CrossThreadCopier: Consider arithmetic and enum types as safe.

The original code assumed that types convertible to int were safe to
copy cross-thread. However, this condition is too narrow; any
arithmetic types (integers and floats) and enum types are actually
safe. This is fixed by this patch.

Additionally, this patch rearranges the order of CrossThreadCopierBase's
template parameters, because putting T in the last looks unusual. Also,
the specializations of CrossThreadCopierBase are updated so that the
readers can understand the specializations cover all the possible
combinations of template parameters (except for <T, false, false,
false>, which is intentionally undefined in the header).

BUG=565765
R=haraken@chromium.org, hiroshige@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/1701013003

Cr-Commit-Position: refs/heads/master@{#376129}

third_party/WebKit/Source/platform/CrossThreadCopier.cpp

@@ -87,7 +87,7 @@ static_assert((std::is_same<
 
 
 // Add a generic specialization which will let's us verify that no other template matches.
-template<typename T> struct CrossThreadCopierBase<false, false, false, T> {
+template<typename T> struct CrossThreadCopierBase<T, false, false, false> {
     typedef int Type;
 };
 

third_party/WebKit/Source/platform/CrossThreadCopier.h

@@ -63,18 +63,18 @@ struct CrossThreadCopierPassThrough {
     }
 };
 
-template <bool isConvertibleToInteger, bool isThreadSafeRefCounted, bool isGarbageCollected, typename T>
+template <typename T, bool isArithmeticOrEnum, bool isThreadSafeRefCounted, bool isGarbageCollected>
 struct CrossThreadCopierBase;
 
-// Integers get passed through without any changes.
-template <typename T>
-struct CrossThreadCopierBase<true, false, false, T> : public CrossThreadCopierPassThrough<T> {
+// Arithmetic values (integers or floats) and enums can be safely copied.
+template <typename T, bool isThreadSafeRefCounted, bool isGarbageCollected>
+struct CrossThreadCopierBase<T, true, isThreadSafeRefCounted, isGarbageCollected> : public CrossThreadCopierPassThrough<T> {
     STATIC_ONLY(CrossThreadCopierBase);
 };
 
-// Custom copy methods.
-template <typename T>
-struct CrossThreadCopierBase<false, true, false, T> {
+// Custom copy method for ThreadSafeRefCounted.
+template <typename T, bool isGarbageCollected>
+struct CrossThreadCopierBase<T, false, true, isGarbageCollected> {
     STATIC_ONLY(CrossThreadCopierBase);
     typedef typename WTF::RemoveTemplate<T, RefPtr>::Type TypeWithoutRefPtr;
     typedef typename WTF::RemoveTemplate<TypeWithoutRefPtr, PassRefPtr>::Type TypeWithoutPassRefPtr;
@@ -93,8 +93,9 @@ struct CrossThreadCopierBase<false, true, false, T> {
     }
 };
 
+// A pointer to GarbageCollected.
 template <typename T>
-struct CrossThreadCopierBase<false, false, true, T> {
+struct CrossThreadCopierBase<T, false, false, true> {
     STATIC_ONLY(CrossThreadCopierBase);
     typedef typename std::remove_pointer<T>::type TypeWithoutPointer;
     typedef RawPtr<TypeWithoutPointer> Type;
@@ -105,12 +106,13 @@ struct CrossThreadCopierBase<false, false, true, T> {
 };
 
 template <typename T>
-struct CrossThreadCopier : public CrossThreadCopierBase<std::is_convertible<T, int>::value,
+struct CrossThreadCopier : public CrossThreadCopierBase<
+    T,
+    std::is_arithmetic<T>::value || std::is_enum<T>::value,
     WTF::IsSubclassOfTemplate<typename WTF::RemoveTemplate<T, RefPtr>::Type, ThreadSafeRefCounted>::value
     || WTF::IsSubclassOfTemplate<typename std::remove_pointer<T>::type, ThreadSafeRefCounted>::value
     || WTF::IsSubclassOfTemplate<typename WTF::RemoveTemplate<T, PassRefPtr>::Type, ThreadSafeRefCounted>::value,
-    WTF::IsSubclassOfTemplate<typename std::remove_pointer<T>::type, GarbageCollected>::value,
-    T> {
+    WTF::IsSubclassOfTemplate<typename std::remove_pointer<T>::type, GarbageCollected>::value> {
     STATIC_ONLY(CrossThreadCopier);
 };