[interop] SourceKit should retry generating module interface with C++ interop enabled

Sometimes the request might not have it enabled explicitly

Author: hyp

test/SourceKit/InterfaceGen/gen_clang_cxx_module.swift

@@ -0,0 +1,44 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %sourcekitd-test -req=interface-gen -module CxxModule -- -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -I %t/Inputs -target %target-triple %clang-importer-sdk-nosource | %FileCheck %s
+// RUN: %sourcekitd-test -req=interface-gen -module CxxModule -- -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -cxx-interoperability-mode=swift-5.9 -I %t/Inputs -target %target-triple %clang-importer-sdk-nosource | %FileCheck %s
+
+
+// RUN: not %sourcekitd-test -req=interface-gen -module CxxModule -- -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -Xcc -DERROR  -I %t/Inputs -target %target-triple %clang-importer-sdk-nosource 2>&1 | %FileCheck --check-prefix=NOLOAD %s
+
+
+//--- Inputs/module.modulemap
+module CxxModule {
+    header "headerA.h"
+    requires cplusplus
+}
+
+//--- Inputs/headerA.h
+
+namespace ns {
+
+#ifdef ERROR
+#error "Unluckee"
+#endif
+
+class CxxClass {
+public:
+    int x;
+
+    CxxClass(): x(0) {}
+
+    inline void method() const {}
+};
+
+} // ns
+
+using ClassType = ns::CxxClass;
+
+// CHECK: public enum ns {
+// CHECK: public struct CxxClass {
+// CHECK: public init()
+// CHECK: public func method()
+// CHECK: public typealias ClassType = ns.CxxClass
+
+// NOLOAD: Could not load module: CxxModule

tools/SourceKit/lib/SwiftLang/SwiftEditorInterfaceGen.cpp

@@ -694,8 +694,26 @@ void SwiftLangSupport::editorOpenInterface(EditorConsumer &Consumer,
                                                       SynthesizedExtensions,
                                                       InterestedUSR);
   if (!IFaceGenRef) {
-    Consumer.handleRequestError(ErrMsg.c_str());
-    return;
+      // Retry to generate a module interface with C++ interop enabled,
+      // if the first attempt failed.
+      bool retryWithCxxEnabled = true;
+      for (const auto &arg: Args) {
+          if (StringRef(arg).startswith("-cxx-interoperability-mode=") ||
+              StringRef(arg).startswith("-enable-experimental-cxx-interop")) {
+              retryWithCxxEnabled = false;
+              break;
+          }
+      }
+      if (retryWithCxxEnabled) {
+          std::vector<const char *> AdjustedArgs(Args.begin(), Args.end());
+          AdjustedArgs.push_back("-cxx-interoperability-mode=swift-5.9");
+          return editorOpenInterface(Consumer, Name, ModuleName, Group, AdjustedArgs,
+                                     SynthesizedExtensions, InterestedUSR);
+      }
+      else {
+          Consumer.handleRequestError(ErrMsg.c_str());
+          return;
+      }
   }
 
   IFaceGenRef->reportEditorInfo(Consumer);