Project Elvis

[Ola Fosheim Grøstad]

On Sunday, 29 October 2017 at 15:57:19 UTC, Steven Schveighoffer 
wrote:
> I would have expected Nullable!int to fit the bill, but 
> unfortunately, opCast(bool) doesn't work on a null Nullable.
>
> But certainly you can construct a type that does work.

The right thing to do is to create a type that you cannot cast to 
bool, but where you can test for invalid values and substitute in 
a default.

Forcing people to have a boolean interpretation of a type mean 
valid/invalid state has viral consequences. It means that if the 
type has a zero value then a boolean interpretation of zero now 
should give true. That's not good for generic code.

Float and NaN is another use case.