Only check for on-heap Member<T>s iff T is in scope.

Source/platform/heap/Handle.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 771 matching lines @@
         if (reinterpret_cast<intptr_t>(m_raw) % allocationGranularity)
             return;
 
         // TODO(haraken): What we really want to check here is that the pointer
         // is a traceable object. In other words, the pointer is either of:
         //
         //   (a) a pointer to the head of an on-heap object.
         //   (b) a pointer to the head of an on-heap mixin object.
         //
         // We can check it by calling Heap::isHeapObjectAlive(m_raw),
-        // but we cannot call it here because it requres to include T.h.
-        // So we currently implement only the check for (a).
-        if (!IsGarbageCollectedMixin<T>::value)
+        // but we cannot call it here because it requires to include T.h.
+        // So we currently only try to implement the check for (a), but do
+        // not insist that T's definition is in scope.
+        if (IsFullyDefined<T>::value && !IsGarbageCollectedMixin<T>::value)

[Nico, 2015/06/30 20:04:45]

If this assert fires now depends on .h include order, right? If so, that seems
kind of bad too.

(If not:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...
just inlines the static_assert – not sure this IsFullyDefined class is needed)

Did you try my suggestion of out-of-lining a few destructors?

[sof, 2015/06/30 20:10:59]

That's entirely to be expected. e.g., TrackBase doesn't #include
HTMLMediaElement, because it cannot without stepping into a cyclic dependency. 

So, I don't know what your concern is wrt this particular check.

             ASSERT(HeapObjectHeader::fromPayload(m_raw)->checkHeader());
 #endif
     }
 
     T* m_raw;
 
     template<bool x, WTF::WeakHandlingFlag y, WTF::ShouldWeakPointersBeMarkedStrongly z, typename U, typename V> friend struct CollectionBackingTraceTrait;
     friend class Visitor;
 
 };
@@ skipping 482 matching lines @@
 struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif