following case:
Framework with Library Evolution enabled:
open class Super { ... }
public struct Gen<T : Super> {
}
Target in a different resilience domain from that framework:
class Sub : Super {
var gen: Gen<Sub>?
}
Here, the mechanism for checking whether the generic argument K at which
the metadata for Gen is being instantiated handles the case where K's
metadata is incomplete. At worst, every superclass name from super(K)
up to Super are demangled to instantiate metadata. A number of faster
paths are included as well.
rdar://problem/60790020
Metadata uniquing might encounter witness tables that were distinctly
generated but come from identical descriptors. Handle this case in metadata
uniquing be looking into the protocol conformance descriptors themselves.
Clang-importer-synthesized declarations get an extra tag character included in their mangling, which was not being preserved in type context descriptors. This caused runtime lookup for these synthesized types to fail. Fix this by adding the tag information to type context descriptors and teaching the runtime to match it up when fetching metadata by mangled name. Fixes rdar://problem/40878715.
