CTFE limitation on reinterpreting cast

[Rambutan]

Here's a tiny version of something I ran into working on a 
project with both C and D source, while moving into D a C global 
array of structs that was initialized in place.
```
struct B {}
struct C {}

__gshared B Bobj;

auto ptr = cast(C*)&Bobj;

void main() {}
```
Without the cast the initializer `&Bobj` is fine, but with it 
this does not compile.
```
$ gdc ctfecast.d
ctfecast.d:6:20: error: reinterpreting cast from 'B*' to 'C*' is 
not supported in CTFE
     6 | auto ptr = cast(C*)&Bobj;
       |                    ^
```
Why? What can you tell me about this? Is there a workaround? Can 
you point me at connected documentation?

Any information about this will be gratefully received.

R.