I am not sure if it is entirely proper to post this here, as I have compiled GDC from svn head with DMD 0.161 support (and I'm new to this GDC game), but the code below (which works with DMD) gives the following assert:

cc1d: ../../gcc/d/d-codegen.cc:2933: virtual void AggLayout::doFields(Array*, AggregateDeclaration*): Assertion `((field_decl)->decl.mode) != VOIDmode' failed.
gdctest.d:0: internal compiler error: Aborted

class Foo{ struct Bar { } private Bar bar; }

This is as minimal as I get it, but the non-minimal case actually involved an enum, but since it was the same assert that was tripped, I didn't look further into it.