[ffigen] Functions using pointers to a C++ class are skipped unless cpp: class support is configured #3592

Summary

When a header is parsed as C++ (-x c++ in compilerOptions) but cpp: class support is not configured, any function whose signature mentions a pointer to a class is dropped from the bindings:

[WARNING] Skipped Function 'my_handle_create', function has unsupported return type or parameter type.

A pointer to a class is exactly as expressible as a pointer to a struct — it should be emitted as an Opaque, which is what already happens for struct OtherHandle;. Only the keyword differs.

Reproduction

src/handle.h:

class MyHandle;          // forward-declared, used only through pointers
struct OtherHandle;      // same thing, spelled `struct`

class DefinedHandle {    // defined in the TU, still only used through pointers
 public:
  virtual ~DefinedHandle() = default;
  virtual int value() const = 0;
 private:
  int state_;
};

MyHandle* my_handle_create(void);
void my_handle_destroy(MyHandle* h);
OtherHandle* other_handle_create(void);
DefinedHandle* defined_handle_create(void);

(In a real header these would sit in an extern "C" block; that is left out here because extern "C" is currently ignored entirely, which is a separate issue)

tool/ffigen.dart:

import 'dart:io';

import 'package:ffigen/ffigen.dart';

Future<void> main() async {
  final root = Platform.script.resolve('../');
  await FfiGenerator(
    output: Output(dart: DartOutput(path: root.resolve('lib/bindings.dart'))),
    input: Input(
      entryPoints: [root.resolve('src/handle.h')],
      compilerOptions: ['-x', 'c++', '-std=c++17'],
    ),
    // No `cpp:` configured.
    visitors: [
      Visitor(
        func: (node) => node.isIncluded = {
          'my_handle_create',
          'my_handle_destroy',
          'other_handle_create',
          'defined_handle_create',
        }.contains(node.originalName),
      ),
    ],
  ).generate();
}

Actual (main @ f914da8):

[WARNING] Skipped Function 'my_handle_create', function has unsupported return type or parameter type.
[WARNING] Skipped Function 'my_handle_destroy', function has unsupported return type or parameter type.
[WARNING] Skipped Function 'defined_handle_create', function has unsupported return type or parameter type.

@ffi.Native<ffi.Pointer<OtherHandle> Function()>()
external ffi.Pointer<OtherHandle> other_handle_create();

final class OtherHandle extends ffi.Opaque {}

Expected: all four functions are generated, with MyHandle and DefinedHandle emitted as ffi.Opaque just like OtherHandle.

Cause

In _extractfromRecord (pkgs/ffigen/lib/src/header_parser/type_extractor/extractor.dart), a CXCursor_ClassDecl is only handled by the C++ class parser, which is a no-op when config.cpp == null (and also returns null for anonymous or unavailable classes). Whatever it declines falls through to the UnimplementedType default instead of the StructDecl/UnionDecl fallback, so the enclosing function is removed by parseFunctionDeclaration.

Treating CXCursor_ClassDecl the same as CXCursor_StructDecl in that fallback fixes it: the class goes through importType / parseStructDeclaration and, since it is only ever referenced through pointers, ends up as an Opaque.