AST: Clang enum elements can't be unavailable during lowering.

There aren't sufficiently strong compiler checks that would prevent unavailable
elements of clang enums from being instantiated at runtime, so don't consider
any clang enum elements as unavailable during lowering. Since there aren't any
symbols associated with clang enum elements, we don't have to worry about
linker failures that might result from leaving references to them in SIL.

Author: tshortli

lib/AST/Availability.cpp

@@ -312,6 +312,9 @@ bool Decl::isAvailableDuringLowering() const {
       UnavailableDeclOptimization::Complete)
     return true;
 
+  if (hasClangNode())
+    return true;
+
   return !isUnconditionallyUnavailable(this);
 }
 

test/SILOptimizer/Inputs/switch_enum_objc.h

@@ -12,3 +12,18 @@ enum Coin {
   CoinHeads,
   CoinTails
 } __attribute__((enum_extensibility(closed)));
+
+// Swift should preserve branches matching the unavailable elements in clang
+// enums since there are not strong compiler protections preventing these values
+// from being instantiated at runtime.
+
+enum Dimension {
+  DimensionX __attribute__((swift_name("x"))),
+  DimensionY __attribute__((swift_name("y"))),
+  DimensionZ __attribute__((swift_name("z"))) __attribute__((unavailable)),
+} __attribute__((enum_extensibility(open)));
+
+enum UnfairCoin {
+  UnfairCoinHeads,
+  UnfairCoinTails __attribute__((unavailable)),
+} __attribute__((enum_extensibility(closed)));

test/SILOptimizer/unavailable_enum_element_mandatory_objc.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-emit-sil -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=none -Onone -enable-objc-interop -disable-objc-attr-requires-foundation-module -import-objc-header %S/Inputs/switch_enum_objc.h | %FileCheck %s
+// RUN: %target-swift-emit-sil -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=complete -Onone -enable-objc-interop -disable-objc-attr-requires-foundation-module -import-objc-header %S/Inputs/switch_enum_objc.h | %FileCheck %s
+
+// CHECK-LABEL: sil @$s4Test30testFullyCoveredSwitchOpenEnumyySo9DimensionVF : $@convention(thin) (Dimension) -> () {
+// CHECK:         switch_enum %0 : $Dimension, case #Dimension.x!enumelt: [[XBB:bb[0-9]+]], case #Dimension.z!enumelt: [[ZBB:bb[0-9]+]], case #Dimension.y!enumelt: [[YBB:bb[0-9]+]], default [[DEFAULTBB:bb[0-9]+]]
+// CHECK:       [[ZBB]]:
+// CHECK-NOT:     unreachable
+// CHECK:       [[YBB]]:
+// CHECK:       } // end sil function '$s4Test30testFullyCoveredSwitchOpenEnumyySo9DimensionVF'
+public func testFullyCoveredSwitchOpenEnum(_ e: Dimension) {
+  switch e {
+  case .x: ()
+  case .z: ()
+  case .y: ()
+  }
+}
+
+// CHECK-LABEL: sil @$s4Test32testFullyCoveredSwitchClosedEnumyySo10UnfairCoinVF : $@convention(thin) (UnfairCoin) -> () {
+// CHECK:         switch_enum %0 : $UnfairCoin, case #UnfairCoin.heads!enumelt: [[HEADSBB:bb[0-9]+]], case #UnfairCoin.tails!enumelt: [[TAILSBB:bb[0-9]+]], default [[DEFAULTBB:bb[0-9]+]]
+// CHECK:       [[TAILSBB]]:
+// CHECK-NOT:     unreachable
+// CHECK:       [[DEFAULTBB]]:
+// CHECK:       } // end sil function '$s4Test32testFullyCoveredSwitchClosedEnumyySo10UnfairCoinVF'
+public func testFullyCoveredSwitchClosedEnum(_ e: UnfairCoin) {
+  switch e {
+  case .heads: ()
+  case .tails: ()
+  }
+}