From 270ea8b921d280b48a54621e2823320a7a104e07 Mon Sep 17 00:00:00 2001
From: Allan Shortlidge <shortlidge@apple.com>
Date: Fri, 3 Oct 2025 14:57:32 -0700
Subject: [PATCH] SILOptimizer: Fix mandatory switch dispatch elimination for
 unreachable cases.

https://github.com/swiftlang/swift/pull/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.
---
 .../Mandatory/DiagnoseUnreachable.cpp         |  4 +-
 ...enum_element_mandatory_custom_domain.swift | 63 +++++++++++++++++++
 2 files changed, 64 insertions(+), 3 deletions(-)
 create mode 100644 test/SILOptimizer/unavailable_enum_element_mandatory_custom_domain.swift

diff --git a/lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp b/lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp
index ea9051d3156ba..f2ddc950a02f9 100644
--- a/lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp
+++ b/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)) {
diff --git a/test/SILOptimizer/unavailable_enum_element_mandatory_custom_domain.swift b/test/SILOptimizer/unavailable_enum_element_mandatory_custom_domain.swift
new file mode 100644
index 0000000000000..30c4bd5b5d26d
--- /dev/null
+++ b/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: ()
+  }
+}