Some examples of awkwardness:

1. C tag name space

2. enum values are placed into the enclosing scope

3. structs defined within a struct definition don't go into that definition's scope

4. `if (a = b)` is not allowed in D

5. unprototyped C function declarations

6. C bit fields

7. C struct declarations without a definition

8. oddities of _Alignas and _Alignof behavior

9. _Generic

10. all the weird C compiler extensions

11. etc.

All these things wind up making translation about 95% feasible, and the rest is a mess.