cppgc: Allow BasicPersistent::Clear() with incomplete type

This allows construction and destruction of empty Persistent and
friends, which simplifiest the use for embedders.

Bug: chromium:1056170
Change-Id: I4286639aa5d50f9f98654b859de10bb80cbada21
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2655505
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72396}

include/cppgc/cross-thread-persistent.h

@@ -168,7 +168,18 @@ class BasicCrossThreadPersistent final : public PersistentBase,
   /**
    * Clears the stored object.
    */
-  void Clear() { Assign(nullptr); }
+  void Clear() {
+    // Simplified version of `Assign()` to allow calling without a complete type
+    // `T`.
+    const void* old_value = GetValue();
+    if (IsValid(old_value)) {
+      PersistentRegionLock guard;
+      PersistentRegion& region = this->GetPersistentRegion(old_value);
+      region.FreeNode(GetNode());
+      SetNode(nullptr);
+    }
+    SetValue(nullptr);
+  }
 
   /**
    * Returns a pointer to the stored object and releases it.

include/cppgc/persistent.h

@@ -194,7 +194,15 @@ class BasicPersistent final : public PersistentBase,
     return static_cast<T*>(const_cast<void*>(GetValue()));
   }
 
-  void Clear() { Assign(nullptr); }
+  void Clear() {
+    // Simplified version of `Assign()` to allow calling without a complete type
+    // `T`.
+    if (IsValid()) {
+      WeaknessPolicy::GetPersistentRegion(GetValue()).FreeNode(GetNode());
+      SetNode(nullptr);
+    }
+    SetValue(nullptr);
+  }
 
   T* Release() {
     T* result = Get();

test/unittests/heap/cppgc/persistent-family-unittest.cc

@@ -864,5 +864,18 @@ TEST_F(PersistentTest, PersistentTraceLocation) {
   }
 }
 
+namespace {
+class IncompleteType;
+}  // namespace
+
+TEST_F(PersistentTest, EmptyPersistentConstructDestructWithoutCompleteType) {
+  // Test ensures that empty constructor and destructor compile without having
+  // a complete type available.
+  Persistent<IncompleteType> p1;
+  WeakPersistent<IncompleteType> p2;
+  subtle::CrossThreadPersistent<IncompleteType> p3;
+  subtle::WeakCrossThreadPersistent<IncompleteType> p4;
+}
+
 }  // namespace internal
 }  // namespace cppgc