Argument S to typeof is not an expression

[Meta]

On Tuesday, 7 May 2013 at 10:33:58 UTC, Dicebot wrote:
> On Tuesday, 7 May 2013 at 06:41:25 UTC, Meta wrote:
>> I don't really understand why either of these error messages 
>> are occurring. The first is just incomprehensible, and the 
>> second seems like it should work. In this case, rhs is fully 
>> accessible at compile time in the expression (a + b), so why 
>> does the compiler complain?
>
> 1) typeof don't work on types. Period. It is often inconvenient 
> in generic code (I'd love it typeof(T) to be T for simplicity) 
> but it is how it is done now. In your case pragma(msg, T) will 
> be correct one.
>
> 2) template parameter must be known at compile time. "rhs" is a 
> plain function parameter, so compiler can't be sure it is known 
> at compile time, so it is an error. Same goes for n. In 
> general, only enum's and template parameters can be assumed to 
> be known at compile-time. Your code seems to wrongly mix plain 
> variables with template code all over.
>
> If you can explain for behavior/API you are trying to achieve, 
> most likely I'll be able to provide a more idiomatic D solution.

This is a little test to see if I can wrap types in structs and
do some checking at compile time. For example, statically
verifying in opAdd that neither number being added is negative. I
added in the pragma to see what T's type is because I had the
example compiling before, but two S structs added together was
giving a result of void, and I was trying to figure out why that
was happening.