Give cpp::Function a null mPtr for generational write barriers.

include/cpp/Pointer.h

@@ -426,6 +426,12 @@ class Function
 public:
    T *call;
 
+   #ifdef HXCPP_GC_GENERATIONAL
+   // Haxe versions without HaxeFoundation/haxe#13002 write barrier cpp.Function
+   // fields as objects. A null mPtr turns those barriers into no-ops.
+   static constexpr hx::Object *mPtr = 0;
+   #endif
+
    inline Function( ) { }
    inline Function( const Function &inRHS ) : call(inRHS.call) {  }
    inline Function( const Dynamic &inRHS) { call = inRHS==null()?0: (T*)inRHS->__GetHandle(); }

test/haxe/TestTypes.hx

@@ -12,6 +12,7 @@ class TestTypes extends Test
    var i6:cpp.UInt8 = 1;
    var i7:cpp.Int16 = 1;
    var i8:cpp.UInt16 = 1;
+   var callback:cpp.Function<Int->Int, cpp.abi.Abi>;
 
    public function new() super();
 
@@ -49,4 +50,15 @@ class TestTypes extends Test
       Assert.equals(d.i6, 1);
       Assert.equals(d.i7, 1);
    }
+
+   static function increment(value:Int):Int
+   {
+      return value + 1;
+   }
+
+   public function testFunctionField()
+   {
+      callback = cpp.Function.fromStaticFunction(increment);
+      Assert.equals(callback.call(41), 42);
+   }
 }