It’s a bug that you can use 0.0. The compiler implicitly treats all constant expressions with a value of zero as just 0.
Now, it’s correct for the compiler to allow an implicit conversion from a constant int expression of 0 to your enum as per section 6.1.3 of the C# 5 specification:
An implicit enumeration conversion permits the decimal-integer-literal 0 to be converted to any enum-type and to any nullable-type whose underlying type is an enum-type. In the latter case the conversion is evaluated by converting to the underlying enum-type and wrapping the result (ยง4.1.10).
I’ve spoken with the C# team about this before: they’d have liked to have removed the accidental conversion from 0.0 (and indeed 0.0m and 0.0f) to enum values, but unfortunately I gather it broke too much code – even though it should never have been allowed in the first place.
The Mono mcs compiler prohibits all of these floating point conversions, although it does allow:
const int Zero = 0;
...
SomeEnum x = Zero;
despite the fact that Zero is a constant expression but not a decimal-integer-literal.
I wouldn’t be surprised to see the C# specification change in the future to allow any integer constant expression with a value of 0 (i.e. to mimic mcs), but I wouldn’t expect the floating point conversions to ever officially be correct. (I’ve been wrong before about predicting the future of C#, of course…)