[TorchBind] Support using lambda function as TorchBind constructor/in…

…it method

test/cpp/jit/test_custom_class_registrations.cpp

@@ -33,6 +33,14 @@ struct Foo : torch::CustomClassHolder {
   }
 };
 
+struct LambdaInit : torch::CustomClassHolder {
+  int x, y;
+  LambdaInit(int x_, int y_) : x(x_), y(y_) {}
+  int64_t diff() {
+    return this->x - this->y;
+  }
+};
+
 struct NoInit : torch::CustomClassHolder {
   int64_t x;
 };
@@ -202,6 +210,16 @@ TORCH_LIBRARY(_TorchScriptTesting, m) {
       .def("add", &Foo::add)
       .def("combine", &Foo::combine);
 
+  m.class_<LambdaInit>("_LambdaInit")
+      .def(torch::init([](int64_t x, int64_t y, bool swap) {
+        if (swap) {
+          return c10::make_intrusive<LambdaInit>(y, x);
+        } else {
+          return c10::make_intrusive<LambdaInit>(x, y);
+        }
+      }))
+      .def("diff", &LambdaInit::diff);
+
   m.class_<NoInit>("_NoInit").def(
       "get_x", [](const c10::intrusive_ptr<NoInit>& self) { return self->x; });
 

test/jit/test_torchbind.py

@@ -338,3 +338,10 @@ def test_torchbind_attr_exception(self):
         foo = torch.classes._TorchScriptTesting._StackString(["test"])
         with self.assertRaisesRegex(AttributeError, 'does not have a field'):
             foo.bar
+
+    def test_lambda_as_constructor(self):
+        obj_no_swap = torch.classes._TorchScriptTesting._LambdaInit(4, 3, False)
+        self.assertEqual(obj_no_swap.diff(), 1)
+
+        obj_swap = torch.classes._TorchScriptTesting._LambdaInit(4, 3, True)
+        self.assertEqual(obj_swap.diff(), -1)

torch/custom_class.h

@@ -27,6 +27,21 @@ detail::types<void, Types...> init() {
   return detail::types<void, Types...>{};
 }
 
+template <typename Func, typename... ParameterTypeList>
+struct InitLambda {
+  Func f;
+};
+
+template <typename Func>
+decltype(auto) init(Func&& f) {
+  using InitTraits =
+      c10::guts::infer_function_traits_t<std::decay_t<Func>>;
+  using ParameterTypeList = typename InitTraits::parameter_types;
+
+  InitLambda<Func, ParameterTypeList> init{std::forward<Func>(f)};
+  return init;
+}
+
 /// Entry point for custom C++ class registration. To register a C++ class
 /// in PyTorch, instantiate `torch::class_` with the desired class as the
 /// template parameter. Typically, this instantiation should be done in
@@ -95,6 +110,24 @@ class class_ {
     return *this;
   }
 
+  // Used in combination with torch::init([]lambda(){......})
+  template <typename Func, typename... ParameterTypes>
+  class_& def(
+      InitLambda<Func, c10::guts::typelist::typelist<ParameterTypes...>> init,
+      std::string doc_string = "") {
+    auto init_lambda_wrapper = [func = std::move(init.f)](
+                                   c10::tagged_capsule<CurClass> self,
+                                   ParameterTypes... arg) {
+      c10::intrusive_ptr<CurClass> classObj =
+          at::guts::invoke(func, std::forward<ParameterTypes>(arg)...);
+      auto object = self.ivalue.toObject();
+      object->setSlot(0, c10::IValue::make_capsule(classObj));
+    };
+    defineMethod("__init__", std::move(init_lambda_wrapper), std::move(doc_string));
+
+    return *this;
+  }
+
   /// This is the normal method registration API. `name` is the name that
   /// the method will be made accessible by in Python and TorchScript.
   /// `f` is a callable object that defines the method. Typically `f`