[cxx-interop] Forward declare classes

The code already forward declared strutcs and enums. This patch extends
the logic to also forward declare classes. Unfortunately, there was some
fallout because some traits ended up defined multiple times for some
classes, so the code is now extended to only conditionally emit these
traits if no forward declaration was emitted for the type yet.

rdar://124022242

Author: Gabor Horvath

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -325,8 +325,6 @@ class DeclAndTypePrinter::Implementation
 
     if (outputLang == OutputLanguageMode::Cxx) {
       // FIXME: Non objc class.
-      // FIXME: forward decl should be handled by ModuleWriter.
-      ClangValueTypePrinter::forwardDeclType(os, CD, owningPrinter);
       ClangClassTypePrinter(os).printClassTypeDecl(
           CD, [&]() { printMembers(CD->getMembers()); }, owningPrinter);
       recordEmittedDeclInCurrentCxxLexicalScope(CD);

lib/PrintAsClang/DeclAndTypePrinter.h

@@ -15,6 +15,7 @@
 
 #include "OutputLanguageMode.h"
 
+#include "swift/AST/Decl.h"
 #include "swift/AST/Type.h"
 // for OptionalTypeKind
 #include "swift/ClangImporter/ClangImporter.h"

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -117,11 +117,7 @@ class ReferencedTypeFinder : public TypeDeclFinder {
 };
 
 class ModuleWriter {
-  enum class EmissionState {
-    NotYetDefined = 0,
-    DefinitionRequested,
-    Defined
-  };
+  enum class EmissionState { NotYetDefined = 0, DefinitionRequested, Defined };
 
   raw_ostream &os;
   SmallPtrSetImpl<ImportModuleTy> &imports;
@@ -309,7 +305,7 @@ class ModuleWriter {
 
   void forwardDeclareType(const TypeDecl *TD) {
     if (outputLangMode == OutputLanguageMode::Cxx) {
-      if (isa<StructDecl>(TD) || isa<EnumDecl>(TD)) {
+      if (isa<StructDecl>(TD) || isa<EnumDecl>(TD) || isa<ClassDecl>(TD)) {
         auto *NTD = cast<NominalTypeDecl>(TD);
         if (!addImport(NTD))
           forwardDeclareCxxValueTypeIfNeeded(NTD);
@@ -474,7 +470,12 @@ class ModuleWriter {
       return false;
 
     (void)forwardDeclareMemberTypes(CD->getMembers(), CD);
-    seenTypes[CD] = { EmissionState::Defined, true };
+    auto [it, inserted] =
+        seenTypes.try_emplace(CD, EmissionState::NotYetDefined, false);
+    if (outputLangMode == OutputLanguageMode::Cxx &&
+        (inserted || !it->second.second))
+      ClangValueTypePrinter::forwardDeclType(os, CD, printer);
+    it->second = {EmissionState::Defined, true};
     os << '\n';
     printer.print(CD);
     return true;

lib/PrintAsClang/PrintClangValueType.h

@@ -41,7 +41,6 @@ class ClangValueTypePrinter {
   /// corresponds to the given structure or enum declaration.
   void printValueTypeDecl(const NominalTypeDecl *typeDecl,
                           llvm::function_ref<void(void)> bodyPrinter,
-
                           DeclAndTypePrinter &declAndTypePrinter);
 
   /// Print the use of a C++ struct/enum parameter value as it's passed to the

test/Interop/SwiftToCxx/class/swift-class-ordering.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -typecheck -module-name Class -clang-header-expose-decls=all-public -emit-clang-header-path %t/class.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/class.h)
+
+public class SwiftNode {
+}
+
+public struct SwiftLinkedList {
+  public var head: SwiftNode
+}