[Clang importer] Infer @_downgrade_exhaustivity_check for new-to-2017 enum elements in Swift 3.

In Swift 3 mode, infer @_downgrade_exhaustivity_check for any enum
elements that were introduced in the 2017 SDKs (macOS 10.13, iOS 11.0,
tvOS 11.0, watchOS 4.0). This is a stop-gap until we get "open" enums,
but serves an important source-compatibility use case.

Fixes rdar://problem/32824207.

(cherry picked from commit f855d2f)

Author: DougGregor

lib/ClangImporter/ImportDecl.cpp

@@ -7016,6 +7016,41 @@ void ClangImporter::Implementation::importAttributes(
                                           PlatformAgnostic, /*Implicit=*/false);
 
       MappedDecl->getAttrs().add(AvAttr);
+
+      // For enum cases introduced in the 2017 SDKs, add
+      // @_downgrade_exhaustivity_check in Swift 3.
+      if (C.LangOpts.isSwiftVersion3() && isa<EnumElementDecl>(MappedDecl)) {
+        bool downgradeExhaustivity = false;
+        switch (*platformK) {
+        case PlatformKind::OSX:
+        case PlatformKind::OSXApplicationExtension:
+          downgradeExhaustivity = (introduced.getMajor() == 10 &&
+                                   introduced.getMinor() &&
+                                   *introduced.getMinor() == 13);
+          break;
+
+        case PlatformKind::iOS:
+        case PlatformKind::iOSApplicationExtension:
+        case PlatformKind::tvOS:
+        case PlatformKind::tvOSApplicationExtension:
+          downgradeExhaustivity = (introduced.getMajor() == 11);
+          break;
+
+        case PlatformKind::watchOS:
+        case PlatformKind::watchOSApplicationExtension:
+          downgradeExhaustivity = (introduced.getMajor() == 4);
+          break;
+
+        case PlatformKind::none:
+          break;
+        }
+
+        if (downgradeExhaustivity) {
+          auto attr =
+            new (C) DowngradeExhaustivityCheckAttr(/*isImplicit=*/true);
+          MappedDecl->getAttrs().add(attr);
+        }
+      }
     }
   }
 

test/ClangImporter/Inputs/custom-modules/AvailabilityExtras.h

@@ -1,3 +1,5 @@
+@import Foundation;
+
 void unavail1(void) __attribute__((unavailable("first")));
 void unavail1(void);
 void unavail1(void);
@@ -84,3 +86,10 @@ struct NSSwiftUnavailableStruct {
 } __attribute__((availability(swift, unavailable)));
 
 void unavailableWithOS() __attribute__((availability(ios, deprecated=8.0))) __attribute__((availability(swift, unavailable))) __attribute__((availability(macosx, deprecated=10.10))) ;
+
+typedef NS_ENUM(NSInteger, NSEnumAddedCasesIn2017) {
+    NSEnumAddedCasesIn2017ExistingCaseOne,
+    NSEnumAddedCasesIn2017ExistingCaseTwo,
+    NSEnumAddedCasesIn2017ExistingCaseThree,
+    NSEnumAddedCasesIn2017NewCaseOne __attribute__((availability(macosx,introduced=10.13))) __attribute__((availability(ios,introduced=11.0))) __attribute__((availability(tvos,introduced=11.0))) __attribute__((availability(watchos,introduced=4.0)))
+};

test/ClangImporter/availability_open_enums_macos.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 4 -target x86_64-apple-macosx10.13 %s 
+
+// REQUIRES: OS=macosx
+// REQUIRES: objc_interop
+
+import Foundation
+import AvailabilityExtras
+
+func exhaustiveSwitch(e: NSEnumAddedCasesIn2017) {
+  switch e { // expected-error{{switch must be exhaustive}}
+    // expected-note@-1{{add missing case: '.newCaseOne'}}
+  case .existingCaseOne:
+    return
+  case .existingCaseTwo:
+    return
+  case .existingCaseThree:
+    return    
+  }
+}

test/ClangImporter/availability_open_enums_macos_swift3.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 3 -target x86_64-apple-macosx10.13 %s 
+
+// REQUIRES: OS=macosx
+// REQUIRES: objc_interop
+
+import Foundation
+import AvailabilityExtras
+
+func exhaustiveSwitch(e: NSEnumAddedCasesIn2017) {
+  switch e { // expected-warning{{switch must be exhaustive}}
+    // expected-note@-1{{add missing case: '.newCaseOne'}}
+  case .existingCaseOne:
+    return
+  case .existingCaseTwo:
+    return
+  case .existingCaseThree:
+    return    
+  }
+}