wwwdocs isn't branched, but there should be a version of codingconventions.html with the conventions being followed for the use of C++ on the branch. (Parts of the libstdc++ coding style may be relevant.)
I think a more conservative approach is needed to being buildable with a range of versions than making sure version N is buildable with N-1; I suggest choosing a GCC version between three and five years old and saying that code must be in the intersection of ISO C++98 and the parts of C++ supported by that version (and testing bootstrap with a non-GNU compiler, as well as with the GCC version chosen, before any merge to trunk). For comparison, 2.95 is the current minimum version for building the non-C, non-Ada front ends (when building a cross compiler), while 3.4 is required for building Ada, and all non-Ada front ends can be bootstrapped starting from a non-GNU C compiler. The versions chosen should of course go in gcc/doc/install.texi on the branch. -- Joseph S. Myers [EMAIL PROTECTED]