Fix template bugs that prevent rolling gtest DEPS in Chrome.

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 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 121 matching lines @@
 template<typename T> class GarbageCollected;
 class GarbageCollectedMixin;
 template<typename T, bool = WTF::IsSubclassOfTemplate<typename WTF::RemoveConst<T>::Type, GarbageCollected>::value> class NeedsAdjustAndMark;
 
 template<typename T>
 class NeedsAdjustAndMark<T, true> {
 public:
     static const bool value = false;
 };
 
+template <typename T> const bool NeedsAdjustAndMark<T, true>::value;

[Ken Russell (switch to Gerrit), 2014/07/30 22:21:29]

Why is this out-of-line definition needed? Will this cause the variable to be
instantiated into every translation unit which includes this header?

[Nico, 2014/07/30 22:26:02]

Yes, that seems wrong.

[Nick Bray (chromium), 2014/07/30 22:33:45]

It isn't technically "out-of-line" because the inline definition is incomplete
on its own.  static variables in classes are essentially an "extern" declaration
for storage that is declared elsewhere.  Normally that "elsewhere" is in a .cc
file, but for templates you don't want to declare the storage for each concrete
instantiation.  Declaring it in a header file would normally cause a multiple
definition problem because each source file that included the header file would
create its own version, but C++ cheats and has a "one definition rule" that
covers this case.  The templated instantiations of this variable are stored in
the COMDAT section and deduplicated, similar to inline method definitions in a
header file.

http://stackoverflow.com/questions/1834597/what-is-the-comdat-section-used-for

Without this declaration, compilers _may_ be able to const fold accesses to this
field out of existence before the link step, so the missing storage declaration
is not noticed.  No such luck with gtest, however, there was a link error.

+
 template<typename T>
 class NeedsAdjustAndMark<T, false> {
 public:
     static const bool value = WTF::IsSubclass<typename WTF::RemoveConst<T>::Type, GarbageCollectedMixin>::value;
 };
 
+template <typename T> const bool NeedsAdjustAndMark<T, false>::value;
+
 template<typename T, bool = NeedsAdjustAndMark<T>::value> class DefaultTraceTrait;
 
 // The TraceTrait is used to specify how to mark an object pointer and
 // how to trace all of the pointers in the object.
 //
 // By default, the 'trace' method implemented on an object itself is
 // used to trace the pointers to other heap objects inside the object.
 //
 // However, the TraceTrait can be specialized to use a different
 // implementation. A common case where a TraceTrait specialization is
@@ skipping 472 matching lines @@
 struct GCInfoTrait {
     static const GCInfo* get()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
     }
 };
 
 }
 
 #endif