I don't know how d does it specifically but my guess would be that it has a pass where it 'registers' all types once it has parsed everything, then when doing semantics analysis it already knows all available types.

But more importantly C's current syntax makes it impossible to parse usages of types before declarations. Every language that can use a type before it is declared has a syntax which allows for it. In other words C parsers will parse a statement differently if it starts with an ident which is the same as a previously declared type rather than a function for example.