[5.4] Narrowly Fix a Crash in Availability Checking of Defer Bodies #36108
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NIO exposed an issue in the new availability walkers where an implicit
check was not being performed. They were able to get this to crash by
using a defer statement - the body of which contains implicit
declarations that got run through the walker. This exposed a wider hole
in availability checking of defer statements. Namely, that it wasn't
happening.
This is a narrow fix (as opposed to the broader fix in
#36102) that is safer to take for
Swift 5.4.
rdar://74484150