A declaration like this: bool m_foo : 1; Produces a warning: ERROR: Source/WebCore/platform/animation/Animation.h:202: Please declare enum bitfields as unsigned integral types. [runtime/enum_bitfields] [5] The source code talks about VS8, which is very very old. Also, it seems many people were ignoring this rule anyway.
This warning is no longer relevant. Visual Studio 2013 (our only supported Windows compiler) and newer all support this syntax. This warning should be removed!