I think the main reason is that DMD front end sources are dual licensed
with GPL and Artistic License. The DMD backend is not under an open
source license (personal use only), so the Artistic License is how the
two are integrated. The fork is required to allow DMD to continue under
its current license scheme.
It also means that fixes to the GCC front end would not be copyable to
the DMD front end going forward.
Strictly speaking, that's not true. Even if the submitter would still
be required to have copyright assignment for the FSF, they could be
copyable to the DMD front-end _as long as the submitter himself sends
them for inclusion there too_. This is the practical significance of
the license grantback from the FSF to the author.
I'm not sure whether it suffices to otherwise specify the intention to
release the changes under the dual license in the message, and I don't
want to imply in any way that this is possible since IANAL.
That said, 1) I don't think the FSF would be very happy; 2) the question
still stands of whether/how to assign copyright to the FSF for changes
before the inclusion in the gcc.gnu.org repository.
A related topic is this: when is the copyright assigned to the FSF for a
particular patch---for example, when the patch is posted or when it is
committed?(*) In other words, do I have to ask the poster for
permission if I want to get into GCC a patch that was sent to the
mailing list but never committed?
(*) And how does this change when the submitter doesn't have write
access to the repository)?
Paolo
