[C++-Interop] Fix EffectiveClangContext for NS_OPTIONS EnumDecl lookup.

lib/ClangImporter/SwiftLookupTable.h

@@ -21,6 +21,7 @@
 #include "swift/Basic/LLVM.h"
 #include "swift/AST/Identifier.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ModuleFileExtension.h"
@@ -197,6 +198,8 @@ class EffectiveClangContext {
       DC = omDecl->getCanonicalDecl();
     } else if (auto fDecl = dyn_cast<clang::FunctionDecl>(dc)) {
       DC = fDecl->getCanonicalDecl();
+    } else if (auto externCDecl = dyn_cast<clang::LinkageSpecDecl>(dc)) {
+      DC = externCDecl->getLexicalDeclContext();
     } else {
       assert(isa<clang::TranslationUnitDecl>(dc) ||
              isa<clang::LinkageSpecDecl>(dc) ||

test/Interop/Cxx/enum/Inputs/c-enums-NS_OPTIONS.h

@@ -0,0 +1,29 @@
+// Enum usage that is bitwise-able and assignable in C++, aka how CF_OPTIONS
+// does things.
+
+#if __has_attribute(enum_extensibility)
+#define __CF_ENUM_ATTRIBUTES __attribute__((enum_extensibility(open)))
+#define __CF_CLOSED_ENUM_ATTRIBUTES __attribute__((enum_extensibility(closed)))
+#define __CF_OPTIONS_ATTRIBUTES __attribute__((flag_enum,enum_extensibility(open)))
+#else
+#define __CF_ENUM_ATTRIBUTES
+#define __CF_CLOSED_ENUM_ATTRIBUTES
+#define __CF_OPTIONS_ATTRIBUTES
+#endif
+
+// explicitly use extern "C" rather than setting it in the modulemap file as
+// would be the case with Foundation's modulemap.
+extern "C" {
+
+#define CF_OPTIONS(_type, _name) _type __attribute__((availability(swift, unavailable))) _name; enum __CF_OPTIONS_ATTRIBUTES : _name
+#define NS_OPTIONS(_type, _name) CF_OPTIONS(_type, _name)
+
+typedef unsigned long NSUInteger;
+
+typedef NS_OPTIONS(NSUInteger, NSBinarySearchingOptions) {
+	NSBinarySearchingFirstEqual = (1UL << 8),
+	NSBinarySearchingLastEqual = (1UL << 9),
+	NSBinarySearchingInsertionIndex = (1UL << 10),
+};
+
+}

test/Interop/Cxx/enum/Inputs/module.modulemap

@@ -17,3 +17,8 @@ module CenumsWithOptionsOmit {
   header "c-enums-withOptions-omit.h"
   requires cplusplus
 }
+
+module CenumsNSOptions {
+  header "c-enums-NS_OPTIONS.h"
+  requires cplusplus
+}

test/Interop/Cxx/enum/c-enums-NS_OPTIONS.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=CenumsNSOptions -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+// REQUIRES: objc_interop
+
+import CenumsNSOptions
+
+// CHECK: typealias NSBinarySearchingOptions = UInt
+// CHECK-NEXT: struct NSBinarySearchingOptions : OptionSet, @unchecked Sendable {
+// CHECK-NEXT:   init(rawValue: UInt)
+// CHECK-NEXT:   let rawValue: UInt
+// CHECK-NEXT:   typealias RawValue = UInt
+// CHECK-NEXT:   typealias Element = NSBinarySearchingOptions
+// CHECK-NEXT:   typealias ArrayLiteralElement = NSBinarySearchingOptions
+// CHECK-NEXT:   static var firstEqual: NSBinarySearchingOptions { get }
+// CHECK-NEXT:   @available(swift, obsoleted: 3, renamed: "firstEqual")
+// CHECK-NEXT:   static var FirstEqual: NSBinarySearchingOptions { get }
+// CHECK-NEXT:   static var lastEqual: NSBinarySearchingOptions { get }
+// CHECK-NEXT:   @available(swift, obsoleted: 3, renamed: "lastEqual")
+// CHECK-NEXT:   static var LastEqual: NSBinarySearchingOptions { get }
+// CHECK-NEXT:   static var insertionIndex: NSBinarySearchingOptions { get }
+// CHECK-NEXT:   @available(swift, obsoleted: 3, renamed: "insertionIndex")
+// CHECK-NEXT:   static var InsertionIndex: NSBinarySearchingOptions { get }
+// CHECK-NEXT: }