Restore Prior Behavior for NSCopying Lookup in Subscript Type Mapping #42275
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@swift-ci smoke test |
hborla
approved these changes
Apr 9, 2022
Bear with me, this is going to get weird. Swift has to import -[NSDictionary objectForKeyedSubscript:] with a special NSCopying bound that otherwise does not appear in source. This involves the clang importer looking very specifically for this selector, on this type, in Foundation. The old behavior was to try to run a type lookup for NSCopying then bail if that failed. With the introduction of explicit existentials, the failure path changed to check if the existential itself is NULL. That's currently an impossible condition. What instead happens is that if your SDK is broken we submit a NULL NSCopying primitive to the ExistentialType constructor and crash in +asserts compilers. Quite how you wind up in this position in real world setups is a mystery - if the Foundation module were somehow corrupt, or the module cache returned an incoherent type entry lacking a definition for NSCopying we could crash here. In fact, you can replicate this crash by removing the definition for NSCopying in the mock SDK and leaving behind a forward declaration! Since this condition is not representative of an expected setup, restoring the old behavior sans test is probably the right way to go. rdar://91062370
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@swift-ci smoke test and merge |
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Bear with me, this is going to get weird.
Swift has to import -[NSDictionary objectForKeyedSubscript:] with a special NSCopying
bound that otherwise does not appear in source. This involves the clang importer
looking very specifically for this selector, on this type, in Foundation. The old
behavior was to try to run a type lookup for NSCopying then bail if that failed.
With the introduction of explicit existentials, the failure path changed to check
if the existential itself is NULL. That's currently an impossible condition. What instead
happens is that if your SDK is broken we submit a NULL NSCopying primitive to the
ExistentialType constructor and crash in +asserts compilers.
Quite how you wind up in this position in real
world setups is a mystery - if the Foundation module were somehow corrupt, or the module
cache returned an incoherent type entry lacking a definition for NSCopying we could crash
here. In fact, you can replicate this crash by removing the definition for NSCopying in the
mock SDK and leaving behind a forward declaration!
Since this condition is not representative of an expected setup, restoring the old behavior
sans test is probably the right way to go.
rdar://91062370