One interesting issue is whether it is important for gcc to be able to be compiled with foreign compilers (other than gcc).
I know that historically this has been an important requirement, but I wonder whether it is still relevant. Gcc is very widespread at this point. Yes, there is the issue of completely new targets, but these can be easily handled by building cross-compilers. I suspect that the use of C++ will make the use of foreign compilers harder, but I suspect also that it doesn't matter. When we first wrote the front end of GNAT in Ada, we worried about the fact that it meant that GNAT needed a version of gcc to be compiled. We had various schemes to get around this (generate C, generate MIPS code and use a MIPS simulator etc). But as it turned out, the requirement for a previous version of gcc did not turn out to be an impediment in getting GNAT spread to all sorts of strange machines. We do take care to ensure that the front end and critical tools can be compiled with an old version of GNAT, so e.g. we avoid using new features like conditional expressions in the GNAT front end.
