Fix instances of clang -Wbitfield-enum-conversion in Blink

third_party/WebKit/Source/core/svg/properties/SVGAnimatedProperty.cpp

 /*
  * 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 22 matching lines @@
 #include "core/svg/SVGElement.h"
 
 namespace blink {
 
 SVGAnimatedPropertyBase::SVGAnimatedPropertyBase(
     AnimatedPropertyType type,
     SVGElement* contextElement,
     const QualifiedName& attributeName,
     CSSPropertyID cssPropertyId)
     : m_type(type),
-      m_cssPropertyId(cssPropertyId),
+      // Explicitly cast cssPropertyId to silence warnings about bitfield
+      // truncation.

[Nico, 2017/03/14 18:50:00]

Maybe also say why this is safe?

[Reid Kleckner, 2017/03/14 19:37:25]

The DCHECK_EQ is already checking that this is safe.

I'm also not really a fan of these sorts of comments. I'd rather let someone who
wants to know why there's a cast try to remove it and find out what the compiler
says. If someone makes this not a bitfield, for example, this comment will
become stale, and the cast superfluous.

[Nico, 2017/03/14 19:52:24]

Which dcheck? The `DCHECK_EQ(this->cssPropertyId(), cssPropertyId);`?

Imho `// Safe because values that don't fit in bitfield are aliases which aren't
passed to this function, see static_assert in header` or something like that
makes it obvious that the author has thought about this. When i blame code, I
often wonder if something's intentional the way it is or it just happens to be
that way, and "just happens to be that way" is true fairly often. So I find
comments like this helpful.

+      m_cssPropertyId(static_cast<unsigned>(cssPropertyId)),
       m_contextElement(contextElement),
       m_attributeName(attributeName) {
   DCHECK(m_contextElement);
   DCHECK(m_attributeName != QualifiedName::null());
   DCHECK_EQ(this->type(), type);
   DCHECK_EQ(this->cssPropertyId(), cssPropertyId);
 }
 
 SVGAnimatedPropertyBase::~SVGAnimatedPropertyBase() {}
 
 void SVGAnimatedPropertyBase::animationEnded() {
   synchronizeAttribute();
 }
 
 void SVGAnimatedPropertyBase::synchronizeAttribute() {
   AtomicString value(currentValueBase()->valueAsString());
   m_contextElement->setSynchronizedLazyAttribute(m_attributeName, value);
 }
 
 bool SVGAnimatedPropertyBase::isSpecified() const {
   return isAnimating() || contextElement()->hasAttribute(attributeName());
 }
 
 }  // namespace blink