[c10] Add default constructor to Maybeowned (pytorch#55128)

Summary: Pull Request resolved: pytorch#55128

Test Plan: CI

Reviewed By: swolchok

Differential Revision: D27495079

fbshipit-source-id: 3bd01956a8b65170d6b38096dbd15c4809904f88

Author: Hao Lu

Diff for: c10/test/util/MaybeOwned_test.cpp

@@ -35,6 +35,17 @@ TEST(MaybeOwnedTest, SimpleDereferencingString) {
   EXPECT_EQ(owned2->size(), x.size());
 }
 
+TEST(MaybeOwnedTest, DefaultCtorInt) {
+  int x = 123;
+  MaybeOwned<int> borrowed, owned;
+  borrowed = MaybeOwned<int>::borrowed(x);
+  owned = MaybeOwned<int>::owned(c10::in_place, x);
+  EXPECT_EQ(*borrowed, x);
+  EXPECT_EQ(*owned, x);
+  EXPECT_EQ(&*borrowed, &x);
+  EXPECT_NE(&*owned, &x);
+}
+
 TEST(MaybeOwnedTest, MoveConstructor) {
   std::string x = "hello";
   auto borrowed = MaybeOwned<std::string>::borrowed(x);

Diff for: c10/util/MaybeOwned.h

@@ -37,7 +37,7 @@ class MaybeOwned final {
   , own_(std::forward<Args>(args)...) {}
 
  public:
-
+  explicit MaybeOwned(): isBorrowed_(true), borrow_(nullptr) {}
   MaybeOwned(const MaybeOwned&) = delete;
   MaybeOwned& operator=(const MaybeOwned&) = delete;
 
@@ -91,10 +91,16 @@ class MaybeOwned final {
   }
 
   const T& operator*() const {
+    if (isBorrowed_) {
+      TORCH_INTERNAL_ASSERT_DEBUG_ONLY(borrow_ != nullptr);
+    }
     return isBorrowed_ ? *borrow_ : own_;
   }
 
   const T* operator->() const {
+    if (isBorrowed_) {
+      TORCH_INTERNAL_ASSERT_DEBUG_ONLY(borrow_ != nullptr);
+    }
     return isBorrowed_ ? borrow_ : &own_;
   }
 };