g++ might emit one of the following lines as part of an error messages:

    foo.hpp:1:4:   required from ‘FOO’
    bar.cpp:2:5:   required from here
    rms.hpp:3:6:   [ skipping EMACS instantiation contexts ]

Right now (GNU Emacs 24.2.50.3 (x86_64-unknown-linux-gnu, GTK+ Version
2.24.10) of 2012-08-26) these are picked up by
compilation-error-regexp-alist-alist as errors although these messages
just provide context to the real error.