http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53930
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-07-12 10:09:10 UTC --- ISO 14882:2011 3.2 "There can be more than one definition of a class type (Clause 9), [...] in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then — each definition of D shall consist of the same sequence of tokens; [...]" So you can have A defined in two different files, but it must be defined identically, because it refers to the same class.
