cppgc: Check for correct base class inheritance

The only valid way to define a GCed type T is by inheriting from
GarbageCollected<T>. Since this is prone to typos (see tests), add a
simple check that covers most interesting use cases.

The static assert covers
  A -> B -> GarbageCollected<C>

The static assert does not cover
 A -> B -> C -> GarbageCollected<B>

(In order to do so, we would need __direct_bases() support which is
not yet available for C++.)

Bug: pdfium:1670, chromium:1056170
Change-Id: I494de48992f8ba9a1f0f9daad60584d828717403
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2810415Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73854}

include/cppgc/allocation.h

@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include <atomic>
+#include <type_traits>
 
 #include "cppgc/custom-space.h"
 #include "cppgc/garbage-collected.h"
@@ -103,6 +104,10 @@ class MakeGarbageCollectedTraitBase
    * \returns the memory to construct an object of type T on.
    */
   V8_INLINE static void* Allocate(AllocationHandle& handle, size_t size) {
+    static_assert(
+        std::is_base_of<typename T::ParentMostGarbageCollectedType, T>::value,
+        "U of GarbageCollected<U> must be a base of T. Check "
+        "GarbageCollected<T> base class inheritance.");
     return SpacePolicy<
         typename internal::GCInfoFolding<
             T, typename T::ParentMostGarbageCollectedType>::ResultType,

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

@@ -48,7 +48,7 @@ class Foo : public GarbageCollected<Foo> {
 size_t Foo::destructor_callcount;
 
 template <size_t Size>
-class GCed : public GarbageCollected<Foo> {
+class GCed : public GarbageCollected<GCed<Size>> {
  public:
   void Trace(cppgc::Visitor*) const {}
   char buf[Size];

test/unittests/heap/cppgc/name-trait-unittest.cc

@@ -23,7 +23,7 @@ struct OtherNoName : public GarbageCollected<OtherNoName> {
   virtual void Trace(Visitor*) const {}
 };
 
-class ClassWithName final : public GarbageCollected<OtherNoName>,
+class ClassWithName final : public GarbageCollected<ClassWithName>,
                             public NameProvider {
  public:
   explicit ClassWithName(const char* name) : name_(name) {}

test/unittests/heap/cppgc/write-barrier-unittest.cc

@@ -387,7 +387,7 @@ struct InlinedObject {
   Member<GCed> ref;
 };
 
-class GCedWithInlinedArray : public GarbageCollected<GCed> {
+class GCedWithInlinedArray : public GarbageCollected<GCedWithInlinedArray> {
  public:
   static constexpr size_t kNumReferences = 4;