extern opaque struct

[John Colvin via Digitalmars-d]

Let's say I have some C headers that have code like this in:
extern struct UndeclaredStruct blah;
Undeclared *p = &blah;

which would naïvely translate to D as:

struct UndeclaredStruct;
extern UndeclaredStruct blah;
auto p = &blah;

which doesn't compile. Why not? Neither the size nor any default 
initialiser is needed.