SILOptimizer: Fix mandatory switch dispatch elimination for unreachable cases.

#67920 introduced a mandatory SIL
optimization that eliminates switch statement dispatch for enum elements that
are provably unavailable at runtime. This optimization helps eliminate
unreachable code, but it's also load bearing because it removes references to
enum element symbols that may not preset at link time. The optimization was
checking the wrong condition to determine whether an enum element is
unavailable at runtime and so the optimization wasn't working for code using
custom availability domains.

Resolves rdar://161846311.

Author: tshortli

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -926,9 +926,7 @@ static bool eliminateSwitchDispatchOnUnavailableElements(
   SmallVector<std::pair<EnumElementDecl *, SILBasicBlock *>, 4> NewCaseBBs;
   for (unsigned i : range(SWI.getNumCases())) {
     auto CaseBB = SWI.getCase(i);
-    auto availableAtr = CaseBB.first->getUnavailableAttr();
-
-    if (availableAtr && availableAtr->isUnconditionallyUnavailable()) {
+    if (!CaseBB.first->isAvailableDuringLowering()) {
       // Mark the basic block as potentially unreachable.
       SILBasicBlock *UnreachableBlock = CaseBB.second;
       if (!State->PossiblyUnreachableBlocks.contains(UnreachableBlock)) {

test/SILOptimizer/unavailable_enum_element_mandatory_custom_domain.swift

@@ -0,0 +1,63 @@
+// RUN: %target-swift-emit-sil -module-name Test %s -verify -parse-as-library \
+// RUN:   -enable-experimental-feature CustomAvailability \
+// RUN:   -define-enabled-availability-domain EnabledDomain \
+// RUN:   -define-always-enabled-availability-domain AlwaysEnabledDomain \
+// RUN:   -define-disabled-availability-domain DisabledDomain \
+// RUN:   -define-dynamic-availability-domain DynamicDomain \
+// RUN:   -Onone \
+// RUN:   | %FileCheck %s --check-prefixes=CHECK,CHECK-NOOPT
+
+// REQUIRES: swift_feature_CustomAvailability
+
+public enum Enum {
+  case alwaysAvailable
+
+  @available(*, unavailable)
+  case alwaysUnavailable
+
+  @available(EnabledDomain)
+  case enabled
+
+  @available(EnabledDomain, unavailable)
+  case enabledUnavailable
+
+  @available(AlwaysEnabledDomain)
+  case alwaysEnabled
+
+  @available(AlwaysEnabledDomain, unavailable)
+  case alwaysEnabledUnavailable
+
+  @available(DisabledDomain)
+  case disabled
+
+  @available(DisabledDomain, unavailable)
+  case disabledUnavailable
+
+  @available(DynamicDomain)
+  case dynamic
+
+  @available(DynamicDomain, unavailable)
+  case dynamicUnavailable
+}
+
+// CHECK-LABEL: sil @$s4Test22testFullyCoveredSwitchyyAA4EnumOF : $@convention(thin) (Enum) -> () {
+// CHECK:         switch_enum %0, case #Enum.alwaysAvailable!enumelt: {{bb[0-9]+}}, case #Enum.alwaysUnavailable!enumelt: {{bb[0-9]+}}, case #Enum.enabled!enumelt: {{bb[0-9]+}}, case #Enum.alwaysEnabled!enumelt: {{bb[0-9]+}}, case #Enum.disabledUnavailable!enumelt: {{bb[0-9]+}}, case #Enum.dynamic!enumelt: {{bb[0-9]+}}, case #Enum.dynamicUnavailable!enumelt: {{bb[0-9]+}}, default [[DEFAULTBB:bb[0-9]+]]
+// CHECK:       [[DEFAULTBB]]:
+// CHECK-NEXT:    integer_literal $Builtin.Int1, -1
+// CHECK-NEXT:    cond_fail {{%.*}}, "unexpected enum value"
+// CHECK-NEXT:    unreachable
+// CHECK:       } // end sil function '$s4Test22testFullyCoveredSwitchyyAA4EnumOF'
+public func testFullyCoveredSwitch(_ e: Enum) {
+  switch e {
+  case .alwaysAvailable: ()
+  case .alwaysUnavailable: ()
+  case .enabled: ()
+  case .enabledUnavailable: ()
+  case .alwaysEnabled: ()
+  case .alwaysEnabledUnavailable: ()
+  case .disabled: ()
+  case .disabledUnavailable: ()
+  case .dynamic: ()
+  case .dynamicUnavailable: ()
+  }
+}