What is the compilation model of D?

[David Piepgrass]

> If you use rdmd to compile (instead of dmd), you *just* give it
> your *one* main source file (typically the one with your 
> "main()"
> function). This file must be the *last* parameter passed to 
> rdmd:
>
> $rdmd --build-only (any other flags) main.d
>
> Then, RDMD will figure out *all* of the source files needed 
> (using
> the full compiler's frontend, so it never gets fooled into 
> missing
> anything), and if any of them have been changed, it will 
> automatically
> pass them *all* into DMD for you. This way, you don't have to
> manually keep track of all your files and pass them all into
> DMD youself. Just give RDMD your main file and that's it, 
> you're golden.

I meant to ask, why would it recompile *all* of the source files 
if only one changed? Seems like it only should recompile the 
changed ones (but still compile them together as a unit.) Is it 
because of bugs (e.g. the template problem you mentioned)?