GDC adds intrinsic support for core.checkedint

[tsbockman via Digitalmars-d]

After thinking about the both the exception issue, and the 
implicit conversion issue, I believe the best solution would be 
something like this:

Have an additional template parameter to CheckedInt!T which 
controls whether an implicit conversion to type T is available or 
not. If there is an implicit conversion, attempting to use it on 
a NaN value will throw an exception.

This approach will keep the user reasonably safe from 
accidentally ignoring a NaN condition for both the throwing and 
the nothrow versions. The throwing version will just be a little 
more syntactically convenient.

This behaviour should probably extend to explicit opCast!V() 
calls as well, where V is an integral type.

Unchecked unwrapping will always be available either way, by 
using CheckedInt.value.

To ease the writing of generic code which might accept a built-in 
integer or a CheckedInt, a template function like this can be 
included:

auto numValue(T)(in T t) pure
	if(isNumeric!T || isCheckedInt!T || isCIStore!T)
{
	static if(isCheckedInt!T || isCIStore!T)
		return t.value;
	else
		return t;
}

(Something like this actually already exists in both my version 
and Robert's version, for internal use.)