Short story: To make delta debugging more useful, gcc's STL system headers should all compile without warnings at the highest error checking level without the use of hardcoded warning suppressions in the compiler based on whether the code is in a system header or not (see http://gcc.gnu.org/ml/gcc-patches/2005-07/msg00049.html for an example of such a suppression).
Long story: Recently I ran into a crash bug in generated code. It was a little hard to figure out, so I decided to try delta debugging (http://www.cs.berkeley.edu/~dsw/) to automatically generate a small testcase. We turned delta loose on the 500KB preprocessed source, with -Wall -Werror to help make sure it didn't generate garbage programs, and it faithfully hacked it down to 8KB which crashed in the same way. Only problem was, because we left the #line directives in the preprocessed source, and delta selectively removed some of them, gcc thought that our entire test case was in a system header. As a result of the DECL_IN_SYSTEM_HEADER warning suppressions, when delta generated garbage, gcc couldn't tell delta it had done so.
