If semantic-analyse-proto-impl-toggle fails, jump to any implementation with the correct class, function name and signature.
In the huge (huge !) majority of cases there is only one such match ; and even if there are more than one and you pick the wrong one, it's not worse than returning an error...
To reduce the scope of my searches (and the number of possible mismatches), I also added a notion of project, which I define as a list of header files and a list of source files. For proto-impl-toggle, I only search inside the header's project.