Is this actually supposed to be legal?

[Jonathan M Davis]

On Tuesday, July 17, 2012 23:11:43 Timon Gehr wrote:
> This issue is unrelated to CRTP. (also, you probably want to negate
> that static if condition, otherwise the code is valid and poses no
> challenge to a compiler.)

It's not that it makes the compiler's life hard. It's the fact that 
conditional compilation relies on state that doesn't exist yet. It's messed up 
to be checking whether an object defines something when you're in the middle of 
defining that object.

Now, as David N. points out in another post, this isn't exactly the only case 
of that. You can make a templated type do it to itself via something like 
__traits(derivedMembers, typeof(this)), but in this case, you're doing it on a 
template argument which may or may not be a derived class (though presumably 
is).

So, in any case, it's a problem in that you do have to be careful about doing 
conditional compilation based on the type, since it's in the middle of being 
defined, but that's not necessarily enough to merit getting rid of the feature 
(especially since you can have essentially the same problem even without a 
base class).

- Jonathan M Davis