SILGen: Don't stub unavailable raw value initializers for clang enums.

We must be conservative when generating SIL for raw value initializers of clang
enums and not insert an unavailable code reached trap, even if the enum is
technically unavailable. It appears that there is a long-standing loophole that
allows a clang module to typedef an unavailable type in order to make that type
available in Swift even though the underlying type is declared to be
unavailable in Swift. This loophole is load-bearing for some existing Swift
overlays.

Resolves rdar://116378269

Author: tshortli

lib/AST/Availability.cpp

@@ -23,6 +23,7 @@
 #include "swift/AST/TypeWalker.h"
 #include "swift/AST/Types.h"
 #include "swift/Basic/Platform.h"
+#include "swift/ClangImporter/ClangModule.h"
 #include <map>
 
 using namespace swift;
@@ -312,7 +313,7 @@ bool Decl::isAvailableDuringLowering() const {
       UnavailableDeclOptimization::Complete)
     return true;
 
-  if (hasClangNode())
+  if (isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext()))
     return true;
 
   return !isUnconditionallyUnavailable(this);
@@ -325,6 +326,9 @@ bool Decl::requiresUnavailableDeclABICompatibilityStubs() const {
       UnavailableDeclOptimization::Stub)
     return false;
 
+  if (isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext()))
+    return false;
+
   return isUnconditionallyUnavailable(this);
 }
 

test/SILGen/unavailable_clang_enum_typedef.swift

@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-emit-silgen %t/Overlay.swift -I %t -import-objc-header %t/MyOptions.h -unavailable-decl-optimization=none | %FileCheck %t/Overlay.swift
+// RUN: %target-swift-emit-silgen %t/Overlay.swift -I %t -import-objc-header %t/MyOptions.h -unavailable-decl-optimization=stub | %FileCheck %t/Overlay.swift
+// RUN: %target-swift-emit-silgen %t/Overlay.swift -I %t -import-objc-header %t/MyOptions.h -unavailable-decl-optimization=complete | %FileCheck %t/Overlay.swift
+
+//--- MyOptions.h
+
+__attribute__((availability(swift, unavailable, message="Unavailable in Swift")))
+typedef enum : int {
+  SomeOption,
+} MyOptions;
+
+typedef MyOptions MyOptionsTypedef;
+
+//--- Overlay.swift
+
+// This really shouldn't be allowed, but it is.
+let _ = MyOptionsTypedef(rawValue: 1)
+
+// CHECK-LABEL: sil shared [transparent] [serialized]{{.*}} @$sSo9MyOptionsa8rawValueABs5Int32V_tcfC : $@convention(method) (Int32, @thin MyOptions.Type) -> MyOptions {
+// CHECK-NOT:     ss31_diagnoseUnavailableCodeReacheds5NeverOyFTwb
+// CHECK:       } // end sil function '$sSo9MyOptionsa8rawValueABs5Int32V_tcfC'