[4.0] [Clang importer] Infer @_downgrade_exhaustivity_check for new-to-2017 enum elements in Swift 3 #10623
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Explanation: In Swift 3 mode, downgrade the "switch is not exhaustive" error to a warning when the missing cases were introduced in the 2017 SDKs.
Scope: Fixes a source-compatibility problem with the newer SDKs for applications that are switching on enums provided by the system.
Radar: rdar://problem/32824207.
Risk: Low; we're inferring a new attribute in specific cases in the Clang importer, which only affects diagnostics.
Testing: Compiler regression tests.