SIL: Fix bug in AbstractionPattern::getFunctionThrownErrorType() #84207
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When storing a closure with type `() throws(any Error) -> ()` as a fully opaque error, we want to leave it unchanged, instead of re-abstracting it to throw the error indirectly. Thus, we had a special carveout here. However, the carveout was too broad, because if the thrown error type contained type parameters, the resulting AbstractionPattern was invalid. While fixing this I realized this entire hack is unsound in some cases, if you view the same value as a `() throws(U) -> ()` vs an `() -> throws(any Error) -> ()`. Perhaps we should always box the thrown error when maximally abstracting a closure, but that would also be an ABI break. - Fixes swiftlang#84051
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When storing a closure with type
() throws(any Error) -> ()as a fully opaque value, we want to leave it unchanged, instead of re-abstracting it to throw the error indirectly. Thus, we had a special carveout here.However, the carveout was too broad, because if the thrown error type contained type parameters, the resulting AbstractionPattern was invalid.
While fixing this I realized this entire hack is unsound in some cases, if you view the same value as a
() throws(U) -> ()vs an() -> throws(any Error) -> ().Perhaps we should always box the thrown error when maximally abstracting a closure, but that would also be an ABI break.